Prephenate dehydratase tyrosine synthesis

In bacteria, two soluble, multiactivity enzymes or enzyme complexes function in the utilization of chorismate for l-phenylalanine and L-tyrosine synthesis. A complex containing chorismate mutase and prephenate dehydratase activities has been observed. Although the reaction proceeds by what appears to be a Claisen rearrangement, the rate-limiting transition state appears to be different in the enzymatically controlled process. It is not known if formation of the higher-energy diaxial conformer (27) occurs after binding to the enzyme or is from an equilibrium mixture (Dewick, 1984 Floss, 1986 Jensen, 1986),... 

In Escherichia coli. Salmonella typhimurium and Aerobacter aerogenes two soluble multi-activity enzymes or enzyme complexes function in the utilisation of chorismate (14) for L-phenyl-alanine and L-tyrosine synthesis An enzyme or enzyme complex (P-protein) containing chorismate mutase and prephenate dehydratase activities has been isolated and partially purified from Escherichia coli. Salmonella typhimurium and Aerobacter aerogenes. The enzyme complex catalyses the transformation of chorismate (14) to phenylpyruvate (32) and both enzymic activities are retained in physical association after chromatography on DEAE cellulose. Kinetic analysis indicated that in isolated enzyme systems direct synthesis of phenylpyruvate (32) from chorismate (14) does not occur. Prephenate (31) once formed dissociates from the enzyme surface and accumulates in the reaction medium. After a lag period it is converted to phenylpyruvate (32). Schmit, Artz and Zalkin also obtained evidence to show that functionally distinct sites (catalytic and regulatory) exist on the P-protein from Salmonella typhimurium for chorismate mutase and prephenate dehydratase activities. The P-protein was obtained from Escherichia coli K-12 by Davidson, Blackburn and Dopheide who showed that it existed in solution mainly as a dimer of similar (and probably identical) sub-units of... 

Until recently, much less information was available concerning the biochemical mechanisms associated with the synthesis of phenylalanine in plants. No definitive reports of prephenate dehydratase (14) have appeared in the literature. Evidence for the existence of arogenate dehydratase (18) in higher plants was first reported by Jensen (1986a). Enzymes from Nieotiana silvestris, spinach, tobacco (Jung et al, 1986), and Sorghum bicolor (Siehl and Conn, 1988) have subsequently been identified and partially characterized. These are typically inhibited by phenylalanine and stimulated by tyrosine. 

Fig. 5. Pretyrosine pathway. An alternate route for synthesis of phenylalanine and tyrosine described in bacteria and fungi. Enzymes labeled (a), (b), and (c) denote prephenate aminotransferase, pretyrosine dehydratase, and pretyrosine dehydrogenase, respectively.

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