enzymology An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

, protochlorophyllide reductases (POR) are

enzymes An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as pro ...

that

catalyze Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...

the conversion from protochlorophyllide to

chlorophyllide Chlorophyllide ''a'' and chlorophyllide ''b'' are the biosynthesis, biosynthetic precursors of chlorophyll a, chlorophyll ''a'' and chlorophyll b, chlorophyll ''b'' respectively. Their propionic acid groups are converted to phytol, phytyl esters b ...

''a''. They are

oxidoreductase In biochemistry, an oxidoreductase is an enzyme that catalyzes the transfer of electrons from one molecule, the reductant, also called the electron donor, to another, the oxidant, also called the electron acceptor. This group of enzymes usually ut ...

s participating in the

biosynthetic pathway In biochemistry, a metabolic pathway is a linked series of chemical reactions occurring within a cell. The reactants, products, and intermediates of an enzymatic reaction are known as metabolites, which are modified by a sequence of chemical re ...

chlorophyll Chlorophyll is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words (, "pale green") and (, "leaf"). Chlorophyll allows plants to absorb energy ...

s. There are two structurally unrelated proteins with this sort of activity, referred to as light-dependent (LPOR) and dark-operative (DPOR). The light- and

NADPH Nicotinamide adenine dinucleotide phosphate, abbreviated NADP or, in older notation, TPN (triphosphopyridine nucleotide), is a cofactor used in anabolic reactions, such as the Calvin cycle and lipid and nucleic acid syntheses, which require N ...

-dependent reductase is part of the short-chain dehydrogenase/reductase (SDR) superfamily and is found in plants and oxygenic

photosynthetic Photosynthesis ( ) is a Biological system, system of biological processes by which Photoautotrophism, photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical ener ...

bacteria, while the ATP-dependent dark-operative version is a completely different protein, consisting of three subunits that exhibit significant sequence and

quaternary structure Protein quaternary structure is the fourth (and highest) classification level of protein structure. Protein quaternary structure refers to the structure of proteins which are themselves composed of two or more smaller protein chains (also refe ...

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 ...

.Yuichi Fujita and Carl E. Bauer (2000). Reconstitution of Light-independent Protochlorophyllide Reductase from Purified Bchl and BchN-BchB Subunits. J. Biol. Chem., Vol. 275, Issue 31, 23583-23588

/ref> This enzyme may be evolutionary older; due to its bound Iron–sulfur cluster, iron-sulfur clusters 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 function if the atmospheric oxygen concentration exceeds about 3%. It is possible that evolutionary pressure associated with the

great oxidation event The Great Oxidation Event (GOE) or Great Oxygenation Event, also called the Oxygen Catastrophe, Oxygen Revolution, Oxygen Crisis or Oxygen Holocaust, was a time interval during the Earth's Paleoproterozoic era when the Earth's atmosphere an ...

resulted in the development of the light-dependent system. The light-dependent version () uses

: :protochlorophyllide + NADPH + H⁺

\rightleftharpoons

chlorophyllide ''a'' + NADP⁺ While the light-independent or dark-operative version () uses ATP and

ferredoxin Ferredoxins (from Latin ''ferrum'': iron + redox, often abbreviated "fd") are iron–sulfur proteins that mediate electron transfer in a range of metabolic reactions. The term "ferredoxin" was coined by D.C. Wharton of the DuPont Co. and applied t ...

: :protochlorophyllide a + reduced ferredoxin + 2 ATP + 2 H2O = chlorophyllide a + oxidized ferredoxin + 2 ADP + 2 phosphate

Light-dependent

The light-dependent version has the accepted name protochlorophyllide reductase. The

systematic name A systematic name is a name given in a systematic way to one unique group, organism, object or chemical substance, out of a specific population or collection. Systematic names are usually part of a nomenclature. A semisystematic name or semitrivi ...

is chlorophyllide-''a'' :NADP+ 7,8-oxidoreductase. Other names in common use include NADPH2-protochlorophyllide oxidoreductase, NADPH-protochlorophyllide oxidoreductase, NADPH-protochlorophyllide reductase, protochlorophyllide oxidoreductase, and protochlorophyllide photooxidoreductase. LPOR is one of only three known light-dependent enzymes. The enzyme enables light-dependent protochlorophyllide reduction via direct local hydride transfer from NADPH and a longer-range proton transfer along a defined structural pathway. LPOR is a ~40kDa monomeric enzyme, for which the structure has been solved by

X-ray crystallography X-ray crystallography is the experimental science of determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract in specific directions. By measuring th ...

