Gramicidin
Encyclopedia
Gramicidin is a heterogeneous mixture of six antibiotic compounds, gramicidins A, B and C, making up 80%, 6%,
and 14% respectively, all of which are obtained from the soil bacterial species Bacillus brevis
and called collectively gramicidin D. Gramicidin D are linear pentadecapeptides; that is chains made up of 15 amino acids. This is in contrast to gramicidin S
which is a cyclic peptide chain.
bacteria, except for the Gram-positive bacilli
, and against select Gram-negative
organisms, such as Neisseria
bacteria. Its therapeutic use is limited to topical application, as it induces hemolysis
in lower concentrations than bacteria cell death so cannot be administered internally. Since the exterior epidermis
is composed of dead cells, applying it to the surface of the skin will not cause harm.
It is used primarily as a topical antibiotic and is one of the three constituents of consumer antibiotic polysporin
ophthalmic solution.
isolated the substance tyrothricin and later showed that it was composed of two substances, gramicidin (20%) and tyrocidine
(80%). These were the first antibiotics to be manufactured commercially.
formyl-L-X-Gly-L-Ala-D-Leu-L-Ala-D-Val-L-Val-D-Val-L-Trp-D-Leu-L-Y-D-Leu-L-Trp-D-Leu-L-Trp-ethanolamine
X and Y depend upon the gramicidin molecule. There exist valine
and isoleucine
variants of all three gramicidin species, and 'X' can be either. Y determines which is which; as Y gramicidin A contains tryptophan
, B phenylalanine
, and C tyrosine
. Also note the alternating stereochemical
configurations (in the form of D and L) of the amino acids; this is vital to the formation of the β-helix
.
The chain assembles inside of the hydrophobic interior of the cellular lipid bilayer
to form a β-helix. The helix itself is not long enough to span the membrane but it dimerizes
to form the elongated channel needed to span the whole membrane.
The structure of the gramicidin head-to-head dimer in micelles and lipid bilayers was determined by solution and solid-state NMR. The structure was first proposed by D. W. Urry in 1971. In organic solvents and crystals this peptide forms different types of non-native double helices.
cations (e.g. Na+) to travel through unrestricted and thereby destroying the ion gradient between the cytoplasm
and the extracellular
environment.
That gramicidin D functions as a channel was demonstrated by Hladky and Haydon, who investigated the unit conductance channel. In general, gramicidin channels are ideally selective for monovalent cations and the single-channel conductances for the alkali cations are ranked in the same order as the aqueous mobilities of these ions. Divalent cations like block the channel by binding near its mouth so that it is essentially impermeable to divalent cations and also excludes anions. in particular is excluded from the channel because its hydration shell is thermodynamically stronger than that of most monovalent cations. The channel is permeable to most monovalent cations, which move through the channel in single file. The channel is filled with about six water molecules, almost all of which must be displaced when an ion is transported. Thus, ions moving through the gramicidin pore carry along a single file of water molecules. Such a flux of ion and water molecules is known as flux coupling
. In the presence of a second type of permeable ion, the two ions couple their flux as well. Like valinomycin
and nonactin
, the gramicidin channel is selective for potassium over sodium but only slightly so. It has a permeability ratio of 2.9. Though it is impermeable to anions, there are conditions under which some anion permeation may be observed. Its ability to bind and transport cations is due to the presence of cation-binding sites, one strong and the other weak, in the channel.
and 14% respectively, all of which are obtained from the soil bacterial species Bacillus brevis
Bacillus brevis
Aneurinibacillus migulanus , formerly known as Bacillus brevis is a Gram-positive aerobic spore-forming bacillus commonly found in soil, air, water, and decaying matter. It is rarely associated with infectious diseases...
and called collectively gramicidin D. Gramicidin D are linear pentadecapeptides; that is chains made up of 15 amino acids. This is in contrast to gramicidin S
Gramicidin S
Gramicidin S or Gramicidin Soviet is an antibiotic effective against some Gram positive and Gram negative bacteria as well as some fungi. It is a derivative of gramicidin, produced by the Gram positive bacterium Bacillus brevis...
which is a cyclic peptide chain.
