L-amino acid decarboxylase Articles 2

© 2012

Evolutionary relationships among pyridoxal-5'-phosphate-dependent enzymes. Regio-specific alpha, beta and gamma families

            (Alexander, Sandmeier et al. 1994) Download

Pyridoxal-5'-phosphate-dependent enzymes catalyze manifold reactions in the metabolism of amino acids. A comprehensive comparison of amino acid sequences has shown that most of these enzymes can be assigned to one of three different families of homologous proteins. The sequences of the enzymes of each family were aligned and their homology confirmed by profile analysis. Scrutiny of the reactions catalyzed by the enzymes showed that their affiliation with one of the three structurally defined families correlates in most cases with their regio-specificity. In the largest family, the covalency changes of the substrate occur at the same carbon atom that carries the amino group forming the imine linkage with the coenzyme. This family was thus named alpha family. It comprises glycine hydroxymethyltransferase, glycine C-acetyltransferase, 5-aminolevulinate synthase, 8-amino-7-oxononanoate synthase, all aminotransferases (with the possible exception of subgroup III), a number of other enzymes relatively closely related with the aminotransferases and very likely a certain group of amino acid decarboxylases as well as tryptophanase and tyrosine phenol-lyase which, however, catalyze beta-elimination reactions. The beta family includes L- and D-serine dehydratase, threonine dehydratase, the beta subunit of tryptophan synthase, threonine synthase and cysteine synthase. These enzymes catalyze beta-replacement or beta-elimination reactions. The gamma family incorporates O-succinylhomoserine (thiol-lyase, O-acetylhomoserine (thiol)-lyase, and cystathionine gamma-lyase, which catalyze gamma-replacement or gamma-elimination reactions, as well as cystathionine beta-lyase. The alpha and gamma family might be distantly related with one another, but are clearly not homologous with the beta family. Apparently, the primordial pyridoxal-5'-phosphate-dependent enzymes were regio-specific catalysts, which first specialized for reaction specificity and then for substrate specificity. The following pyridoxal-5'-phosphate-dependent enzymes seem to be unrelated with the alpha, beta or gamma family by the criterion of profile analysis:alanine racemase, selenocysteine synthase, and many amino acid decarboxylases. These enzymes may represent yet other families of B6 enzymes.


Autoantibodies to human tryptophan hydroxylase and aromatic L-amino acid decarboxylase

            (Dal Pra, Chen et al. 2004) Download

OBJECTIVE: To assess the prevalence of autoantibodies (Abs) to tryptophan hydroxylase (TPH) and aromatic l-amino acid decarboxylase (AADC) in patients with different autoimmune diseases and to analyse their respective epitopes. DESIGN: TPH and AADC Abs were measured in an immunoprecipitation assay using (35)S-labelled full-length and fragments of TPH and AADC. METHODS: Patients with different autoimmune adrenal diseases (n=84), non-adrenal autoimmune diseases (n=37), idiopathic vitiligo (n=8) and 56 healthy blood donors were studied. RESULTS: Fourteen of twenty-three (61%) of patients with autoimmune polyglandular syndrome (APS) type I and 1/34 (3%) of patients with isolated Addison's disease (AD) were positive for TPH Abs. None of the patients with APS type II (n=27), coeliac disease (n=10), autoimmune thyroid disease (AITD) (n=11), type 1 diabetes mellitus (DM) (n=16) or idiopathic vitiligo (n=8) was positive for TPH Abs. AADC Abs were detected in 12/23 (52%) patients with APS type I, in 1/29 (3%) patients with APS type II and 1/34 (3%) patients with isolated AD. None of the patients with coeliac disease, type 1 DM, AITD or idiopathic vitiligo was positive for AADC Abs. TPH Abs were found to interact with the C-terminal amino acids (aa) 308-423, central aa 164-205 and N-terminal aa 1-105 of the TPH molecule. AADC Ab binding epitopes were within the C-terminal aa 382-483, the central aa 243-381 and the N-terminal aa 1-167. CONCLUSIONS: Our study suggests that TPH Abs and AADC Abs react with several different epitopes and that different epitopes are recognized by different sera. The prevalence of TPH Abs and AADC Abs in patients with APS type I in our study is in agreement with previous reports. TPH Abs and AADC Abs were found very rarely in patients with other forms of autoimmune adrenal disease and were not detected in patients with non-adrenal autoimmune diseases.

