Kataoka M, Ikemi M, Morikawa T, Miyoshi T, Nishi K, Wada M, Yamada H, Shimizu S
Division of Applied Life Sciences, Graduate School of Agriculture, Kyoto University, Japan.
Eur J Biochem. 1997 Sep 1;248(2):385-93. doi: 10.1111/j.1432-1033.1997.00385.x.
D-Threonine aldolase is an enzyme that catalyzes the cleavage of D-threonine into glycine and acetaldehyde. Its activity was found in several genera of bacteria such as Arthrobacter, Alcaligenes, Xanthomonas, and Pseudomonas, but not in yeasts or fungi. The enzyme was purified to homogeneity from one strain, Arthrobacter sp. DK-38. The enzyme appeared to consist of a single polypeptide chain with an apparent molecular mass of 51 kDa. This enzyme, as well as L-threonine aldolase, requires pyridoxal 5'-phosphate (pyridoxal-P) as a coenzyme. Unlike other pyridoxal-P enzymes, D-threonine aldolase also requires a divalent cation such as Co2+, Ni2+, Mn2+, or Mg2+ for its catalytic activity. The enzyme completely lost its activity in the absence of either pyridoxal-P or a divalent cation. A divalent cation was also essential for the thermal stability of the enzyme. The metal-free enzyme tends to become thermally unstable, resulting in the irreversible loss of its catalytic activity. The enzyme is strictly D-specific for the alpha-position, whereas it cannot distinguish between threo and erythro forms at the beta-position. Thus, D-threonine and D-allothreonine act as substrates of the enzyme, but their kinetic parameters are different; the Km and Vmax values are 3.81 mM and 38.8 micromol x min(-1) x mg(-1) toward D-threonine, and 14.0 mM and 102 micromol x min(-1) x mg(-1) toward D-allothreonine. respectively. The aldolase reaction is reversible, and the enzyme is therefore able to produce nearly equimolar amounts of D-threonine and D-allothreonine through C-C bond formation between glycine and acetaldehyde. The enzyme also acts, in the same manner, on several other D-beta-hydroxy-alpha-amino acids, including D-beta-phenylserine, D-beta-hydroxy-alpha-aminovaleric acid, D-beta-3,4-dihydroxyphenylserine, and D-beta-3,4-methylenedioxyphenylserine.
D-苏氨酸醛缩酶是一种催化D-苏氨酸裂解为甘氨酸和乙醛的酶。在节杆菌属、产碱菌属、黄单胞菌属和假单胞菌属等几类细菌中发现了该酶的活性，但在酵母或真菌中未发现。该酶从节杆菌属菌株DK-38中纯化至同质。该酶似乎由一条表观分子量为51 kDa的单一多肽链组成。这种酶以及L-苏氨酸醛缩酶需要磷酸吡哆醛（吡哆醛-P）作为辅酶。与其他磷酸吡哆醛酶不同，D-苏氨酸醛缩酶的催化活性还需要二价阳离子，如Co2+、Ni2+、Mn2+或Mg2+。在没有磷酸吡哆醛或二价阳离子的情况下，该酶会完全失去活性。二价阳离子对该酶的热稳定性也至关重要。无金属的酶往往会变得热不稳定，导致其催化活性不可逆地丧失。该酶对α-位严格具有D-特异性，而在β-位无法区分苏式和赤式形式。因此，D-苏氨酸和D-别苏氨酸是该酶的底物，但它们的动力学参数不同；对D-苏氨酸的Km和Vmax值分别为3.81 mM和38.8 μmol·min-1·mg-1，对D-别苏氨酸的Km和Vmax值分别为14.0 mM和102 μmol·min-1·mg-1。醛缩酶反应是可逆的，因此该酶能够通过甘氨酸和乙醛之间的C-C键形成产生几乎等摩尔量的D-苏氨酸和D-别苏氨酸。该酶还以相同的方式作用于其他几种D-β-羟基-α-氨基酸，包括D-β-苯丝氨酸、D-β-羟基-α-氨基戊酸、D-β-3,4-二羟基苯丝氨酸和D-β-3,4-亚甲二氧基苯丝氨酸。
