Histidine decarboxylase active site

As is the case for most enzyme activities measured in vitro, there is some doubt whether the histidine decarboxylase activities determined as above in various organs truly reflect the contribution of these organs to histidine decarboxylation in the intact animal. In vivo measurements give an overall picture of histidine decarboxylation in the living animal, but they can give little indication of the contribution made by individual organs. Moreover, the interpretation of such measurements is rendered difficult by bacterial decarboxylation of histidine in the gut, by metabolic destruction of histamine, and by the release of histamine from storage sites. Nevertheless, such measurements have provided much useful information, and they are particularly suited to the study of the effectiveness of histidine decarboxylase inhibitors in intact animals. As with in vitro methods the in vivo measurements can, in theory, be made either on the carbon dioxide or on the histamine formed in the decarboxylation. 

Non-pyridoxal Phosphate Dependent. Figure 2 depicts the postulated mechanism for a non-pyridoxal phosphate catal) zed decarboxylation of histidine to histamine involving a pyruvoyl residue instead of pyridoxal -5 - phosphate (20). Histidine decarboxylases from Lactobacillus 30a and a Micrococcus sp. have been shown to contain a covalently bound pyruvoyl residue on the active site. The pyruvoyl group is covalently bound to the amino group of a phenylalanine residue on the enzyme, and is derived from a serine residue (21) of an inactive proenzyme (22). The pyruvoyl residue acts in a manner similar to pyridoxal phosphate in the decarboxylation reaction. 

Figure 14-11 Schematic diagram of the active site of the pyruvoyl enzyme histidine decarboxylase showing key polar interactions between the pyruvoyl group and groups of the inhibitor O-methylhistidine and surrounding enzyme groups. Aspartate 63 appears to form an ion pair with the imidazolium group of the substrate.268 Hydrogen bonds are indicated by dotted lines. See Gallagher et al.269...

Nevertheless, malonyl-CoA is a major metabolite. It is an intermediate in fatty acid synthesis (see Fig. 17-12) and is formed in the peroxisomal P oxidation of odd chain-length dicarboxylic acids.703 Excess malonyl-CoA is decarboxylated in peroxisomes, and lack of the decarboxylase enzyme in mammals causes the lethal malonic aciduria.703 Some propionyl-CoA may also be metabolized by this pathway. The modified P oxidation sequence indicated on the left side of Fig. 17-3 is used in green plants and in many microorganisms. 3-Hydroxypropionyl-CoA is hydrolyzed to free P-hydroxypropionate, which is then oxidized to malonic semialdehyde and converted to acetyl-CoA by reactions that have not been completely described. Another possible pathway of propionate metabolism is the direct conversion to pyruvate via a oxidation into lactate, a mechanism that may be employed by some bacteria. Another route to lactate is through addition of water to acrylyl-CoA, the product of step a of Fig. 17-3. Tire water molecule adds in the "wrong way," the OH ion going to the a carbon instead of the P (Eq. 17-8). An enzyme with an active site similar to that of histidine ammonia-lyase (Eq. 14-48) could... 

The pH dependence of the kinetics of histidine decarboxylase (127) demonstrates that the histidine is zwitterionic when it binds to the enzyme. The extra proton on nitrogen must, of course, be removed before the Schiff base is formed. The carboxylate of Glu-197 at the active site may accept this proton. In turn, this same group may then be responsible for proton donation to the Schiff base following decarboxylation. This is consistent with the occurrence of retention of configuration in the overall replacement of-C02 by -H (128) and with studies of enzymes altered at Glu-197 (129). When Glu-197 is replaced by Asp, the protonation that follows decarboxylation occasionally occurs on the pyruvate side, thus giving rise to decarboxylation-dependent transamination (129). 

Aside from the reduction experiments with or without substrate, and other carbonyl group reactions, evidence has accrued also for the presence of a thiol group at the active site of histidine decarboxylase [12]. It has been demonstrated, for example, that approximately one thiol group per a-fi subunit is titrated with iodoacetamide or p-chloromercuribenzoate, with concomitant inactivation of the enzyme [113]. This same active-site thiol group is titrated with DTNB with loss of activity. Interestingly, the competitive inhibitors histamine and imidazole enhance the reactivity of these thiol residues toward DTNB. Upon denaturation, the enzyme... 

Other examples of electrophilic metal catalysis are given under section 2.3.3.1. Electrophilic reactions are also carried out by enzymes which have an a-keto acid (pyruvic acid or a-keto butyric acid) at the transforming locus of the active site. One example of such an enzyme is histidine decarboxylase in which the N-terminal amino acid residue is bound to pyruvate. Histidine decarboxylation is initiated by the formation of a Schiff base by the reaction mechanism in Fig. 2.20. 

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