Fatty acid synthesis
Encyclopedia
Fatty acid synthesis is the creation of fatty acid
s from acetyl-CoA
and malonyl-CoA
precursors through action of enzymes called fatty acid synthase
s. It is an important part of the lipogenesis
process, which - together with glycolysis
- stands behind creating fats
from blood sugar in living organisms.
is produced.
The diagrams presented show how fatty acids are synthesized in microorganisms and list the enzymes found in Escherichia coli
. These reactions are performed by fatty acid synthase
II (FASII), which contain multiple enzymes that generally act as one complex. FASII is present in prokaryotes, plants, fungi, parasites, as well as in mitochondria.
In animals, as well as yeast and some fungi, these same reactions occur on fatty acid synthase I (FASI), a large dimeric protein, which has all of the enzymatic activities required to create a fatty acid. FASI is less efficient than FASII, however, it allows for the formation of more molecules, including “medium-chain” fatty acids via early chain termination.
Once a 16:0 carbon fatty acid has been formed it can undergo a number of modifications; particularly, by fatty acid synthase III (FASIII), which uses 2 carbon molecules to elongate preformed fatty acids.
Regulation
Acetyl-CoA is formed into malonyl-CoA by acetyl-CoA carboxylase
, at which point malonyl-CoA is destined to feed into the fatty acid synthesis pathway. Acetyl-CoA carboxylase is the point of regulation in saturated straight-chain fatty acid synthesis, by both phosphorylation
and allosteric regulation
. Regulation by phosphorylation occurs mostly in mammals, while allosteric regulation occurs in most organisms. Allosteric control occurs as feedback inhibition by palmitol-CoA and activation by citrate. When there are high levels of palmitol-CoA, the final product of saturated fatty acid synthesis, it allosterically inactivates acetyl-CoA carboxylase to prevent a build up of fatty acids in cells. Citrate acts to activate acetyl-CoA carboxylase under high levels, because high levels indicate there is enough acetyl-CoA to feed into the Krebs cycle and produce energy.
De Novo Synthesis in Humans
In humans, fatty acids are predominantly formed in the liver
and lactating mammary glands, and, to a lesser extent, the adipose tissue
. Most acetyl-CoA
is formed from pyruvate by pyruvate dehydrogenase
in the mitochondria. Acetyl-CoA
produced in the mitochondria is condensed with oxaloacetate by citrate synthase
to form citrate
, which is then transported into the cytosol and broken down to yield acetyl-CoA
and oxaloacetate by ATP citrate lyase
. Oxaloacetate in the cytosol is reduced to malate
by cytoplasmic malate dehydrogenase
, and malate is transported back into the mitochondria to participate in the Citric acid cycle
.
Most bacteria that undergo anaerobic desaturation contain homologues of FabA and FabB. Clostridia are the main exception; they have a novel enzyme, yet to be identified, that catalyzes the formation of the cis double bond.
Regulation
This pathway undergoes transcriptional regulation
by FadR
and FabR. FadR is the more extensively studied protein and has been attributed bifunctional characteristics. It acts as an activator of fabA and fabB transcription and as a repressor
for the β-oxidation regulon
. FabR, conversely, acts as a repressor for the transcription of fabA and fabB.
, the desaturase, Δ5-Des, is specific for inducing a cis-double bond at the Δ5 position. Saccharomyces cerevisiae
contains one desaturase, Ole1p, which induces the cis-double bond at Δ9.
Regulation
In B. subtilis, this pathway is regulated by a two-component system: DesK and DesR. DesK is a membrane associated kinase and DesR is a transcriptional regulator of the des gene. The regulation responds to temperature; when there is a drop in temperature this gene is upregulated. Unsaturated fatty acids decrease the fluidity of the membrane and stabilize it under lower temperatures. DesK is the sensor protein that when there is a decrease in temperature, will autophosphorylate. DesK-P will transfer its phosphoryl group to DesR. Two DesR-P proteins will dimerize and bind to the DNA promoters of the des gene and recruit RNA polymerase to begin transcription.
Pseudomonas aeruginosa
Generally, both anaerobic and aerobic unsaturated fatty acid synthesis will not occur within the same system, however Pseudomonas aeruginosa
and Vibrio ABE-1 are exceptions.
