Kredich N M, Hulanicka M D, Hallquist S G
Ciba Found Symp. 1979(72):87-99. doi: 10.1002/9780470720554.ch6.
In Salmonella typhimurium and Escherichia coli the biosynthesis of L-cysteine from L-serine and inorganic sulphate proceeds along a branched convergent pathway along one arm of which sulphate is reduced to sulphide, while on the other L-serine is acetylated to O-acetyl-L-serine. This system is subject to positive genetic control in which growth on a poor sulphur source, O-acetyl-L-serine and the product of the cysB regulatory gene are all required for derepression. The final step consists of the formation of L-cysteine from O-acetyl-L-serine and sulphide. We find that in S. typhimurium this reaction is catalysed by two different enzymes, O-acetylserine sulphydrylase A and O-acetylserine sulphydrylase B, coded for by cysK and cysM respectively. Both enzymes are under the control of the cysteine regulon, and either alone is sufficient for cysteine prototrophy during aerobic growth. Although the advantage to the bacterium of having two separate enzymes to carry out the same reaction is unclear, preliminary data suggest that O-acetylserine sulphydrylase B is preferentially utilized for cysteine biosynthesis during anaerobic growth. We speculate that one enzyme may prefer free sulphide as a substrate while the other may use a bound form of sulphide.
在鼠伤寒沙门氏菌和大肠杆菌中，由L-丝氨酸和无机硫酸盐生物合成L-半胱氨酸是沿着一条分支汇聚途径进行的。在该途径的一个分支中，硫酸盐被还原为硫化物，而在另一个分支中，L-丝氨酸被乙酰化为O-乙酰-L-丝氨酸。该系统受到正向遗传控制，在这种控制中，在贫硫源上生长、O-乙酰-L-丝氨酸以及cysB调节基因的产物都是去阻遏所必需的。最后一步是由O-乙酰-L-丝氨酸和硫化物形成L-半胱氨酸。我们发现，在鼠伤寒沙门氏菌中，该反应由两种不同的酶催化，分别是由cysK和cysM编码的O-乙酰丝氨酸巯基化酶A和O-乙酰丝氨酸巯基化酶B。这两种酶都受半胱氨酸调节子的控制，并且在有氧生长期间，单独任何一种酶都足以实现半胱氨酸原养型。虽然细菌拥有两种单独的酶来进行相同反应的优势尚不清楚，但初步数据表明，在厌氧生长期间，O-乙酰丝氨酸巯基化酶B优先用于半胱氨酸的生物合成。我们推测，一种酶可能更喜欢游离硫化物作为底物，而另一种酶可能使用结合形式的硫化物。
