AHo-threonine

The metabolism of P-hydroxy-a-amino adds involves pyridoxal phosphate-dependent enzymes, dassified as serine hydroxymethyltransferase (SHMT) (EC 2.1.2.1) or threonine aldolases (ThrA L-threonine selective = EC 4.1.2.5, L-aHo-threonine selective = EC 4.1.2.6). Both enzymes catalyze reversible aldol-type deavage reactions yielding glycine (120) and an aldehyde (Eigure 10.45) [192]. 

Diastereoisomers. Whereas compounds with one chiral center exist as an enantiomorphic pair, molecules with two or more chiral centers also exist as diastereoisomers (diastereomers). These are pairs of isomers with an opposite configuration at one or more of the chiral centers, but which are not complete mirror images of each other. An example is L-threonine which has the 2S, 3R configuration. The diastereoisomer with the 2S, 3S configuration is known as L-aHo-threonine. L-isoleucine, whose side chain is -CH(CH3) CH2CH3, has the 2S, 3R configuration. It can be called 2(S)-amino-3(l )-methyl-valeric acid but the simpler name L-isoleucine implies the correct configuration at both chiral centers. 

The furan derivative 72 was prepared by organometallic addition of furyl-lithium (60) to N,0-protected o-aHo-threoninals 73 (Scheme 17). Alcohols 72 were obtained as major products in a very good yield. 

Threonine aldolases catalyze the reversible addition of 134 to acetaldehyde to give threonine (137), but the distinction bet veen L-threonine aldolase (ThrA EC 4.1.2.5) and L-aHo-threonine aldolase (EC 4.1.2.6) is vague, because many catalysts have only poor capacity for threo/ erythro discrimination (137/138 Figure 5.61) [284, 291, 292]. In fact, both l- and o-selective enzymes have been purified and cloned from a variety of organisms [284]. The first stereospecific L-aHo-threonine aldolase from Aeromonas jandad has recently been cloned and characterized [293]. X-ray structures of Thermotoga... 

---