While, P. aeruginosa primarily undergoes anaerobic desaturation, it also undergoes two aerobic pathways. One pathway utilizes a Δ9 desaturase (DesA) that catalyzes a double bond formation in membrane lipids. Another pathway uses two proteins, DesC and DesB, together to act as a Δ9 desaturase which inserts a double bond into a saturated fatty acid-CoA molecule. This second pathway is regulated by repressor protein, DesT. DesT is also a repressor of fabAB expression for anaerobic desaturation when in presence of exogenous unsaturated fatty acids. This functions to coordinate the expression of the two pathways within the organism.
contain unique branch chain fatty acid synthesis mechanisms, including that which forms tuberculosteric acid.
The branched-chain fatty acid synthesizing system uses α-keto acids as primers. This system is distinct from the branched-chain fatty acid synthetase which utilizes short-chain acyl-CoA esters as primers. α-keto acid primers are derived from the transamination
and decarboxylation
of valine
, leucine
, and isoleucine
to form 2-methylpropanyl-CoA, 3-methylbutyryl-CoA, and 2-Methylbutyryl-CoA, respectively. 2-methylpropanyl-CoA primers derived from valine are elongated to produce even-numbered iso-series fatty acids such as 14-methyl-pentadecanoic (isopalmitic) acid, and 3-methylbutyryl-CoA primers from leucine may be used to form odd numbered iso-series fatty acids such as 13-methyl-tetradecanoic acid. 2-Methylbutyryl-CoA primers from isoleucine are elongated to form anteiso-series fatty acids containing an odd number of carbon atoms such as 12-Methyl tetradecanoic acid. Decarboxylation of the primer precursors occurs through the branched-chain α-keto acid decarboxylase
(BCKA) enzyme. Elongation of the fatty acid follows the same biosynthetic pathway in Escherichia coli used to produce straight-chain fatty acids where malonyl-CoA is used as a chain extender. The major end products are 12-17 carbon branched-chain fatty acids and their composition tends to be uniform and characteristic for many bacterial species.
BCKA decarboxylase and relative activities of α-keto acid substrates
The BCKA decarboxylase enzyme is composed of two subunits in a tetrameric structure (A2B2) and is essential for the synthesis of branched-chain fatty acids. It is responsible for the decarboxylation of α-keto acids formed by the transamination of valine, leucine, and isoleucine and produces the primers used for branched-chain fatty acid synthesis. The activity of this enzyme is much higher with branched-chain α-keto acid substrates than straight-chain substrates, and in Bacillus
species its specificity is highest for the isoleucine-derived α-keto-β-methylvaleric acid, followed by α-ketoisocaproate and α-ketoisovalerate. The enzyme’s high affinity toward branched-chain α-keto acids allows it to function as the primer donating system for branched-chain fatty acid synthetase.
Factors affecting chain length and pattern distribution
α-keto acid primers are used to produce branched-chain fatty acids that are generally between 12 and 17 carbons in length. The proportions of these branched-chain fatty acids tend to be uniform and consistent among a particular bacterial species but may be altered due to changes in malonyl-CoA concentration, temperature, or heat-stable factors (HSF) present. All of these factors may affect chain length, and HSF’s have been demonstrated to alter the specificity of BCKA decarboxylase for a particular α-keto acid substrate, thus shifting the ratio of branched-chain fatty acids produced. An increase in malonyl-CoA concentration has been shown to result in a larger proportion of C17 fatty acids produced, up until the optimal concentration (≈20μM) of malonyl-CoA is reached. Decreased temperatures also tend to shift the fatty-acid distribution slightly toward C17 fatty-acids in Bacillus species.
