Protochlorophyllide
Encyclopedia
Protochlorophyllide, or monovinyl protochlorophyllide, is an immediate precursor of chlorophyll a
that lacks the phytol
side-chain of chlorophyll. Unlike chlorophyll, protochlorophyllide is highly fluorescent; mutants that accumulate it glow red if irradiated with blue light. In Angiosperms, the last step, conversion of protochlorophyllide to chlorophyll, is light-dependent, and such plants are pale (chlorotic) if grown in the darkness. Gymnosperms, algae
, and photosynthetic bacteria have another, light-independent enzyme and grow green in the darkness as well.
, EC
1.3.1.33. There are two structurally unrelated proteins with this activity: the light-dependent and the dark-operative. The light-dependent reductase needs light to operate. The dark-operative version is a completely different protein, consisting of three subunits that exhibit significant sequence similarity to the three subunits of nitrogenase
, which catalyzes the formation of ammonia from dinitrogen. This enzyme might be evolutionary older but (being similar to nitrogenase) is highly sensitive to free oxygen
and does not work if its concentration exceeds about 3%. Hence, the alternative, light-dependent version needed to evolve.
Most of the photosynthetic bacteria have both light-dependent and light-independent reductases. Angiosperms have lost the dark-operative form and rely on 3 slightly different copies of light-dependent version, frequently abbreviated as POR A, B, and C. Gymnosperms have much more copies of the similar gene (Loblolly pine
has about 11 http://pcp.oxfordjournals.org/cgi/content/abstract/39/8/795). In plants, POR is encoded in the cell nucleus
and only later transported to its place of work, chloroplast
. Unlike with POR, in plants and algae that have the dark-operative enzyme it is at least partially encoded in the chloroplast genome.
Arabidopsis FLU mutant with damaged regulation can survive only either in a continuous darkness (protochlorophyllide is not dangerous in the darkness) or under continuous light, when the plant is capable to convert all produced protochlorophyllide into chlorophyll and do not overaccumulate it despite of the lack of regulation. In barley
Tigrina mutant (mutated on the same gene,) light kills the majority of the leaf tissue that has developed in the darkness, but part of the leaf that originated during the day survive. As a result, the leaves are covered by white stripes of necrotic regions, and the number of the white stripes is close to the age of the leaf in days. Green regions survive the subsequent nights, likely because the synthesis of chlorophyll in the mature leaf tissue is greatly reduced anyway.
membrane. Later, it was discovered that Tigrina mutants in barley, known a long time ago, are also mutated in the same gene. It is not obvious why no mutants of any other gene were observed; maybe mutations in other proteins, involved into the regulatory chain, are fatal. Flu is a single gene, not a member of the gene family
.
Later, by the sequence similarity, a similar protein was found in Chlamydomonas
algae, showing that this regulatory subsystem existed a long time before the angiosperms lost the independent conversion enzyme. In a different manner, the Chlamydomonas regulatory protein is more complex: It is larger, crosses the thylakoid membrane twice rather than once, contains more protein-protein interactions sites, and even undergoes alternative splicing
. It appears that the regulatory system underwent simplification during evolution.
Chlorophyll
Chlorophyll is a green pigment found in almost all plants, algae, and cyanobacteria. Its name is derived from the Greek words χλωρος, chloros and φύλλον, phyllon . Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to obtain energy from light...
that lacks the phytol
Phytol
Phytol is an acyclic diterpene alcohol that can be used as a precursor for the manufacture of synthetic forms of vitamin E and vitamin K1. In ruminants, the gut fermentation of ingested plant materials liberates phytol, a constituent of chlorophyll, which is then converted to phytanic acid and...
side-chain of chlorophyll. Unlike chlorophyll, protochlorophyllide is highly fluorescent; mutants that accumulate it glow red if irradiated with blue light. In Angiosperms, the last step, conversion of protochlorophyllide to chlorophyll, is light-dependent, and such plants are pale (chlorotic) if grown in the darkness. Gymnosperms, algae
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...
, and photosynthetic bacteria have another, light-independent enzyme and grow green in the darkness as well.
Conversion to chlorophyll
The enzyme that converts protochlorophyllide to chlorophyllide is protochlorophyllide reductaseProtochlorophyllide reductase
In enzymology, a protochlorophyllide reductase is an enzyme that catalyzes the chemical reactionThus, the two substrates of this enzyme are chlorophyllide a and NADP+, whereas its 3 products are protochlorophyllide, NADPH, and H+....
, EC
EC number
The Enzyme Commission number is a numerical classification scheme for enzymes, based on the chemical reactions they catalyze....
