ABSTRACT
p
-Cresol methylhydroxylases (PCMH) from aerobic and facultatively anaerobic bacteria are soluble, periplasmic flavocytochromes that catalyze the first step in biological
p
-cresol degradation, the hydroxylation of the substrate with water. Recent results suggested that
p
-cresol degradation in the strictly anaerobic
Geobacter metallireducens
involves a tightly membrane-bound PCMH complex. In this work, the soluble components of this complex were purified and characterized. The data obtained suggest a molecular mass of 124 ± 15 kDa and a unique αα′β
2
subunit composition, with α and α′ representing isoforms of the flavin adenine dinucleotide (FAD)-containing subunit and β representing a
c
-type cytochrome. Fluorescence and mass spectrometric analysis suggested that one FAD was covalently linked to Tyr
394
of the α subunit. In contrast, the α′ subunit did not contain any FAD cofactor and is therefore considered to be catalytically inactive. The UV/visible spectrum was typical for a flavocytochrome with two heme
c
cofactors and one FAD cofactor.
p
-Cresol reduced the FAD but only one of the two heme cofactors. PCMH catalyzed both the hydroxylation of
p
-cresol to
p
-hydroxybenzyl alcohol and the subsequent oxidation of the latter to
p
-hydroxybenzaldehyde in the presence of artificial electron acceptors. The very low
K
m
values (1.7 and 2.7 μM, respectively) suggest that the in vivo function of PCMH is to oxidize both
p
-cresol and
p
-hydroxybenzyl alcohol. The latter was a mixed inhibitor of
p
-cresol oxidation, with inhibition constants of a
K
ic
(competitive inhibition) value of 18 ± 9 μM and a
K
iu
(uncompetitive inhibition) value of 235 ± 20 μM. A putative functional model for an unusual PCMH enzyme is presented.
摘要
p
好氧细菌和兼性厌氧细菌中的甲酚甲基羟化酶(PCMH)是一种可溶性的、质外黄变色团,可催化生物过程中的第一步--甲酚甲基羟化。
p
-甲酚降解的第一步,即底物与水发生羟基化反应。最近的研究结果表明
p
-甲酚在严格厌氧的
Geobacter metallireducens
中的对甲酚降解涉及一个紧密结合膜的 PCMH 复合物。在这项工作中,对该复合物的可溶性成分进行了纯化和表征。获得的数据表明,该复合物的分子质量为 124 ± 15 kDa,具有独特的 αα′β
2
亚基组成,其中 α 和 α′ 代表含黄素腺嘌呤二核苷酸 (FAD) 亚基的同工型,而 β 则代表一种含黄素腺嘌呤二核苷酸 (FAD) 亚基的同工型。
c
-型细胞色素。荧光和质谱分析表明,一个 FAD 与 Tyr
394
亚基的 Tyr 394 共价连接。相反,α′亚基不含任何 FAD 辅助因子,因此被认为不具有催化活性。紫外/可见光谱是典型的具有两个血红素
c
辅助因子和一个 FAD 辅助因子。
p
-甲酚还原了 FAD,但只还原了两个血红素辅助因子中的一个。PCMH 同时催化了
p
-甲酚羟化为
p
-羟基苄醇,并随后将后者氧化为
p
-羟基苯甲醛。极低的
K
m
值非常低(分别为 1.7 和 2.7 μM),这表明 PCMH 在体内的功能是同时氧化
p
-甲酚和
p
-羟基苯甲醇。后者是对甲酚和对羟基苯甲醇的混合抑制剂。
p
-甲酚氧化的混合抑制剂,其抑制常数为
K
ic
(竞争性抑制)值为 18 ± 9 μM,抑制常数为
K
iu
(非竞争性抑制)值为 235 ± 20 μM。本文提出了一种不常见的 PCMH 酶的推定功能模型。