The over all reaction is:
Isobutyryl-CoA + 6 malonyl-CoA +12 NADPH + 12H+ = Isoplmitic acid + 6CO2 12NADP + 5H2O + 7CoA
The difference between (straight-chain) fatty acid synthase and branch-chain fatty acid synthase is substrate specificity of the enzyme that catalyzes the reaction of acyl-CoA to acyl-ACP.
which is then converted to cyclohexylcarboxylic acid-CoA esters that serve as primers for omega-alicyclic fatty acid synthesis
(D-10-Methylstearic acid) is a saturated fatty acid that is known to be produced by Mycobacterium
spp. and two species of Streptomyces
. It is formed from the precursor oleic acid (a monosaturated fatty acid). After oleic acid is esterified to a phospholipid, S-adenosyl-methionine donates a methyl group to the double bond of oleic acid. This methylation reaction forms the intermediate 10-methylene-octadecanoyal. Successive reduction of the residue, with NADPH as a cofactor, results in 10-methylstearic acid
Fatty acid
In chemistry, especially biochemistry, a fatty acid is a carboxylic acid with a long unbranched aliphatic tail , which is either saturated or unsaturated. Most naturally occurring fatty acids have a chain of an even number of carbon atoms, from 4 to 28. Fatty acids are usually derived from...
s from acetyl-CoA
Acetyl-CoA
Acetyl coenzyme A or acetyl-CoA is an important molecule in metabolism, used in many biochemical reactions. Its main function is to convey the carbon atoms within the acetyl group to the citric acid cycle to be oxidized for energy production. In chemical structure, acetyl-CoA is the thioester...
and malonyl-CoA
Malonyl-CoA
Malonyl-CoA is a coenzyme A derivative.-Functions:It plays a key role in chain elongation in fatty acid biosynthesis and polyketide biosynthesis....
precursors through action of enzymes called fatty acid synthase
Fatty acid synthase
Fatty acid synthase is an enzyme that in humans is encoded by the FASN gene.Fatty acid synthase is a multi-enzyme protein that catalyzes fatty acid synthesis...
s. It is an important part of the lipogenesis
Lipogenesis
Lipogenesis is the process by which acetyl-CoA is converted to fats. The former is an intermediate stage in metabolism of simple sugars, such as glucose, a source of energy of living organisms. Through lipogenesis, the energy can be efficiently stored in the form of fats...
process, which - together with glycolysis
Glycolysis
Glycolysis is the metabolic pathway that converts glucose C6H12O6, into pyruvate, CH3COCOO− + H+...
- stands behind creating fats
Fats
Fats may refer to:*Fat, a generic term for a class of lipids in biochemistry*Fats Domino, rock and roll pianist*Fats Waller, jazz pianist*Fats Navarro, jazz trumpeter*Singing Fats, master of ceremonies*Obesity...
from blood sugar in living organisms.
Straight-Chain Fatty Acids
Straight-chain fatty acids occur in two types; saturated and unsaturated.Saturated Straight-Chain Fatty Acids
Much like β-oxidation, straight-chain fatty acid synthesis occurs via the six recurring reactions shown below, until the 16 carbon palmitic acidPalmitic acid
Palmitic acid, or hexadecanoic acid in IUPAC nomenclature, is one of the most common saturated fatty acids found in animals and plants. Its molecular formula is CH314CO2H. As its name indicates, it is a major component of the oil from palm trees . Palmitate is a term for the salts and esters of...
is produced.
The diagrams presented show how fatty acids are synthesized in microorganisms and list the enzymes found in Escherichia coli
Escherichia coli
Escherichia coli is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls...
. These reactions are performed by fatty acid synthase
Fatty acid synthase
Fatty acid synthase is an enzyme that in humans is encoded by the FASN gene.Fatty acid synthase is a multi-enzyme protein that catalyzes fatty acid synthesis...
II (FASII), which contain multiple enzymes that generally act as one complex. FASII is present in prokaryotes, plants, fungi, parasites, as well as in mitochondria.
In animals, as well as yeast and some fungi, these same reactions occur on fatty acid synthase I (FASI), a large dimeric protein, which has all of the enzymatic activities required to create a fatty acid. FASI is less efficient than FASII, however, it allows for the formation of more molecules, including “medium-chain” fatty acids via early chain termination.
