Protochlorophyllide,KEGG compound database entr

/ref> or monovinyl protochlorophyllide, is an intermediate in the

biosynthesis Biosynthesis, i.e., chemical synthesis occurring in biological contexts, is a term most often referring to multi-step, enzyme-Catalysis, catalyzed processes where chemical substances absorbed as nutrients (or previously converted through biosynthe ...

of chlorophyll ''a''. It lacks the

phytol Phytol (florasol, phytosol) is an acyclic hydrogenated diterpene alcohol that is used as a precursor for the manufacture of synthetic forms of vitamin E and vitamin K1, as well as in the fragrance industry. Its other commercial uses include co ...

side-chain of chlorophyll and the

reduced Reduction, reduced, or reduce may refer to: Science and technology Chemistry * Reduction (chemistry), part of a reduction-oxidation (redox) reaction in which atoms have their oxidation state changed. ** Organic redox reaction, a redox reacti ...

pyrrole Pyrrole is a heterocyclic, aromatic, organic compound, a five-membered ring with the formula . It is a colorless volatile liquid that darkens readily upon exposure to air. Substituted derivatives are also called pyrroles, e.g., ''N''-methylpyrrol ...

in ring D. Protochlorophyllide is highly

fluorescent Fluorescence is one of two kinds of photoluminescence, the emission of light by a substance that has absorbed light or other electromagnetic radiation. When exposed to ultraviolet radiation, many substances will glow (fluoresce) with color ...

; 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 (). The term angiosperm is derived from the Greek words (; 'container, vessel') and (; 'seed'), meaning that the seeds are enclosed within a fruit. T ...

, 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 The gymnosperms ( ; ) are a group of woody, perennial Seed plant, seed-producing plants, typically lacking the protective outer covering which surrounds the seeds in flowering plants, that include Pinophyta, conifers, cycads, Ginkgo, and gnetoph ...

algae Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...

, 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 two ...

,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 a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

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 pi ...

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 The cell nucleus (; : nuclei) is a membrane-bound organelle found in eukaryote, eukaryotic cell (biology), cells. Eukaryotic cells usually have a single nucleus, but a few cell types, such as mammalian red blood cells, have #Anucleated_cells, ...

and only later transported to its place of work,

chloroplast A chloroplast () is a type of membrane-bound organelle, organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. Chloroplasts have a high concentration of chlorophyll pigments which captur ...

. 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), also known as plastid DNA (ptDNA) 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 s ...

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 chlorophyl ...

(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 can convert all produced protochlorophyllide into chlorophyll and does not over accumulate it despite the lack of regulation. In

barley Barley (), a member of the grass family, is a major cereal grain grown in temperate climates globally. It was one of the first cultivated grains; it was domesticated in the Fertile Crescent around 9000 BC, giving it nonshattering spikele ...

''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 overaccumulates 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 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 frequently u ...

protein that is located in the

thylakoid Thylakoids are membrane-bound compartments inside chloroplasts and cyanobacterium, cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. Thylakoids consist of a #Membrane, thylakoid membrane surrounding a #Lumen, ...

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 A gene family is a set of several similar genes, formed by duplication of a single original gene, and generally with similar biochemical functions. One such family are the genes for human hemoglobin subunits; the ten genes are in two clusters on ...

. Later, by the sequence similarity, a similar protein was found in ''

Chlamydomonas ''Chlamydomonas'' ( ) is a genus of green algae consisting of about 150 species of unicellular organism, unicellular flagellates, found in stagnant water and on damp soil, in freshwater, seawater, and even in snow as "snow algae". ''Chlamydom ...

'' 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