General · EdgepediaSeptember 17, 20267 min read

Pyridoxal phosphate

Pyridoxal phosphate (PLP), also called pyridoxal 5′-phosphate, is the biologically active coenzyme form of vitamin B6 and one of the most versatile organic cofactors in biology. It is a derivative of pyridoxal in which phosphoric acid is esterified with the primary hydroxy group, giving the formula C₈H₁₀NO₆P and an average mass of 247.143¹. Enzymes that use PLP catalyze transamination, decarboxylation, deamination, racemization, elimination, retro-aldol cleavage, and Claisen-like condensation reactions, chiefly on amino acid and amine substrates²^•³. More than 140 distinct enzymatic activities cataloged by the Enzyme Commission are PLP-dependent, corresponding to about 4% of all classified activities⁴, and one review counts PLP as a cofactor in over 160 enzyme activities⁵.

Key fact	Detail
Identity	Monophosphate ester of pyridoxal; formula C₈H₁₀NO₆P, average mass 247.143, CAS 54-47-7¹
Role	Active coenzyme form of vitamin B6; cofactor for more than 140 EC-cataloged activities, about 4% of all classified enzyme activities⁴^•⁶
Common step	Every PLP-catalyzed reaction begins with formation of an external aldimine (Schiff base) between substrate and PLP²
Chemical basis	The pyridine ring with conjugated double bonds acts as an electron sink, stabilizing carbanionic intermediates through resonance⁷
Human supply	Recycled through a salvage pathway requiring pyridoxal kinase, pyridoxine 5′-phosphate oxidase, and phosphatases⁵
Genomic footprint	In many free-living prokaryotes almost 1.5% of all genes encode PLP-dependent enzymes; the fraction is lower in higher eukaryotes⁴
Clinical marker	Plasma PLP is the most common measure of vitamin B6 status; the Food and Nutrition Board used 20 nmol/L as the adequacy indicator for adult RDAs⁶

Chemical mechanism

PLP consists of a central pyridine ring bearing four substituents at the 2′ to 5′ positions: methyl, hydroxyl, formyl, and phosphomethyl groups⁷. The ring's conjugated double bonds and its ring nitrogen allow electron movement to extend through the Schiff-base linkage to substrates, so PLP works as an efficient electron sink that stabilizes the carbanionic intermediates of substrates through its quinonoid structure⁷.

The first and common step in all PLP-catalyzed reactions is formation of an external aldimine intermediate. The unprotonated amino group of the substrate attacks the protonated Schiff base formed between an active-site lysine and the aldehyde group of PLP; after proton transfers and collapse, a Schiff base links the substrate directly to PLP². From this shared intermediate the different reaction types diverge, and the enzyme's active-site residues determine which bond of the substrate is broken². Reaction specificity is shaped by stereoelectronic effects, the protonation state of the external aldimine, and the interaction of the carbanionic intermediate with protein side chains in the active site².

This shared chemistry has a cost: the high reactivity of PLP permits catalytic promiscuity, meaning a single enzyme can catalyze different chemical reactions, and relatively low specificity complicates the design of drugs against PLP-dependent enzymes⁷. The phosphate group also matters structurally; replacing PLP with pyridoxal in serine palmitoyltransferase lowered enzyme activity more than 10-fold even though transaldimination still proceeded⁸.

Reaction repertoire

PLP enzymes catalyze racemization, transamination, decarboxylation, elimination, retro-aldol cleavage, Claisen condensation, and other reactions on substrates containing an amino group, most commonly α-amino acids². In aminotransferases, PLP undergoes reversible conversion into pyridoxamine 5′-phosphate (PMP), which has coenzyme activity for the aminotransferases but not for other types of vitamin B6-dependent enzymes⁹.

The repertoire keeps expanding. A small number of PLP-dependent enzymes employ molecular oxygen as a cosubstrate, and these enzymes occur in multiple protein families, suggesting that reaction with oxygen emerged de novo several times¹⁰. In natural product biosynthesis, PLP-dependent enzymes carry out transaminations, Claisen-like condensations, β- and γ-eliminations and substitutions, epimerizations, decarboxylations, and transaldolations¹¹. As of 2023, no PLP-dependent enzyme was known to catalyze Mannich-type reactions until the report of the PLP-dependent Mannich cyclase LolT¹².