Once a 16:0 carbon fatty acid has been formed it can undergo a number of modifications; particularly, by fatty acid synthase III (FASIII), which uses 2 carbon molecules to elongate preformed fatty acids.
| Step | Enzyme | Reaction | Description |
|---|---|---|---|
| (a) | Acetyl CoA:ACP transacylase | Activates acetyl CoA for reaction with malonyl-ACP | |
| (b) | Malonyl CoA:ACP transacylase | Activates malonyl CoA for reaction with acetyl-ACP | |
| (c) | 3-ketoacyl-ACP synthetase | Reacts priming acetyl-ACP with chain extending malonyl-ACP. | |
| (d) | 3-ketoacyl-ACP reductase | Reduces the carbon 3 ketone to a hydroxyl group | |
| (e) | 3-hydroxyacyl-ACP dehydrase | Removes water | |
| (f) | Enoyl-ACP reductase | Reduces the C3-C4 double bond. |
Regulation
Acetyl-CoA is formed into malonyl-CoA by acetyl-CoA carboxylase
Acetyl-CoA carboxylase
Acetyl-CoA carboxylase is a biotin-dependent enzyme that catalyzes the irreversible carboxylation of acetyl-CoA to produce malonyl-CoA through its two catalytic activities, biotin carboxylase and carboxyltransferase...
, at which point malonyl-CoA is destined to feed into the fatty acid synthesis pathway. Acetyl-CoA carboxylase is the point of regulation in saturated straight-chain fatty acid synthesis, by both phosphorylation
Phosphorylation
Phosphorylation is the addition of a phosphate group to a protein or other organic molecule. Phosphorylation activates or deactivates many protein enzymes....
and allosteric regulation
Allosteric regulation
In biochemistry, allosteric regulation is the regulation of an enzyme or other protein by binding an effector molecule at the protein's allosteric site . Effectors that enhance the protein's activity are referred to as allosteric activators, whereas those that decrease the protein's activity are...
. Regulation by phosphorylation occurs mostly in mammals, while allosteric regulation occurs in most organisms. Allosteric control occurs as feedback inhibition by palmitol-CoA and activation by citrate. When there are high levels of palmitol-CoA, the final product of saturated fatty acid synthesis, it allosterically inactivates acetyl-CoA carboxylase to prevent a build up of fatty acids in cells. Citrate acts to activate acetyl-CoA carboxylase under high levels, because high levels indicate there is enough acetyl-CoA to feed into the Krebs cycle and produce energy.
De Novo Synthesis in Humans
In humans, fatty acids are predominantly formed in the liver
Liver
The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
and lactating mammary glands, and, to a lesser extent, the adipose tissue
Adipose tissue
In histology, adipose tissue or body fat or fat depot or just fat is loose connective tissue composed of adipocytes. It is technically composed of roughly only 80% fat; fat in its solitary state exists in the liver and muscles. Adipose tissue is derived from lipoblasts...
. Most acetyl-CoA
Acetyl-CoA
Acetyl coenzyme A or acetyl-CoA is an important molecule in metabolism, used in many biochemical reactions. Its main function is to convey the carbon atoms within the acetyl group to the citric acid cycle to be oxidized for energy production. In chemical structure, acetyl-CoA is the thioester...
is formed from pyruvate by pyruvate dehydrogenase
Pyruvate dehydrogenase
Pyruvate dehydrogenase complex is a complex of three enzymes that transform pyruvate into acetyl-CoA by a process called pyruvate decarboxylation. Acetyl-CoA may then be used in the citric acid cycle to carry out cellular respiration, and this complex links the glycolysis metabolic pathway to the...
in the mitochondria. Acetyl-CoA
Acetyl-CoA
Acetyl coenzyme A or acetyl-CoA is an important molecule in metabolism, used in many biochemical reactions. Its main function is to convey the carbon atoms within the acetyl group to the citric acid cycle to be oxidized for energy production. In chemical structure, acetyl-CoA is the thioester...
produced in the mitochondria is condensed with oxaloacetate by citrate synthase
Citrate synthase
The enzyme citrate synthase exists in nearly all living cells and stands as a pace-making enzyme in the first step of the Citric Acid Cycle . Citrate synthase is localized within eukaryotic cells in the mitochondrial matrix, but is encoded by nuclear DNA rather than mitochondrial...
to form citrate
Citrate
A citrate can refer either to the conjugate base of citric acid, , or to the esters of citric acid. An example of the former, a salt is trisodium citrate; an ester is triethyl citrate.-Other citric acid ions:...