Multiple evolutionary origin of pyridoxal-5'-phosphate-dependent amino acid decarboxylases

            (Sandmeier, Hale et al. 1994) Download

Comparison of the amino acid sequences of nine different pyridoxal-5'-phosphate-dependent amino acid decarboxylases indicated that they can be subdivided into four different groups that seem to be evolutionarily unrelated to each other. Group I is represented by glycine decarboxylase, a component of a multienzyme system; group II comprises glutamate, histidine, tyrosine, and aromatic-L-amino-acid decarboxylases; group III, procaryotic ornithine and lysine decarboxylase as well as the procaryotic biodegradative type of arginine decarboxylase; group IV, eucaryotic ornithine and arginine decarboxylase as well as the procaryotic biosynthetic type of arginine decarboxylase and diaminopimelate decarboxylase. (N-1) profile analysis, a more stringent application of profile analysis, established the homology among the enzymes of each group. A search with the profile of group II indicated a distant relationship with aminotransferases and thus with the alpha family of pyridoxal-5'-phosphate-dependent enzymes. No evidence was obtained that groups I, III and IV were related with other pyridoxal-5'-phosphate-dependent enzymes or any other protein in the database. Unlike the aminotransferases, which, with few possible exceptions, constitute a single group of homologous proteins, the amino acid decarboxylases, by the criterion of profile analysis, have evolved along multiple lineages, in some cases even if they have the same substrate specificity.

Molecular evolution of B6 enzymes: binding of pyridoxal-5'-phosphate and Lys41Arg substitution turn ribonuclease A into a model B6 protoenzyme

            (Vacca, Giannattasio et al. 2008) Download

BACKGROUND: The pyridoxal-5'-phosphate (PLP)-dependent or vitamin B6-dependent enzymes that catalyze manifold reactions in the metabolism of amino acids belong to no fewer than four evolutionarily independent protein families. The multiple evolutionary origin and the essential mechanistic role of PLP in these enzymes argue for the cofactor having arrived on the evolutionary scene before the emergence of the respective apoenzymes and having played a dominant role in the molecular evolution of the B6 enzyme families. Here we report on an attempt to re-enact the emergence of a PLP-dependent protoenzyme. The starting protein was pancreatic ribonuclease A (RNase), in which active-site Lys41 or Lys7 readily form a covalent adduct with PLP. RESULTS: We screened the PLP adduct of wild-type RNase and two variant RNases (K7R and K41R) for catalytic effects toward L- and D-amino acids. RNase(K41R)-PLP, in which the cofactor is bound through an imine linkage to Lys7, qualifies for a model proto-B6 enzyme by the following criteria: (1) covalent linkage of PLP (internal aldimine); (2) catalytic activity toward amino acids that depends on formation of an imine linkage with the substrate (external aldimine); (3) adjoining binding sites for the cofactor and amino acid moiety that facilitate the transimination reaction of the internal to the external aldimine and stabilize the resulting noncovalent complex of the coenzyme-substrate adduct with the protein; (4) reaction specificity, the only detectable reactions being racemization of diverse amino acids and beta-decarboxylation of L-aspartate; (5) acceleration factors for racemization and beta-decarboxylation of >103 over and above that of PLP alone; (6) ribonuclease activity that is 103-fold lower than that of wild-type RNase, attenuation of a pre-existing biological activity being indispensable for the further evolution as a PLP-dependent protoenzyme. CONCLUSION: A single amino acid substitution (Lys41Arg) and covalent binding of PLP to active-site Lys7 suffice to turn pancreatic ribonuclease A into a protein catalyst that complies with all plausible criteria for a proto-B6 enzyme. The study thus retraces in a model system what may be considered the committed step in the molecular evolution of a potential ancestor of a B6 enzyme family.


References

Alexander, F. W., E. Sandmeier, et al. (1994). "Evolutionary relationships among pyridoxal-5'-phosphate-dependent enzymes. Regio-specific alpha, beta and gamma families." Eur J Biochem 219(3): 953-60.

Dal Pra, C., S. Chen, et al. (2004). "Autoantibodies to human tryptophan hydroxylase and aromatic L-amino acid decarboxylase." Eur J Endocrinol 150(3): 313-21.

Sandmeier, E., T. I. Hale, et al. (1994). "Multiple evolutionary origin of pyridoxal-5'-phosphate-dependent amino acid decarboxylases." Eur J Biochem 221(3): 997-1002.

Vacca, R. A., S. Giannattasio, et al. (2008). "Molecular evolution of B6 enzymes: binding of pyridoxal-5'-phosphate and Lys41Arg substitution turn ribonuclease A into a model B6 protoenzyme." BMC Biochem 9: 17.