![]() The third metabolic fate of ACC occurs in microorganisms where it is converted to α-ketobutyrate and ammonia by the action of ACC deaminase (Honma, 1985).Īlthough ACC has been used in pharmacological studies of the mammalian N-methyl- d-aspartate (NMDA) receptor (i.e. In plants, ACC is either oxidized to form ethylene by ACC oxidase or it is covalently modified by malonylation at the α-amino group as a mechanism for controlling the amount of ACC available for ethylene biosynthesis (Hoffman et al., 1982). There are currently only three known metabolic fates for ACC within these organisms. The biosynthesis and utilization of ACC appear to be limited to plants and microorganisms. Ethylene is subsequently produced by ACC oxidase. ACC synthase (ACS) (methylthioadenosine lyase EC 4.4.1.14) catalyzes the penultimate and rate-determining step of ethylene biosynthesis, which is the conversion of S-adenosyl- l-methionine (SAM) to 5′-methylthioadenosine (MTA) and ACC (Fig. This gaseous hormone is a vital signaling molecule that regulates many facets of plant growth and development (Abeles et al., 1992). The amino acid 1-aminocyclopropane-1-carboxylate (ACC) is a key metabolic intermediate in the biosynthesis of the plant hormone ethylene. ![]() Molecular modeling data illustrate that the conservation of residues between PHACS and the plant ACSs is dispersed throughout its structure and that two active site residues that are important for ACS activity in plants are not conserved in PHACS. Bioinformatic analysis indicates that PHACS is a member of the α-family of PLP-dependent enzymes. PHACS does, however, catalyze the deamination of l -vinylglycine, a known side-reaction of apple ACS. Purified recombinant PHACS, expressed in Pichia pastoris, contains bound pyridoxal-5′-phosphate (PLP), but does not catalyze the synthesis of ACC. We describe the cloning of the putative human ACS ( PHACS) cDNA that encodes a 501 amino acid protein that exhibits 58% sequence identity to the putative Fugu ACS and ∼30% sequence identity to plant ACSs. Profound physiological questions would be raised if it could be demonstrated that ACC is formed in animals, because there is no known function for ethylene in these organisms. ACC is oxidized to ethylene in the second and final step of ethylene biosynthesis. ACC synthase (ACS) catalyzes the formation of ACC from S-adenosyl- l-methionine. ![]() The sequences of genes encoding homologues of 1-aminocyclopropane-1-carboxylate (ACC) synthase, the first enzyme in the two-step biosynthetic pathway of the important plant hormone ethylene, have recently been found in Fugu rubripes and Homo sapiens (Peixoto et al., Gene 246 (2000) 275). ![]()
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