, which is then transported into the cytosol and broken down to yield acetyl-CoA
Acetyl-CoA
Acetyl coenzyme A or acetyl-CoA is an important molecule in metabolism, used in many biochemical reactions. Its main function is to convey the carbon atoms within the acetyl group to the citric acid cycle to be oxidized for energy production. In chemical structure, acetyl-CoA is the thioester...
and oxaloacetate by ATP citrate lyase
ATP citrate lyase
ATP citrate lyase is an enzyme that represents an important step in fatty acid biosynthesis. This step in fatty acid biosynthesis occurs because ATP citrate lyase is the link between the metabolism of carbohydrates to the production of fatty acids.-Function:ATP citrate lyase is the primary enzyme...
. Oxaloacetate in the cytosol is reduced to malate
Malate
Malate is the ionized form of malic acid. It is an important chemical compound in biochemistry. In the C4 carbon fixation process, malate is a source of CO2 in the Calvin cycle....
by cytoplasmic malate dehydrogenase
Malate dehydrogenase
Malate dehydrogenase is an enzyme in the citric acid cycle that catalyzes the conversion of malate into oxaloacetate and vice versa...
, and malate is transported back into the mitochondria to participate in the Citric acid cycle
Citric acid cycle
The citric acid cycle — also known as the tricarboxylic acid cycle , the Krebs cycle, or the Szent-Györgyi-Krebs cycle — is a series of chemical reactions which is used by all aerobic living organisms to generate energy through the oxidization of acetate derived from carbohydrates, fats and...
.
Desaturation
Unsaturated fatty acids are essential components to prokaryotic and eukaryotic cell membranes. These fatty acids primarily function in maintaining membrane fluidity. They have also been associated with serving as signaling molecules in other processes such as cell differentiation and DNA replication. There are two pathways organisms use for desaturation: Aerobic and Anaerobic.Anaerobic Desaturation
Many bacteria use the anaerobic pathway for synthesizing unsaturated fatty acids. This pathway does not utilize oxygen and is dependent on enzymes to insert the double bond before elongation utilizing the normal fatty acid synthesis machinery. In Escherichia coli, this pathway is well understood.- FabA is a β-hydroxydecanoyl-ACP dehydrase- it is specific for the 10 carbon saturated fatty acid synthesis intermediate (β-hydroxydecanoyl-ACP).
- FabA catalyzes the dehydration of β-hydroxydecanoyl-ACP causing the release of water and insertion of the double bond between C7 and C8 counting from the methyl end. This creates the trans-2-decenoyl intermediate.
- The trans-2-decenoyl intermediate can either be shunted to the normal saturated fatty acid synthesis pathway by FabB, where the double bond will be hydrolyzed and the final product will be a saturated fatty acid, or FabA will catalyze the isomerization into the cis-3-decenoyl intermediate.
- FabB is a β-ketoacyl-ACP synthase which elongates and channels intermediates into the mainstream fatty acid synthesis pathway. When FabB reacts with the cis-decenoyl intermediate the final product after elongation will be an unsaturated fatty acid.
- The two main unsaturated fatty acids made are Palmitoleoyl-ACP (16:1ω7) and cis-vaccenoyl-ACP (18:1ω7).
Most bacteria that undergo anaerobic desaturation contain homologues of FabA and FabB. Clostridia are the main exception; they have a novel enzyme, yet to be identified, that catalyzes the formation of the cis double bond.
Regulation
This pathway undergoes transcriptional regulation
Transcriptional regulation
Transcriptional regulation is the change in gene expression levels by altering transcription rates. -Regulation of transcription:Regulation of transcription controls when transcription occurs and how much RNA is created...
by FadR
Fatty acid metabolism regulator protein FadR
In molecular biology, the fatty acid metabolism regulator protein FadR, is a bacterial transcription factor.Bacteria regulate membrane fluidity by manipulating the relative levels of saturated and unsaturated fatty acids within the phospholipids of their membrane bilayers...
and FabR. FadR is the more extensively studied protein and has been attributed bifunctional characteristics. It acts as an activator of fabA and fabB transcription and as a repressor
Repressor
In molecular genetics, a repressor is a DNA-binding protein that regulates the expression of one or more genes by binding to the operator and blocking the attachment of RNA polymerase to the promoter, thus preventing transcription of the genes. This blocking of expression is called...
for the β-oxidation regulon
Regulon
In cell biology and genetics, a regulon is a collection of genes or operons under regulation by the same regulatory protein. This term is generally used for prokaryotic systems, for example quorum sensing in bacteria. It is a group of operons/genes spread around the chromosome but controlled by a...