Evolution and distribution

PLP-dependent enzymes occur in all organisms and are indispensable in the biosynthesis and degradation of amino compounds¹³. A genomic survey found that in many free-living prokaryotes almost 1.5% of all genes code for PLP-dependent enzymes, while in higher eukaryotes the percentage is substantially lower, consistent with these catalysts being involved mainly in basic metabolism⁴. Most PLP-dependent enzymes can be assigned to one of three families of homologous proteins, designated the α, β, and γ families by regiospecificity, although alanine racemase, selenocysteine synthase, and many amino acid decarboxylases fall outside them¹⁴. The largest family is the type I, or aminotransferase, superfamily¹⁵.

Because their catalytic mechanisms almost invariably involve cofactor-stabilized carbanionic intermediates, enzymes with identical or highly similar activities have apparently arisen several times during evolution, and catalytic promiscuity implies that an organism can have more PLP-dependent activities than it has genes for PLP-dependent enzymes⁴. The genome of Mycobacterium tuberculosis encodes 45 bona fide PLP-dependent enzymes, several of them central to the bacterium's survival and pathogenicity and some targeted by the extant drug D-cycloserine¹³.

Biosynthesis, salvage, and metabolism

Two distinct routes produce PLP from scratch, the DXP-dependent route and the R5P route, and the cofactor may likewise be obtained through salvage from the surroundings; among the small number of cofactors whose breakdown has been thoroughly worked out, PLP counts as one.¹⁶ PLP also functions as a singlet oxygen scavenger and has protective effects against oxidative stress in fungi¹⁶.

In humans, PLP is recycled from food and from degraded PLP-dependent enzymes in a salvage pathway requiring pyridoxal kinase, pyridoxine 5′-phosphate oxidase, and phosphatases; once made, PLP is targeted to the dozens of apoenzymes that need it⁵. Of the six interconvertible vitamin B6 vitamers, PLP and pyridoxamine 5′-phosphate are the active coenzyme forms⁶. Plasma PLP is the most common measure of vitamin B6 status, with 20 nmol/L used as the adequacy indicator for calculating adult RDAs, although PLP has limitations as a biomarker because inflammation lowers plasma concentrations⁶^•¹⁷.

Medical relevance

Because PLP is essential for neurotransmitter synthesis and degradation, including the conversion of glutamate into gamma-aminobutyric acid (GABA) and of levodopa into dopamine, inborn errors that reduce intracellular PLP manifest as vitamin B6-responsive epilepsy, usually of early onset¹⁸^•¹⁹. Such disorders include pyridox(am)ine phosphate oxidase (PNPO) deficiency, hypophosphatasia, PLP-binding protein (PLPBP) deficiency, and disorders in which metabolites accumulate that inactivate PLP, such as ALDH7A1 deficiency and hyperprolinemia type II¹⁹. In a systematic review of PNPO deficiency covering 49 patients treated with PLP, clinical seizure responsiveness was observed in 38 patients (77.6%), and PLP treatment significantly improved survival compared with untreated siblings of similar phenotype²⁰.

The metabolic roles of PLP-dependent enzymes also drive inhibitor design. Many PLP-dependent enzymes are current drug targets, and others are envisaged for therapeutic intervention²¹. Current PLP-enzyme-targeting drugs mimic the PLP-attacking amino group of a substrate, and the Parkinson's disease drugs carbidopa and benserazide illustrate the specificity problems such strategies face⁷.

History

In 1934, Paul György showed that the factor curing "rat acrodynia", a condition of severe cutaneous lesions in young rats, was vitamin B6²². Samuel Lepkovsky isolated and crystallized vitamin B6 in 1938, and the following year Leslie Harris and Karl Folkers, and Richard Kuhn and his associates, independently showed it to be a pyridine derivative, for which György proposed the term pyridoxine²². Esmond Snell developed a microbiological growth assay in 1942 that led to the characterization of pyridoxamine and pyridoxal, and further studies showed that pyridoxal, pyridoxamine, and pyridoxine owe their vitamin activity to the organism's ability to convert them into the enzymatically active form pyridoxal 5′-phosphate²².

References

Topic: Encyclopedia › Life and health › Biological foundations › Biochemistry and metabolism › Metabolites, cofactors, and biomolecules › Coenzymes and cofactors › Vitamin-derived coenzymes › Pyridoxal phosphate

Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 21, 2026 · Edited: Oct 15, 2026; Oct 21, 2026 · Last review: Oct 21, 2026

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