1.3.1.33. There are two structurally unrelated proteins with this activity: the light-dependent and the dark-operative. The light-dependent reductase needs light to operate. The dark-operative version is a completely different protein, consisting of three subunits that exhibit significant sequence similarity to the three subunits of nitrogenase
Nitrogenase
Nitrogenases are enzymes used by some organisms to fix atmospheric nitrogen gas . It is the only known family of enzymes that accomplish this process. Dinitrogen is quite inert because of the strength of its N-N triple bond...
, which catalyzes the formation of ammonia from dinitrogen. This enzyme might be evolutionary older but (being similar to nitrogenase) is highly sensitive to free oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
and does not work if its concentration exceeds about 3%. Hence, the alternative, light-dependent version needed to evolve.
Most of the photosynthetic bacteria have both light-dependent and light-independent reductases. Angiosperms have lost the dark-operative form and rely on 3 slightly different copies of light-dependent version, frequently abbreviated as POR A, B, and C. Gymnosperms have much more copies of the similar gene (Loblolly pine
Loblolly Pine
Pinus taeda is one of several pines native to the Southeastern United States, from central Texas east to Florida, and north to Delaware. It is particularly dominant in the eastern half of North Carolina, where there are huge expanses consisting solely of Loblolly Pine trees...
has about 11 http://pcp.oxfordjournals.org/cgi/content/abstract/39/8/795). In plants, POR is encoded in the cell nucleus
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
and only later transported to its place of work, chloroplast
Chloroplast
Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...
. Unlike with POR, in plants and algae that have the dark-operative enzyme it is at least partially encoded in the chloroplast genome.
Potential danger for plant
Chlorophyll itself is bound to proteins and can transfer the absorbed energy in the required direction. Protochlorophyllide, however, occurs mostly in the free form and, under light conditions, acts as a photosensitizer, forming highly toxic free radicals. Hence, plants need an efficient mechanism of regulating the amount of chlorophyll precursor. In angiosperms, this is done at the step of δ-aminolevulinic acid (ALA), one of the intermediate compounds in the biosynthetic pathway. Plants that are fed by ALA accumulate high and toxic levels of protochlorophyllide, as do mutants with a disrupted regulatory system.Arabidopsis FLU mutant with damaged regulation can survive only either in a continuous darkness (protochlorophyllide is not dangerous in the darkness) or under continuous light, when the plant is capable to convert all produced protochlorophyllide into chlorophyll and do not overaccumulate it despite of the lack of regulation. In barley
Barley
Barley is a major cereal grain, a member of the grass family. It serves as a major animal fodder, as a base malt for beer and certain distilled beverages, and as a component of various health foods...
Tigrina mutant (mutated on the same gene,) light kills the majority of the leaf tissue that has developed in the darkness, but part of the leaf that originated during the day survive. As a result, the leaves are covered by white stripes of necrotic regions, and the number of the white stripes is close to the age of the leaf in days. Green regions survive the subsequent nights, likely because the synthesis of chlorophyll in the mature leaf tissue is greatly reduced anyway.
Biosynthesis regulatory protein FLU
Despite of numerous past attempts to find the mutant that overacumulates protochlorophyllide under usual conditions, only one such gene (flu) is currently (2009) known. Flu (first described in ) is a nuclear-encoded, chloroplast-located protein that appears containing only protein-protein interaction sites. It is currently not known which other proteins interact through this linker. The regulatory protein is a transmembrane protein that is located in the thylakoidThylakoid
A thylakoid is a membrane-bound compartment inside chloroplasts and cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. Thylakoids consist of a thylakoid membrane surrounding a thylakoid lumen. Chloroplast thylakoids frequently form stacks of disks referred to as...
membrane. Later, it was discovered that Tigrina mutants in barley, known a long time ago, are also mutated in the same gene. It is not obvious why no mutants of any other gene were observed; maybe mutations in other proteins, involved into the regulatory chain, are fatal. Flu is a single gene, not a member of the gene family
Gene family
A gene family is a set of several similar genes, formed by duplication of a single original gene, and generally with similar biochemical functions...
.
Later, by the sequence similarity, a similar protein was found in Chlamydomonas
Chlamydomonas
Chlamydomonas is a genus of green algae. They are unicellular flagellates. Chlamydomonas is used as a model organism for molecular biology, especially studies of flagellar motility and chloroplast dynamics, biogenesis, and genetics...
algae, showing that this regulatory subsystem existed a long time before the angiosperms lost the independent conversion enzyme. In a different manner, the Chlamydomonas regulatory protein is more complex: It is larger, crosses the thylakoid membrane twice rather than once, contains more protein-protein interactions sites, and even undergoes alternative splicing
RNA splicing
In molecular biology and genetics, splicing is a modification of an RNA after transcription, in which introns are removed and exons are joined. This is needed for the typical eukaryotic messenger RNA before it can be used to produce a correct protein through translation...
. It appears that the regulatory system underwent simplification during evolution.