Uses
Gramicidin is active against Gram-positiveGram-positive
Gram-positive bacteria are those that are stained dark blue or violet by Gram staining. This is in contrast to Gram-negative bacteria, which cannot retain the crystal violet stain, instead taking up the counterstain and appearing red or pink...
bacteria, except for the Gram-positive bacilli
Bacilli
Bacilli refers to a taxonomic class of bacteria. It includes two orders, Bacillales and Lactobacillales, which contain several well-known pathogens like Bacillus anthracis .-Ambiguity:...
, and against select Gram-negative
Gram-negative
Gram-negative bacteria are bacteria that do not retain crystal violet dye in the Gram staining protocol. In a Gram stain test, a counterstain is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color...
organisms, such as Neisseria
Neisseria
The Neisseria is a large genus of commensal bacteria that colonize the mucosal surfaces of many animals. Of the 11 species that colonize humans, only two are pathogens. N. meningitidis and N. gonorrhoeae often cause asymptomatic infections, a commensal-like behavior...
bacteria. Its therapeutic use is limited to topical application, as it induces hemolysis
Hemolysis
Hemolysis —from the Greek meaning "blood" and meaning a "loosing", "setting free" or "releasing"—is the rupturing of erythrocytes and the release of their contents into surrounding fluid...
in lower concentrations than bacteria cell death so cannot be administered internally. Since the exterior epidermis
Squamous epithelium
In anatomy, squamous epithelium is an epithelium characterised by its most superficial layer consisting of flat, scale-like cells called squamous epithelial cells...
is composed of dead cells, applying it to the surface of the skin will not cause harm.
It is used primarily as a topical antibiotic and is one of the three constituents of consumer antibiotic polysporin
Polysporin
Polysporin is a line of antibiotic ointments produced by Johnson & Johnson used in the prevention of infection and speeding the healing of wounds. The original formulation contains bacitracin and polymyxin B....
ophthalmic solution.
History
In 1939 the French microbiologist René DubosRené Dubos
René Jules Dubos was a French-born American microbiologist, experimental pathologist, environmentalist, humanist, and winner of the Pulitzer Prize for General Non-Fiction for his book So Human An Animal. He is credited as an author of a maxim "Think globally, act locally"...
isolated the substance tyrothricin and later showed that it was composed of two substances, gramicidin (20%) and tyrocidine
Tyrocidine
Tyrocidine is a mixture of cyclic decapeptides produced by the bacteria Bacillus brevis found in soil. It can be composed of 4 different amino acid sequences, giving tyrocidine A–D . Tyrocidine is the major constituent of tyrothricin, which also contains gramicidin...
(80%). These were the first antibiotics to be manufactured commercially.
Composition and structure
Gramicidin is a polypeptide with alternating L- and D-amino acids, sharing the general formula:formyl-L-X-Gly-L-Ala-D-Leu-L-Ala-D-Val-L-Val-D-Val-L-Trp-D-Leu-L-Y-D-Leu-L-Trp-D-Leu-L-Trp-ethanolamine
X and Y depend upon the gramicidin molecule. There exist 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...
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....
variants of all three gramicidin species, and 'X' can be either. Y determines which is which; as Y gramicidin A contains tryptophan
Tryptophan
Tryptophan is one of the 20 standard amino acids, as well as an essential amino acid in the human diet. It is encoded in the standard genetic code as the codon UGG...
, B phenylalanine
Phenylalanine
Phenylalanine is an α-amino acid with the formula C6H5CH2CHCOOH. This essential amino acid is classified as nonpolar because of the hydrophobic nature of the benzyl side chain. L-Phenylalanine is an electrically neutral amino acid, one of the twenty common amino acids used to biochemically form...
