ABSTRACT
We have recently cloned a DNA fragment containing a gene cluster that is responsible for the biosynthesis of an antituberculosis antibiotic,
d
-cycloserine. The gene cluster is composed of 10 open reading frames, designated
dcsA
to
dcsJ
. Judging from the sequence similarity between each putative gene product and known proteins, DcsC, which displays high homology to diaminopimelate epimerase, may catalyze the racemization of
O
-ureidoserine. DcsD is similar to
O
-acetylserine sulfhydrylase, which generates
l
-cysteine using
O
-acetyl-
l
-serine with sulfide, and therefore, DcsD may be a synthase to generate
O
-ureido-
l
-serine using
O
-acetyl-
l
-serine and hydroxyurea. DcsG, which exhibits similarity to a family of enzymes with an ATP-grasp fold, may be an ATP-dependent synthetase converting
O
-ureido-
d
-serine into
d
-cycloserine. In the present study, to characterize the enzymatic functions of DcsC, DcsD, and DcsG, each protein was overexpressed in
Escherichia coli
and purified to near homogeneity. The biochemical function of each of the reactions catalyzed by these three proteins was verified by thin-layer chromatography (TLC), high-performance liquid chromatography (HPLC), and, in some cases, mass spectrometry. The results from this study demonstrate that by using a mixture of the three purified enzymes and the two commercially available substrates
O
-acetyl-
l
-serine and hydroxyurea, synthesis of
d
-cycloserine was successfully attained. These
in vitro
studies yield the conclusion that DcsD and DcsG are necessary for the syntheses of
O
-ureido-
l
-serine and
d
-cycloserine, respectively. DcsD was also able to catalyze the synthesis of
l
-cysteine when sulfide was added instead of hydroxyurea. Furthermore, the present study shows that DcsG can also form other cyclic
d
-amino acid analogs, such as
d
-homocysteine thiolactone.
摘要
我们最近克隆了一个 DNA 片段,其中含有一个负责抗结核抗生素生物合成的基因簇、
d
-环丝氨酸。该基因簇由 10 个开放阅读框组成,分别为
dcsA
到
dcsJ
.从每个推定基因产物与已知蛋白质之间的序列相似性来看,DcsC 与二氨基亚硒酸酯外延酶同源性很高,可能会催化 Omega 的外消旋化。
O
-尿苷酸的外消旋化。DcsD 与
O
-乙酰丝氨酸巯基酶相似,后者可生成
l
-半胱氨酸。
O
-乙酰-
l
因此,DcsD 可能是生成 O-乙酰基-l-半胱氨酸的合成酶。
O
-脲
l
-丝氨酸的合成酶。
O
-乙酰-l-丝氨酸
l
-丝氨酸和羟基脲。DcsG 与具有 ATP-抓取折叠的酶家族相似,可能是一种 ATP 依赖性合成酶,能将 O-乙酰基-丝氨酸转化为丝氨酸。
O
-脲
d
-丝氨酸转化为
d
-环丝氨酸。在本研究中,为了鉴定 DcsC、DcsD 和 DcsG 的酶功能,将每种蛋白在
大肠杆菌
并纯化至接近均一。通过薄层色谱法(TLC)、高效液相色谱法(HPLC)和质谱法(在某些情况下)验证了这三种蛋白质催化的每个反应的生化功能。这项研究的结果表明,通过使用三种纯化酶和两种市售底物的混合物
O
-乙酰-
l
-丝氨酸和羟基脲,可合成
d
-环丝氨酸的合成。这些
体外
这些体外研究得出的结论是,DcsD 和 DcsG 是合成 O-环丝氨酸的必要条件。
O
-脲
l
-丝氨酸和
d
-环丝氨酸。DcsD 还能催化 l -丝氨酸和 d -环丝氨酸的合成。
l
当加入硫化物而不是羟基脲时,DcsD 也能催化合成 l -半胱氨酸。此外,本研究还表明,DcsG 还能形成其他环
d
-氨基酸类似物,如
d
-高半胱氨酸硫内酯。