Abstract
Many enzymes require pyridoxal 5’-phosphate (PLP) as an essential cofactor and share active site residues in mediating diverse enzymatic reactions. Methionine can be converted into cysteine by cystathionine γ-lyases (CGLs) through a transsulfuration reaction dependent on PLP. In bacteria, MccB, also known as YhrB, exhibits CGL activity that cleaves the C–S bond of cystathionine at the γ position. In this study, we determined the crystal structure of MccB from Staphylococcus aureus in its apo- and PLP-bound forms. The structures of MccB exhibited similar molecular arrangements to those of MetC-mediating β-elimination with the same substrate and further illustrated PLP-induced structural changes in MccB. A structural comparison to MetC revealed a longer distance between the N-1 atom of the pyridine ring of PLP and the Oδ atom of the Asp residue, as well as a wider and more flexible active site environment in MccB. We also found a hydrogen bond network in Ser-water-Ser-Glu near the Schiff base nitrogen atom of the PLP molecule and propose the Ser-water-Ser-Glu motif as a general base for the γ-elimination process. Our study suggests the molecular mechanism for how homologous enzymes that use PLP as a cofactor catalyze different reactions with the same active site residues.
Highlights
Cysteine and methionine are sulfur-containing amino acids, which are prone to oxidization by reactive oxygen species [1]
These two amino acids can be interconverted by transsulfuration reactions via cystathionine, which is the common intermediate [2]
For methionine biosynthesis from cysteine in bacteria, cysteine and an activated homoserine are first linked by cystathionine γ-synthase
Summary
Cysteine and methionine are sulfur-containing amino acids, which are prone to oxidization by reactive oxygen species [1]. In bacteria, these two amino acids can be interconverted by transsulfuration reactions via cystathionine, which is the common intermediate [2]. Cystathionine is cleaved at the β position from the cysteine by cystathionine β-lyase MetC, producing homocysteine, the precursor of methionine [4]. In the biosynthesis of cysteine, cystathionine β-synthase [5], called MccA or CysM in bacteria, mediates the C-S linkage of homocysteine and serine to produce cystathionine as the first step. Cystathionine is cleaved at the γ position from the homocysteine by cystathionine γ-lyase, called
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