. FabR, conversely, acts as a repressor for the transcription of fabA and fabB.
Aerobic Desaturation
Aerobic desaturation is the most widespread pathway for the synthesis of unsaturated fatty acids. It is utilized in all eukaryotes and some prokaryotes. This pathway utilizes desaturases to synthesize unsaturated fatty acids from full length saturated fatty acid substrates. All desaturases require oxygen and reducing equivalents acquired from the electron transport chain. Desaturases are specific for the double bond they induce in the substrate. In Bacillus subtilisBacillus subtilis
Bacillus subtilis, known also as the hay bacillus or grass bacillus, is a Gram-positive, catalase-positive bacterium commonly found in soil. A member of the genus Bacillus, B. subtilis is rod-shaped, and has the ability to form a tough, protective endospore, allowing the organism to tolerate...
, the desaturase, Δ5-Des, is specific for inducing a cis-double bond at the Δ5 position. Saccharomyces cerevisiae
Saccharomyces cerevisiae
Saccharomyces cerevisiae is a species of yeast. It is perhaps the most useful yeast, having been instrumental to baking and brewing since ancient times. It is believed that it was originally isolated from the skin of grapes...
contains one desaturase, Ole1p, which induces the cis-double bond at Δ9.
Regulation
In B. subtilis, this pathway is regulated by a two-component system: DesK and DesR. DesK is a membrane associated kinase and DesR is a transcriptional regulator of the des gene. The regulation responds to temperature; when there is a drop in temperature this gene is upregulated. Unsaturated fatty acids decrease the fluidity of the membrane and stabilize it under lower temperatures. DesK is the sensor protein that when there is a decrease in temperature, will autophosphorylate. DesK-P will transfer its phosphoryl group to DesR. Two DesR-P proteins will dimerize and bind to the DNA promoters of the des gene and recruit RNA polymerase to begin transcription.
Pseudomonas aeruginosa
Generally, both anaerobic and aerobic unsaturated fatty acid synthesis will not occur within the same system, however Pseudomonas aeruginosa
Pseudomonas aeruginosa
Pseudomonas aeruginosa is a common bacterium that can cause disease in animals, including humans. It is found in soil, water, skin flora, and most man-made environments throughout the world. It thrives not only in normal atmospheres, but also in hypoxic atmospheres, and has, thus, colonized many...
and Vibrio ABE-1 are exceptions.
While, P. aeruginosa primarily undergoes anaerobic desaturation, it also undergoes two aerobic pathways. One pathway utilizes a Δ9 desaturase (DesA) that catalyzes a double bond formation in membrane lipids. Another pathway uses two proteins, DesC and DesB, together to act as a Δ9 desaturase which inserts a double bond into a saturated fatty acid-CoA molecule. This second pathway is regulated by repressor protein, DesT. DesT is also a repressor of fabAB expression for anaerobic desaturation when in presence of exogenous unsaturated fatty acids. This functions to coordinate the expression of the two pathways within the organism.
Branched chain fatty acids
Branched-chain fatty acids are usually saturated and are found in two distinct families; the iso-series and anteiso-series. It has been found that ActinomycetalesActinomycetales
Actinomycetales is an order of Actinobacteria. They are very diverse and contain a variety of subdivisions as well as yet unclassified isolates. This is mainly because some genera are very difficult to classify because of a highly niche-dependent phenotype...
contain unique branch chain fatty acid synthesis mechanisms, including that which forms tuberculosteric acid.
