ABSTRACT
Mycobacterium tuberculosis
is dependent on cysteine biosynthesis, and reduced sulfur compounds such as mycothiol synthesized from cysteine serve in first-line defense mechanisms against oxidative stress imposed by macrophages. Two biosynthetic routes to
l
-cysteine, each with its own specific cysteine synthase (CysK1 and CysM), have been described in
M. tuberculosis
, but the function of a third putative sulfhydrylase in this pathogen, CysK2, has remained elusive. We present biochemical and biophysical evidence that CysK2 is an
S
-sulfocysteine synthase, utilizing
O
-phosphoserine (OPS) and thiosulfate as substrates. The enzyme uses a mechanism via a central aminoacrylate intermediate that is similar to that of other members of this pyridoxal phosphate-dependent enzyme family. The apparent second-order rate of the first half-reaction with OPS was determined as
k
max
/
K
s
= (3.97 × 10
3
) ± 619 M
−1
s
−1
, which compares well to the OPS-specific mycobacterial cysteine synthase CysM with a
k
max
/
K
s
of (1.34 × 10
3
) ± 48.2. Notably, CysK2 does not utilize thiocarboxylated CysO as a sulfur donor but accepts thiosulfate and sulfide as donor substrates. The specificity constant
k
cat
/
K
m
for thiosulfate is 40-fold higher than for sulfide, suggesting an annotation as
S
-sulfocysteine synthase. Mycobacterial CysK2 thus provides a third metabolic route to cysteine, either directly using sulfide as donor or indirectly via
S
-sulfocysteine. Hypothetically,
S
-sulfocysteine could also act as a signaling molecule triggering additional responses in redox defense in the pathogen upon exposure to reactive oxygen species during dormancy.
摘要
结核分枝杆菌
依赖于半胱氨酸的生物合成,由半胱氨酸合成的还原硫化合物(如霉菌硫醇)是抵御巨噬细胞氧化应激的一线防御机制。两种生物合成途径
l
-半胱氨酸的两种生物合成途径,每种途径都有自己特定的半胱氨酸合成酶(CysK1 和 CysM)。
结核杆菌
但该病原体中第三种假定的巯基酶 CysK2 的功能仍未确定。我们提出的生化和生物物理证据表明,CysK2 是一种巯基酶。
S
-硫代半胱氨酸合成酶,利用
O
-磷酸丝氨酸(OPS)和硫代硫酸盐作为底物。该酶通过一个氨基丙烯酸酯中心中间体进行合成,其机制与这个依赖于磷酸吡哆醛的酶家族的其他成员相似。与 OPS 的第一个半反应的表观二阶速率被确定为
k
max
/
K
s
= (3.97 × 10
3
) ± 619 M
-1
s
-1
与OPS特异性分枝杆菌半胱氨酸合成酶CysM的
k
max
/
K
s
的(1.34×10
3
) ± 48.2.值得注意的是,CysK2 并不利用硫代羧化的 CysO 作为硫供体,而是接受硫代硫酸盐和硫化物作为供体底物。特异性常数
k
cat
/
K
m
硫代硫酸盐的 K m 是硫化物的 40 倍,这表明注释为
S
-硫代半胱氨酸合成酶。因此,分枝杆菌的 CysK2 为半胱氨酸提供了第三种代谢途径,即直接使用硫化物作为供体或间接通过
S
-硫代半胱氨酸。假设
S
-硫代半胱氨酸还可以作为一种信号分子,在休眠期接触活性氧时触发病原体的氧化还原防御反应。