Protochlorophyllide,KEGG compound database entr

/ref> or monovinyl protochlorophyllide, is an intermediate in the biosynthesis of chlorophyll ''a''. It lacks the

phytol Phytol (florasol, phytosol) is an acyclic hydrogenated 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 libera ...

side-chain of chlorophyll and the reduced pyrrole in ring D. Protochlorophyllide is highly

fluorescent Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, ...

; mutants that accumulate it glow red if irradiated with blue light.Meskauskiene R, Nater M, Goslings D, Kessler F, op den Camp R, Apel K. FLU: a negative regulator of chlorophyll biosynthesis in Arabidopsis thaliana. Proceedings of the National Academy of Sciences of the United States of America. 2001; 98(22):12826-3
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In

angiosperms Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek words ('container, vessel') and ('seed'), and refers to those plants ...

, the later steps which convert protochlorophyllide to chlorophyll are light-dependent, and such plants are pale (

chlorotic In botany, chlorosis is a condition in which leaves produce insufficient chlorophyll. As chlorophyll is responsible for the green color of leaves, chlorotic leaves are pale, yellow, or yellow-white. The affected plant has little or no ability to ...

) if grown in the darkness. Gymnosperms, algae, 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 ''a'', the next intermediate on the biosynthetic pathway, is

protochlorophyllide reductase In enzymology, protochlorophyllide reductases (POR) are enzymes that catalyze the conversion from protochlorophyllide to chlorophyllide ''a''. They are oxidoreductases participating in the biosynthetic pathway to chlorophylls. There are t ...

,KEGG enzyme entry 1.3.1.3

/ref> Enzyme Commission number, 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 Nitrogenases are enzymes () that are produced by certain bacteria, such as cyanobacteria (blue-green bacteria) and rhizobacteria. These enzymes are responsible for the reduction of nitrogen (N2) to ammonia (NH3). Nitrogenases are the only fa ...

, 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 is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...

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 ''Pinus taeda'', commonly known as loblolly pine, is one of several pines native to the Southeastern United States, from East Texas to Florida, and north to southern New Jersey. The wood industry classifies the species as a southern yellow pine. ...

has about 1
Loblolly Pine (Pinus taeda L.) Contains Multiple Expressed Genes Encoding Light-Dependent NADPH:Protochlorophyllide Oxidoreductase (POR)
. 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 Chloroplast DNA (cpDNA) is the DNA located in chloroplasts, which are photosynthetic organelles located within the cells of some eukaryotic organisms. Chloroplasts, like other types of plastid, contain a genome separate from that in the cell Cel ...

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 δ-Aminolevulinic acid (also dALA, δ-ALA, 5ALA or 5-aminolevulinic acid), an endogenous non-proteinogenic amino acid, is the first compound in the porphyrin synthesis pathway, the pathway that leads to heme in mammals, as well as chlorophyll in ...

(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 (''Hordeum vulgare''), a member of the grass family, is a major cereal grain grown in temperate climates globally. It was one of the first cultivated grains, particularly in Eurasia as early as 10,000 years ago. Globally 70% of barley p ...

''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 survives. 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

In spite 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 A transmembrane protein (TP) is a type of integral membrane protein that spans the entirety of the cell membrane. Many transmembrane proteins function as gateways to permit the transport of specific substances across the membrane. They frequent ...

protein that is located in the

thylakoid Thylakoids are membrane-bound compartments 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 thyl ...

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,A Falciatore, L Merendino, F Barneche, M Ceol, R Meskauskiene, K Apel, JD Rochaix (2005). The FLP proteins act as regulators of chlorophyll synthesis in response to light and plastid signals in ''Chlamydomonas''. Genes & Dev, 19:176-187

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 RNA splicing, splicing. It appears that the regulatory system underwent simplification during evolution.

References

{{tetrapyrroles Porphyrins Plant physiology