. It is part of the SDR superfamily, which includes

alcohol dehydrogenase Alcohol dehydrogenases (ADH) () are a group of dehydrogenase enzymes that occur in many organisms and facilitate the interconversion between alcohols and aldehydes or ketones with the reduction of nicotinamide adenine dinucleotide (NAD+) to N ...

, and consists of a Rossman-fold NADPH-binding site and a substrate-specific C-terminal segment region. The protochlorophyllide substrate is thought to bind to a cavity near the

nicotinamide Nicotinamide (International nonproprietary name, INN, British Approved Name, BAN ) or niacinamide (United States Adopted Name, USAN ) is a form of vitamin B3, vitamin B3 found in food and used as a dietary supplement and medication. As a suppl ...

end of the bound NADPH. LPOR is primarily found in plants and oxygenic photosynthetic bacteria, as well as in some algae.

Light-independent

The light-independent version has the accepted name of ferredoxin:protochlorophyllide reductase (ATP-dependent). Systematically it is known as ATP-dependent ferredoxin:protochlorophyllide-a 7,8-oxidoreductase. Other names in common use include light-independent protochlorophyllide reductase and dark-operative protochlorophyllide reductase (DPOR). DPOR is a nitrogenase homologue and adopts an almost identical overall architecture arrangement to both nitrogenase as well as the downstream chlorophyllide a reductase (COR). The enzyme consists of a catalytic heterotetramer and two transiently-bound ATPase dimers (right). Similar to nitrogenase, the reduction mechanism relies on an electron transfer from the iron-sulfur cluster of the ATPase domain, through a secondary cluster on the catalytic heterotetramer and finally to the protochlorophyllide-bound active site (which, distinct from nitrogenase, does not contain

FeMoco FeMoco ( cofactor) or M-cluster is the primary cofactor of nitrogenase. Nitrogenase is the enzyme that catalyzes the conversion of atmospheric nitrogen molecules N2 into ammonia (NH3) through the process known as nitrogen fixation. Because it con ...

). The reduction requires significantly less input than the nitrogenase reaction, requiring only a 2-electron reduction and 4 ATP equivalents, and as such may require an auto-inhibitory mechanism to avoid over-activity. DPOR can alternatively take as its

substrate Substrate may refer to: Physical layers *Substrate (biology), the natural environment in which an organism lives, or the surface or medium on which an organism grows or is attached ** Substrate (aquatic environment), the earthy material that exi ...

the compound with a second

vinyl group In organic chemistry, a vinyl group (abbr. Vi; IUPAC name: ethenyl group) is a functional group with the formula . It is the ethylene (IUPAC name: ethene) molecule () with one fewer hydrogen atom. The name is also used for any compound contai ...

(instead of an

ethyl group In organic chemistry, an ethyl group (abbr. Et) is an alkyl substituent with the formula , derived from ethane (). ''Ethyl'' is used in the International Union of Pure and Applied Chemistry The International Union of Pure and Applied ...

) in the structure, in which case the reaction is :3,8-divinylprotochlorophyllide + reduced ferredoxin + 2 ATP + 2 H₂O

\rightleftharpoons

3,8-divinylchlorophyllide ''a'' + oxidized ferredoxin + 2 ADP + 2 phosphate This enzyme is present in photosynthetic bacteria,

cyanobacteria Cyanobacteria ( ) are a group of autotrophic gram-negative bacteria that can obtain biological energy via oxygenic photosynthesis. The name "cyanobacteria" () refers to their bluish green (cyan) color, which forms the basis of cyanobacteri ...

green algae The green algae (: green alga) are a group of chlorophyll-containing autotrophic eukaryotes consisting of the phylum Prasinodermophyta and its unnamed sister group that contains the Chlorophyta and Charophyta/ Streptophyta. The land plants ...

and

gymnosperm 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 ...

References

{{MeshName, Ferredoxin:protochlorophyllide+reductase+(ATP-dependent) EC 1.3.1 EC 1.3.7 NADPH-dependent enzymes

Light-dependent

Light-independent

See also

References