Subsystem: Cysteine Biosynthesis

This subsystem's description is:

Cysteine biosynthesis constitutes the primary pathway for incorporation of organic sulfur
into cellular components. Cysteine serves as a precursor for methionine biosynthesis.

The biosynthesis of L-cysteine in enteric bacteria, such as Salmonella typhimurium and Escherichia coli, and in plants proceeds via a two-step pathway. The amino acid precursor of L-cysteine is L-serine, which undergoes a substitution of its {beta}-hydroxyl with a thiol in two steps. Serine acetyltransferase (EC catalyzes the acetylation (by acetyl-CoA) of the {beta}-hydroxyl of L-serine to give O-acetyl-L-serine (OAS). The final step, the {alpha},{beta}-elimination of acetate from OAS and the addition of H2S to give L-cysteine is then catalyzed by O-acetylserine sulfhydrylase (OASS; EC
In enteric bacteria, two isozymes of OASS, A and B, are produced under aerobic and anaerobic growth conditions, respectively. The A and B isozymes are homodimeric with Mr of about 68,900 and 64,000, respectively, and each has one tightly bound pyridoxal 5'-phosphate (PLP) per subunit.
This pathway is regulated at the genetic level in its second step, with both cysteine synthase isozymes being under the positive control of the cysteine-responsive transcription factor CysB. It is also subject to very strong feedback inhibition of its first step by the final pathway product, cysteine.

Although two cysteine synthase isozymes exist, only cysteine synthase A (CysK) forms a complex with serine acetyltransferase. CysK is also the only one of the two cysteine synthases that is required for cell viability on cysteine-free medium.

==========Variant codes:==========

1.0 - complete sulfate reduction (assimilatory) pathway of cysteine biosynthesis:
(CysD) +(CysC) +(CysH) +(*SR) +(CysE) + (*CysK);
1.1 - sulfate reduction pathway and Sulfate_transport system;
1.7 - sulfate reduction pathway and L-Homocysteine_synthesis;
1.17 - sulfate reduction pathway + Sulfate_transport system + L-Homocysteine_synthesis;

2.0 - pathway of cysteine biosynthesis as a two-step conversion starting from L-serine and yielding L-cysteine - (CysE) + (*CysK);
2.1 - same with Sulfate_transport system;
2.17 - same with Sulfate_transport system and L-Homocysteine_synthesis;

3.0 - cysteine synthesized from O-Acetyl-L-serine ( no CysE gene)
3.1 - same with Sulfate_transport system

For more information, please check out the description and the additional notes tabs, below

DiagramFunctional RolesSubsystem SpreadsheetDescriptionAdditional Notes 

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Group Alias
Abbrev.Functional RoleReactionsScenario ReactionsGOLiterature

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