A chromosomal gene, mlr6793, in Mesorhizobium loti was identified as the gene encoding 5-formyl-3-hydroxy-2-methylpyridine 4-carboxylic acid (FHMPC) dehydrogenase (dismutase) involved in the degradation pathway for pyridoxine (vitamin B6). The homogenously purified recombinant enzyme has a molecular mass of 59.1 kDa and is a homodimeric protein. FHMPC dehydrogenase catalyses practically irreversible oxidation (kcat = 204 s–1) of FHMPC (Km = 48.2 μM) by NAD+ (Km = 34.3 μM) to 3-hydroxy-2-methyl-pyridine 4, 5-dicarboxylic acid (HMPDC), and practically irreversible reduction (kcat = 217 s–1) of FHMPC (Km = 24.9 μM) by NADH (Km = 12.4 μM) to 4-pyridoxic acid. When the enzyme reaction was started with the combination of FHMPC and NAD+ or that of FHMPC and NADH, HMPDC and 4-pyridoxic acid were produced in an almost equimolar ratio throughout the reaction. FHMPC dehydrogenase belongs to the 3-hydroxyacyl-CoA dehydrogenase family with 31% identity with the human enzyme: it has probable catalytic diad residues, i.e. His137 and Glu149. The H137L mutant enzyme showed no measurable activity. The E149Q one was stable in contrast to the corresponding human 3-hydroxyacyl-CoA dehydrogenase mutant, and showed unique pH optima depending on the co-substrates used for the reaction.
Mesorhizobium loti中的染色体
基因mlr6793被确定为编码5-甲酰基-3-羟基-2-
甲基吡啶4-
羧酸(FHMPC)脱氢酶(歧化酶)的
基因,该酶参与
吡哆醇(
维生素B6)的降解途径。均相纯化的
重组酶分子量为59.1 kDa,是一种同二聚体蛋白。FHMPC脱氢酶催化
NAD+(Km = 34.3 μM)对FHMPC(Km = 48.2 μM)的几乎不可逆氧化(kcat = 204 s–1)反应,生成3-羟基-2-
甲基吡啶4,5-二
羧酸(HM
PDC),以及催化
NADH(Km = 12.4 μM)对FHMPC(Km = 24.9 μM)的几乎不可逆还原(kcat = 217 s–1)反应,生成
4-吡哆酸。当FHMPC和
NAD+或FHMPC和
NADH的组合开始酶反应时,在整个反应过程中,HM
PDC和
4-吡哆酸以几乎等摩尔比生成。FHMPC脱氢酶属于3