Branch-Chain Fatty Acid Synthesizing System
The branched-chain fatty acid synthesizing system uses α-keto acids as primers. This system is distinct from the branched-chain fatty acid synthetase which utilizes short-chain acyl-CoA esters as primers. α-keto acid primers are derived from the transamination
Transamination
There are two chemical reactions known as transamination . The first is the reaction between an amino acid and an alpha-keto acid...
and decarboxylation
Decarboxylation
Decarboxylation is a chemical reaction that releases carbon dioxide . Usually, decarboxylation refers to a reaction of carboxylic acids, removing a carbon atom from a carbon chain. The reverse process, which is the first chemical step in photosynthesis, is called carbonation, the addition of CO2 to...
of valine
Valine
Valine is an α-amino acid with the chemical formula HO2CCHCH2. L-Valine is one of 20 proteinogenic amino acids. Its codons are GUU, GUC, GUA, and GUG. This essential amino acid is classified as nonpolar...
, leucine
Leucine
Leucine is a branched-chain α-amino acid with the chemical formula HO2CCHCH2CH2. Leucine is classified as a hydrophobic amino acid due to its aliphatic isobutyl side chain. It is encoded by six codons and is a major component of the subunits in ferritin, astacin and other 'buffer' proteins...
, and isoleucine
Isoleucine
Isoleucine is an α-amino acid with the chemical formula HO2CCHCHCH2CH3. It is an essential amino acid, which means that humans cannot synthesize it, so it must be ingested. Its codons are AUU, AUC and AUA....
to form 2-methylpropanyl-CoA, 3-methylbutyryl-CoA, and 2-Methylbutyryl-CoA, respectively. 2-methylpropanyl-CoA primers derived from valine are elongated to produce even-numbered iso-series fatty acids such as 14-methyl-pentadecanoic (isopalmitic) acid, and 3-methylbutyryl-CoA primers from leucine may be used to form odd numbered iso-series fatty acids such as 13-methyl-tetradecanoic acid. 2-Methylbutyryl-CoA primers from isoleucine are elongated to form anteiso-series fatty acids containing an odd number of carbon atoms such as 12-Methyl tetradecanoic acid. Decarboxylation of the primer precursors occurs through the branched-chain α-keto acid decarboxylase
Branched-chain-2-oxoacid decarboxylase
In enzymology, a branched-chain-2-oxoacid decarboxylase is an enzyme that catalyzes the chemical reaction-3-methyl-2-oxopentanoate \rightleftharpoons 2-methylbutanal + CO2...
(BCKA) enzyme. Elongation of the fatty acid follows the same biosynthetic pathway in Escherichia coli used to produce straight-chain fatty acids where malonyl-CoA is used as a chain extender. The major end products are 12-17 carbon branched-chain fatty acids and their composition tends to be uniform and characteristic for many bacterial species.
BCKA decarboxylase and relative activities of α-keto acid substrates
The BCKA decarboxylase enzyme is composed of two subunits in a tetrameric structure (A2B2) and is essential for the synthesis of branched-chain fatty acids. It is responsible for the decarboxylation of α-keto acids formed by the transamination of valine, leucine, and isoleucine and produces the primers used for branched-chain fatty acid synthesis. The activity of this enzyme is much higher with branched-chain α-keto acid substrates than straight-chain substrates, and in Bacillus
Bacillus
Bacillus is a genus of Gram-positive, rod-shaped bacteria and a member of the division Firmicutes. Bacillus species can be obligate aerobes or facultative anaerobes, and test positive for the enzyme catalase. Ubiquitous in nature, Bacillus includes both free-living and pathogenic species...
species its specificity is highest for the isoleucine-derived α-keto-β-methylvaleric acid, followed by α-ketoisocaproate and α-ketoisovalerate. The enzyme’s high affinity toward branched-chain α-keto acids allows it to function as the primer donating system for branched-chain fatty acid synthetase.
| Substrate | BCKA activity | CO2 Produced (nmol/min mg) | Km (μM) | Vmax (nmol/min mg) |
|---|---|---|---|---|
| L-α-keto-β-methyl-valerate | 100% | 19.7 | <1 | 17.8 |
| α-Ketoisovalerate | 63% | 12.4 | <1 | 13.3 |
| α-Ketoisocaproate | 38% | 7.4 | <1 | 5.6 |
| Pyruvate | 25% | 4.9 | 51.1 | 15.2 |
Factors affecting chain length and pattern distribution
α-keto acid primers are used to produce branched-chain fatty acids that are generally between 12 and 17 carbons in length. The proportions of these branched-chain fatty acids tend to be uniform and consistent among a particular bacterial species but may be altered due to changes in malonyl-CoA concentration, temperature, or heat-stable factors (HSF) present. All of these factors may affect chain length, and HSF’s have been demonstrated to alter the specificity of BCKA decarboxylase for a particular α-keto acid substrate, thus shifting the ratio of branched-chain fatty acids produced. An increase in malonyl-CoA concentration has been shown to result in a larger proportion of C17 fatty acids produced, up until the optimal concentration (≈20μM) of malonyl-CoA is reached. Decreased temperatures also tend to shift the fatty-acid distribution slightly toward C17 fatty-acids in Bacillus species.
