ABSTRACT
We found the occurrence of thermophilic reversible γ-resorcylate decarboxylase (γ-RDC) in the cell extract of a bacterium isolated from natural water,
Rhizobium
sp. strain MTP-10005, and purified the enzyme to homogeneity. The molecular mass of the enzyme was determined to be about 151 kDa by gel filtration, and that of the subunit was 37.5 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis; in other words, the enzyme was a homotetramer. The enzyme was induced specifically by the addition of γ-resorcylate to the medium. The enzyme required no coenzyme and did not act on 2,4-dihydroxybenzoate, 2,5-dihydroxybenzoate, 3,4-dihydroxybenzoate, 3,5-dihydroxybenzoate, 2-hydroxybenzoate, or 3-hydroxybenzoate. It was relatively thermostable to heat treatment, and its half-life at 50°C was estimated to be 122 min; furthermore, it catalyzed the reverse carboxylation of resorcinol. The values of
k
cat
/
K
m
(mΜ
−1
· s
−1
) for γ-resorcylate and resorcinol at 30°C and pH 7 were 13.4 and 0.098, respectively. The enzyme contains 327 amino acid residues, and sequence identities were found with those of hypothetical protein AGR C 4595p from
Agrobacterium tumefaciens
strain C58 (96% identity), 5-carboxyvanillate decarboxylase from
Sphingomonas paucimobilis
(32%), and 2-amino-3-carboxymuconate-6-semialdehyde decarboxylases from
Bacillus cereus
ATCC 10987 (26%),
Rattus norvegicus
(26%), and
Homo sapiens
(25%). The genes (
graA
[1,230 bp],
graB
[888 bp], and
graC
[1,056 bp]) that are homologous to those in the resorcinol pathway also exist upstream and downstream of the γ-RDC gene. Judging from these results, the resorcinol pathway also exists in
Rhizobium
sp. strain MTP-10005, and γ-RDC probably catalyzes a reaction just before the hydroxylase in it does.
摘要
我们在从天然
水中分离出的一种细菌的细胞
提取物中发现了嗜热可逆性γ-
还原酸脱羧酶(γ-RDC)、
根瘤菌
sp.菌株
MTP-10005 的细胞
提取物中的γ-resylate decarboxylase (γ-RDC),并将该酶纯化至均一。经凝胶过滤测定,该酶的分子质量约为 151 kDa,经
十二烷基硫酸钠-聚
丙烯酰胺凝胶电泳测定,该酶亚基的分子质量为 37.5 kDa;换言之,该酶为同源四聚体。在
培养基中加入 γ-去甲酰基酯可特异性诱导该酶。该酶不需要辅酶,也不作用于
2,4-二羟基苯甲酸酯、
2,5-二羟基苯甲酸酯、3,4-二羟基
苯甲酸酯、
3,5-二羟基苯甲酸酯、
2-羟基苯甲酸酯或 3-羟基
苯甲酸酯。它对热处理具有较好的热稳定性,在 50°C 时的半衰期估计为 122 分钟;此外,它还能催化
间苯二酚的反向羧化反应。其
k
cat
/
K
m
(mΜ
-1
- s
-1
)分别为 13.4 和 0.098。该酶含有 327 个
氨基酸残基,与来自农杆菌的假定蛋白 AGR C 4595p 的序列相同。
农杆菌
菌株 C58 的假定蛋白 AGR C 4595p(相同度为 96%)、Sphingomonas paucimas 的
5-羧基香草酸脱羧酶
的 5-羧基
香草醛脱羧酶
(的 2-
氨基-3-羧基
琥珀酸-6-半
乳糖醛脱羧酶(32%),以及来自
蜡样芽孢杆菌
atcc 10987(26%)、
鼠
(26%) 和
智人
(25%).
基因
graA
[1,230 bp]、
graB
[888 bp] 和
graC
1,230bp]、graB [888bp] 和 graC [1,056 bp])与
间苯二酚途径中的同源
基因也存在于 γ-RDC
基因的上下游。从这些结果来看,
间苯二酚途径也存在于
根瘤菌
菌株
MTP-10005 中也存在
间苯二酚途径,而且γ-RDC 很可能就在其中的羟化酶之前催化了一个反应。