ABSTRACT
The ubiquitous algal metabolite dimethylsulfoniopropionate (DMSP) is a major source of carbon and reduced sulfur for marine bacteria. Recently, the enzyme responsible for the demethylation of DMSP, designated DmdA, was identified, and homologs were found to be common in marine bacterioplankton cells. The recombinant DmdA proteins from the cultured marine bacteria
Pelagibacter ubique
HTCC1062 and
Silicibacter pomeroyi
DSS-3 were purified with a three-step procedure using anion-exchange, hydrophobic interaction, and hydroxyapatite chromatographies. The
P. ubique
enzyme possessed an
M
r
on sodium dodecyl sulfate-polyacrylamide gel electrophoresis of 38,500. Under nondenaturing conditions, the
M
r
was 68,000, suggesting that the enzyme was likely to be a dimer. The purified enzyme exhibited strict substrate specificity for DMSP, as DmdA from both
S. pomeroyi
and
P. ubique
possessed no detectable demethylase activity with glycine betaine, dimethyl glycine, methylmercaptopropionate, methionine, or dimethylsulfonioacetate. Less than 1% activity was found with dimethylsulfoniobutanoate and dimethylsulfoniopentanoate. The apparent
K
m
s for DMSP were 13.2 ± 2.0 and 5.4 ± 2.3 mM for the
P. ubique
and
S. pomeroyi
enzymes, respectively. In cell extracts of
S. pomeroyi
DSS-3, the apparent
K
m
for DMSP was 8.6 ± 1.2 mM, similar to that of purified recombinant DmdA. The intracellular concentration of DMSP in chemostat-grown
S. pomeroyi
DSS-3 was 70 mM. These results suggest that marine bacterioplankton may actively accumulate DMSP to osmotically significant concentrations that favor near-maximal rates of DMSP demethylation activity.
摘要
无处不在的藻类代谢物二甲基硫代丙酸酯(DMSP)是海洋细菌的主要碳源和还原硫。最近,一种被命名为 DmdA 的负责 DMSP 去甲基化的酶被鉴定出来,并且发现同源物在海洋浮游细菌细胞中很常见。从培养的海洋细菌中获得的重组 DmdA 蛋白
Pelagibacter ubique
HTCC1062 和
Silicibacter pomeroyi
DSS-3 的重组 DmdA 蛋白采用阴离子交换、疏水相互作用和羟基磷灰石色谱三步法进行纯化。结果表明
P. ubique
酶具有
M
r
为 38 500。在非变性条件下
M
r
为 68,000,表明该酶可能是二聚体。纯化的酶对 DMSP 表现出严格的底物特异性,因为 DmdA 既来自 S. pomeroyi,也来自 S. pomeroyi。
和
和
P. ubique
在甘氨酸甜菜碱、二甲基甘氨酸、巯基丙酸甲酯、蛋氨酸或二甲基磺酰乙酸中均未检测到去甲基化酶活性。二甲基砜基丁酸酯和二甲基砜基戊酸酯的活性不到 1%。表观活性
K
m
分别为 13.2 ± 2.0 毫摩尔和 5.4 ± 2.3 毫摩尔。
P. ubique
和
pomeroyi
分别为 13.2 ± 2.0 mM 和 5.4 ± 2.3 mM。在
S. pomeroyi
DSS-3 中,表观
K
m
为 8.6 ± 1.2 mM,与纯化的重组 DmdA 相似。在恒温培养箱中生长的
DSS-3 的细胞内浓度为 70 mM。
DSS-3 细胞内的 DMSP 浓度为 70 mM。这些结果表明,海洋浮游细菌可能会积极积累 DMSP,使其达到显著的渗透压浓度,从而有利于接近最大速率的 DMSP 去甲基化活动。