Histamine is a multifunctional biogenic amine with relevant roles in intercellular communication, inflammatory processes and highly prevalent pathologies. Histamine biosynthesis depends on a single decarboxylation step, carried out by a PLP-dependent histidine decarboxylase activity (EC 4.1.1.22), an enzyme that still remains to be fully characterized. Nevertheless, during the last few years, important advances have been made in this field, including the generation and validation of the first three-dimensional model of the enzyme, which allows us to revisit previous results and conclusions. This essay provides a comprehensive review of the current knowledge of the structural and functional characteristics of mammalian histidine decarboxylase.

Mature, active mammalian histidine decarboxylase is a dimeric enzyme of carboxy-truncated monomers ( 53 kDa). By using a biocomputational approach, we have generated a three-dimensional model of a recombinant 1/512 fragment of the rat enzyme, which shows kinetic constants similar to those of the mature enzyme purified from rodent tissues. This model, together with previous spectroscopic data, allowed us to postulate that the occupation of the catalytic center by the natural substrate, or by substrate-analogs, would induce remarkable changes in the conformation of the intact holoenzyme. To investigate the proposed conformational changes during catalysis, we have carried out electrophoretic, chromatographic and spectroscopic analyses of purified recombinant rat 1/512 histidine decarboxylase in the presence of the natural substrate or substrate-analogs. Our results suggest that local changes in the catalytic site indeed affect the global conformation and stability of the dimeric protein. These results provide insights for new alternatives to inhibit histamine production efficiently in vivo.Keywords: histidine decarboxylase; histamine; a-fluoromethylhistidine; L-histidine methyl ester; pyridoxal phosphate-dependent enzymes.Mammalian histidine decarboxylase (HDC), the enzyme responsible for the biosynthesis of histamine, is a pyridoxal 5¢-phosphate (PLP)-dependent enzyme that belongs to the evolutionary group II of L-amino acid decarboxylases [1][2][3]. Histamine is involved in several physiological responses (immune responses, gastric acid secretion, neurotransmission, cell proliferation, etc.) and is also implicated in widely spread human pathologies (inflammation-related diseases, neurological disorders, cancer and invasion) [4][5][6][7][8]. In spite of the importance of these pathologies, HDC has not been fully characterized, and important questions about the regulation of the enzyme expression, sorting, processing, structural characterization and turnover remain unanswered [9][10][11][12][13][14][15].Mature HDC purified from mammalian tissues has been reported to be a dimer. Although the exact sequence of each monomer is not known, it is generally believed that the 74 kDa precursor is processed to a carboxy-truncated form of 53-58 kDa [16,17]. The N-terminus of the polypeptide (residues 1-480) exhibits a moderately high degree of identity with the porcine DOPA decarboxylase (DDC), another dimeric group II L-amino acid decarboxylase for which an X-ray structure has been solved [18]. Recently, we have characterized the catalytic mechanism of a recombinant carboxy-truncated form of the rat enzyme (fragment 1-512, also named HDC 1/512) [19], which shows kinetic constants similar to those of the mature enzyme purified from rodent tissues [16,17].Mammalian HDC and DDC appear to share several catalytic features [19,20]. First, the PLP-enzyme internal Schiff base consists mainly of an enolimine tautomeric form (free holoenzyme). Second, Michaelis complex formation leads to a polarized ketoenamine form of the Schiff base...

HDC (L-histidine decarboxylase), the enzyme responsible for the catalytic production of histamine from L-histidine, belongs to an evolutionarily conserved family of vitamin B6-dependent enzymes known as the group II decarboxylases. Yet despite the obvious importance of histamine, mammalian HDC enzymes remain poorly characterized at both the biochemical and structural levels. By comparison with the recently described crystal structure of the homologous enzyme L-DOPA decarboxylase, we have been able to identify a number of conserved domains and motifs that are important also for HDC catalysis. This includes residues that were proposed to mediate events within the active site, and HDC proteins carrying mutations in these residues were inactive when expressed in reticulocyte cell lysates reactions. Our studies also suggest that a significant change in quartenary structure occurs during catalysis. This involves a protease sensitive loop, and incubating recombinant HDC with an L-histidine substrate analogue altered enzyme structure so that the loop was no longer exposed for tryptic proteolysis. In total, 27 mutant proteins were used to test the proposed importance of 34 different amino acid residues. This is the most extensive mutagenesis study yet to identify catalytically important residues in a mammalian HDC protein sequence and it provides a number of novel insights into the mechanism of histamine biosynthesis.

Protein domains mediate drug-protein interactions and this principle can guide the design of multi-target drugs i.e. polypharmacology. In this study, we associate multi-target drugs with CATH functional families through the overrepresentation of targets of those drugs in CATH functional families. Thus, we identify CATH functional families that are currently enriched in drugs (druggable CATH functional families) and we use the network properties of these druggable protein families to analyse their association with drug side effects. Analysis of selected druggable CATH functional families, enriched in drug targets, show that relatives exhibit highly conserved drug binding sites. Furthermore, relatives within druggable CATH functional families occupy central positions in a human protein functional network, cluster together forming network neighbourhoods and are less likely to be within proteins associated with drug side effects. Our results demonstrate that CATH functional families can be used to identify drug-target interactions, opening a new research direction in target identification.