PLP-dependent reactions

We have just noted the role that pyridoxal phosphate plays as a coenzyme (cofactor) in transamination reactions (see section 15.6). Pyridoxal 5 -phosphate (PLP) is crucial to a number of biochemical reactions. PLP, together with a number of closely related materials that are readily converted into PLP, e.g. pyridoxal, pyridoxine and pyridoxamine, are collectively known as vitamin Bg, which is essential for good health. 

The reversal of this process could potentially occur with reprotonation from either face of the C=N double bond, and a mixture of aldimines would result, leading to generation of a racemic amino acid. This accounts for the mode of action of PLP-dependent amino acid racemase enzymes. Of course, the enzyme controls removal and supply of protons this is not a random event. One important example of this reaction is alanine racemase, employed by bacteria to convert L-alanine into o-alanine for cell-wall synthesis (see Box 13.12). 

In transamination and racemization reactions, we have seen loss of a proton from the aldimine, i.e. breaking of bond a. Let us now consider the two 

Breaking of bond b accounts for PLP-dependent decarboxylations. Decarboxylation of the intermediate aldimine is facilitated in the same way as loss of a proton in the transamination sequence. The pro-tonated nitrogen acts as an electron sink, and the conjugated system allows loss of the carboxyl proton. 

An important example of PLP-dependent amino acid decarboxylation is the conversion of histidine into histamine. Histamine is often involved in human allergic responses, e.g. to insect bites or pollens. Stress stimulates the action of the enzyme histidine decarboxylase and histamine is released from mast cells. Topical antihistamine creams are valuable for pain relief, and oral antihistamines are widely prescribed for nasal allergies such as hay fever. Major effects of histamine include dilation of blood vessels, inflammation and swelling of tissues, and narrowing of airways. In serious cases, life-threatening anaphylactic shock may occur, caused by a dramatic fall in blood pressure. 

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Group d). The fourth group of PLP-dependent reactions are thought to depend upon formation of the ketimine intermediate of Eq. 14-28. In this form the original a-hydrogen of the amino acid has been removed and the C = NH+ bond of the ketimine is polarized in a direction that favors electron withdrawal from the amino acid into the imine group. 

Figure 14-7 Some PLP-dependent reactions involving elimination of a y substituent. Replacement by another y substituent or by a substituent in the (3 position is possible, as is deamination to an a-oxo acid.

PLP. See Pyridoxal phosphate (PLP) PLP-dependent reactions. See Pyridoxal phosphate-dependent reactions PNA, 227s... 

Pyridoxal phosphate (PLP) 121, 679, 737-753, 740s. See also PLP-dependent reactions... 

Ethylene is rather inert, but it is metabolized slowly, some of it to ethylene glycol.326 Plants store N-malonyl-ACC as a metabolically inert pool. Excess ACC can be deaminated in a PLP-dependent reaction to 2-oxobutyrate (step k, Fig. 24-16), a process that also occurs in bacteria able to subsist on ACC.327/327a There may also be other mechanisms for ethylene formation, e.g., peroxidation of lipids during scenescence of leaves.328 See also Chapter 31, Section G. 

L-Serine is converted to pyruvate + NH3 by serine dehydratase (deaminase) in a PLP-dependent reaction. However, using the same coenzyme selenocysteine is converted by selenocysteine lyase into L-alanine + elemental selenium Se°. l-Cysteine may be converted by PLP-dependent enzymes into wither H2S or into S° for transfer into metal clusters. Compare the chemical mecha-... 

The catalytic strategy is familiar from our discussion of PLP-dependent reactions reaction via a Schiff base, probable medium control of the decarboxylation, and desolvation of the carboxyl group on binding to the enzyme. What is most surprising is that pyruvate, with its very small electron sink, works as efficiently as PLP, which allows for more extensive electron delocalization. The specialness of PLP in enzymic catalysis must lie in other factors. 

The salient features of this model involve the binding of the /3 subunit ligand serine and its activation through a PLP-dependent reaction to form a reactive enzyme-bound aminoacrylate (PLP AA) species that in turn triggers a conformational change that promotes the cleavage of IGP to indole at the a subunit. When a molecule of indole is formed it diffuses rapidly through the hydrophobic tunnel to the (3 subunit and reacts with the PLP AA, also very rapidly, to form the product tryptophan. This intersubunit communication keeps the a and /3 reactions in phase such that the intermediate indole does not accumulate. 

Fic. 1. Metabolism of tryptophan to serotonin (5-hydroxytryptamine) and niacin. Fyiidoxal phosphate (PLP) dependent reactions are indicated. Reactions not shown which may result in formation of products excreted in urine include the acetylation of liymuenine and 3-hydroxykynurenine, conjugation of anthranilic acid with glycine (to form o-aminohippuric acid) and with glucuronic acid, and the dehydroxylation of kynurenic acid and xanthurenic add to quinaldic add and 8-hydroxyquinaldic add, respectively. 

Aminolevulinate is the precursor from which the large class of alkaloids called tetrapyrroles are hiosynthesized. It arises by a PLP-dependent reaction of glycine and succinyl CoA. Review the mechanism of the formation of dopamine from L-dopa in Figure 25.7, and propose a mechanism for 5-aminolevulinate biosynthesis. 

Tryptophan synthase catalyzes the last two steps of the biosynthesis of tryptophan. It is typically found as a tetramer. The a subunits catalyze the reversible formation of indole and G3P from InGP. The p subunits catalyze the irreversible condensation of indole and serine to form tryptophan in a PLP-dependent reaction. Each a active site is connected to a p active site in a process known as substrate channeling [18]. Eigure 14.7 represents the stereo view of one ap heterodimer of tryptophan [19]. The a subunit contains bound indole-3-propanol phosphate and p subunit contains L-serine, which forms aldimine with the coenzyme PEP The orange sphere represents the course of tunnel running from the a to the p subunits. This tunnel is a 25 A long hydrophobic channel contained within the enzyme allowing for the diffusion of indole. If the channel did not exist, the indole formed at a active site would quickly diffuse always and be lost to the cell as it is hydrophobic and can easily cross membranes. 

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