Abstract

L-serine deaminase 1 (L-SDI) is one of three enzymes that break down L-serine to produce pyruvate and ammonia in Escherichia coli K-12. L-SD1, a 454 amino acid protein encoded by sdaA gene, contains nine cysteine residues at the 181, 219, 290, 339, 347, 366, 381, 392, and 453 positions. Blast results show that three cysteines at positions 339, 381 and 392 are in what is then a highly conserved motif is found in most L-SDs that have a Fe-S cluster. The other six cysteine residues are also conserved among some bacterial L-SDs. In this study, all nine cysteine residues have been mutated individually. Assay of enzyme activity both in vivo and in vitro shows that: (1) eight of nine cysteine residues play an important role in L-SD1 activity; (2) the cysteine at position 181 might be non-essential for L-SD1 activity; (3) cysteines at positions 339, 381 and 392 are essential for L-SD1 activity. This is consistent with the demonstration by Cicchilo and his colleagues that L-SD1 uses a 4Fe-4S cluster for the deamination of L-serine. If so, cysteine residues 339, 381 and 392 are probably essential for this iron-sulfur cluster and ligate to three iron molecules of a 4Fe-4S cluster.