Branch-Chain Fatty Acid Synthase
This system functions similarly to the branch-chain fatty acid synthesizing system, however it uses short chain carboxylic acids as primers instead of alpha-keto acids. This method is generally used by bacteria that do not have the ability to perform the branch-chain fatty acid system using alpha-keto primers. Typical short chain primers include isovalerate, isobutyrate and 2-methyl butyrate. The acids needed for these primers are generally taken up from the environment; this is often seen in ruminal bacteria.The over all reaction is:
Isobutyryl-CoA + 6 malonyl-CoA +12 NADPH + 12H+ = Isoplmitic acid + 6CO2 12NADP + 5H2O + 7CoA
The difference between (straight-chain) fatty acid synthase and branch-chain fatty acid synthase is substrate specificity of the enzyme that catalyzes the reaction of acyl-CoA to acyl-ACP.
Omega-alicyclic fatty acids
Omega-alicyclic fatty acids typically contain an omega-terminal propyl or butyryl cyclic group and are some of the major membrane fatty acids found in several species of bacteria. The fatty acid synthetase used to produce omega-alicyclic fatty acids is also used to produce membrane branched-chain fatty acids. In bacteria with membranes composed mainly of omega-alicyclic fatty acids, the supply of cyclic carboxylic acid-CoA esters is much greater than that of branched-chain primers. The synthesis of cyclic primers is not well understood but it has been suggested that mechanism involves the conversion of sugars to shikimic acidShikimic acid
Shikimic acid, more commonly known as its anionic form shikimate, is an important biochemical metabolite in plants and microorganisms. Its name comes from the Japanese flower shikimi , from which it was first isolated....
which is then converted to cyclohexylcarboxylic acid-CoA esters that serve as primers for omega-alicyclic fatty acid synthesis
Tuberculostearic Acid Synthesis
Tuberculostearic acidTuberculostearic acid
Tuberculostearic acid is a saturated fatty acid produced by Actinomycetales bacteria....
(D-10-Methylstearic acid) is a saturated fatty acid that is known to be produced by Mycobacterium
Mycobacterium
Mycobacterium is a genus of Actinobacteria, given its own family, the Mycobacteriaceae. The genus includes pathogens known to cause serious diseases in mammals, including tuberculosis and leprosy...
spp. and two species of Streptomyces
Streptomyces
Streptomyces is the largest genus of Actinobacteria and the type genus of the family Streptomycetaceae. Over 500 species of Streptomyces bacteria have been described. As with the other Actinobacteria, streptomycetes are gram-positive, and have genomes with high guanine and cytosine content...
. It is formed from the precursor oleic acid (a monosaturated fatty acid). After oleic acid is esterified to a phospholipid, S-adenosyl-methionine donates a methyl group to the double bond of oleic acid. This methylation reaction forms the intermediate 10-methylene-octadecanoyal. Successive reduction of the residue, with NADPH as a cofactor, results in 10-methylstearic acid
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- Overview at Rensselaer Polytechnic InstituteRensselaer Polytechnic InstituteStephen Van Rensselaer established the Rensselaer School on November 5, 1824 with a letter to the Rev. Dr. Samuel Blatchford, in which van Rensselaer asked Blatchford to serve as the first president. Within the letter he set down several orders of business. He appointed Amos Eaton as the school's...
- Overview at Indiana State UniversityIndiana State UniversityIndiana State University is a public university located in Terre Haute, Indiana, United States.The Princeton Review has named Indiana State as one of the "Best in the Midwest" seven years running, and the College of Education's Graduate Program was recently named as a 'Top 100' by U.S...