, and C tyrosine
Tyrosine
Tyrosine or 4-hydroxyphenylalanine, is one of the 22 amino acids that are used by cells to synthesize proteins. Its codons are UAC and UAU. It is a non-essential amino acid with a polar side group...
. Also note the alternating stereochemical
Stereochemistry
Stereochemistry, a subdiscipline of chemistry, involves the study of the relative spatial arrangement of atoms within molecules. An important branch of stereochemistry is the study of chiral molecules....
configurations (in the form of D and L) of the amino acids; this is vital to the formation of the β-helix
Beta helix
A beta helix is a protein structure formed by the association of parallel beta strands in a helical pattern with either two or three faces. The structure is stabilized by inter-strand hydrogen bonds, protein-protein interactions, and sometimes bound metal ions...
.
The chain assembles inside of the hydrophobic interior of the cellular lipid bilayer
Lipid bilayer
The lipid bilayer is a thin membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around cells. The cell membrane of almost all living organisms and many viruses are made of a lipid bilayer, as are the membranes surrounding the cell nucleus...
to form a β-helix. The helix itself is not long enough to span the membrane but it dimerizes
Protein dimer
In biochemistry, a dimer is a macromolecular complex formed by two, usually non-covalently bound, macromolecules like proteins or nucleic acids...
to form the elongated channel needed to span the whole membrane.
The structure of the gramicidin head-to-head dimer in micelles and lipid bilayers was determined by solution and solid-state NMR. The structure was first proposed by D. W. Urry in 1971. In organic solvents and crystals this peptide forms different types of non-native double helices.
Pharmacological effect
Gramicidin's bactericidal activity is a result of increasing the permeability of the bacterial cell membrane, allowing inorganic monovalentMonovalent
Monovalent may refer to:*In chemistry, valence is a measure of the number of chemical bonds formed by the atoms of a given element. Monovalent is a synonym of univalent.*Monovalent ions contain one valence electron....
cations (e.g. Na+) to travel through unrestricted and thereby destroying the ion gradient between the cytoplasm
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
and the extracellular
Extracellular
In cell biology, molecular biology and related fields, the word extracellular means "outside the cell". This space is usually taken to be outside the plasma membranes, and occupied by fluid...
environment.
That gramicidin D functions as a channel was demonstrated by Hladky and Haydon, who investigated the unit conductance channel. In general, gramicidin channels are ideally selective for monovalent cations and the single-channel conductances for the alkali cations are ranked in the same order as the aqueous mobilities of these ions. Divalent cations like block the channel by binding near its mouth so that it is essentially impermeable to divalent cations and also excludes anions. in particular is excluded from the channel because its hydration shell is thermodynamically stronger than that of most monovalent cations. The channel is permeable to most monovalent cations, which move through the channel in single file. The channel is filled with about six water molecules, almost all of which must be displaced when an ion is transported. Thus, ions moving through the gramicidin pore carry along a single file of water molecules. Such a flux of ion and water molecules is known as flux coupling
Co-transport
Co-transport, also known as coupled transport or secondary active transport, refers to the simultaneous or sequential passive transfer of molecules or ions across biological membranes in a fixed ratio...
. In the presence of a second type of permeable ion, the two ions couple their flux as well. Like valinomycin
Valinomycin
Valinomycin is a dodecadepsipeptide antibiotic.Valinomycin is obtained from the cells of several Streptomyces strains, among which "S. tsusimaensis" and S. fulvissimus....
and nonactin
Nonactin
Nonactin is a member of a family of naturally occurring cyclic ionophores known as the macrotetrolide antibiotics. The other members of nactins homologous family are monactin, dinactin, trinactin and tetranactin which are all neutral ionophoric substances and higher homologs of nonactin....
, the gramicidin channel is selective for potassium over sodium but only slightly so. It has a permeability ratio of 2.9. Though it is impermeable to anions, there are conditions under which some anion permeation may be observed. Its ability to bind and transport cations is due to the presence of cation-binding sites, one strong and the other weak, in the channel.