The pathway of cysteine biosynthesis is a two-step conversion starting from L-serine and yielding L-cysteine. L-serine biosynthesis is shown for context. L-cysteine can also be synthesized from sulfate derivatives. The process through L-serine involves a serine acetyltransferase that produces a O-acetylserine which reacts together with hydrogen sulfide through a cysteine synthase complex in order to produce L-cysteine and acetic acid. Hydrogen sulfide is produced from a sulfate. Sulfate reacts with sulfate adenylyltransferase to produce adenosine phosphosulfate. This compound in turn is phosphorylated through a adenylyl-sulfate kinase into a phosphoadenosine phosphosulfate which in turn reacts with a phosphoadenosine phosphosulfate reductase to produce a sulfite. The sulfite reacts with a sulfite reductase to produce the hydrogen sulfide. This pathway shows the second step of cysteine biosynthesis (at genetic level). Both cysteine synthase isozymes undergo the positive control by the cysteine-responsive transcription factor CysB. Only cysteine synthase A (CysK) forms a complex with serine acetyltransferase and it is the only cysteine synthase that is required for cell viability with cysteine-free medium. Cysteine synthases may also work as the sulfur scavenging systemfor sulfur starvation by taking sulfur off of L-cysteine.

Cysteine Biosynthesis References