Mutational analysis of chitin synthase 2 of Saccharomyces cerevisiae - Identification of additional amino acid residues involved in its catalytic activity

Saccharomyces cerevisiae harbors three chitin synthases termed Chs1p, Chs2p and Chs3p. Previously, we demonstrated that con1, a region that is highly conserved among all chitin synthases, contains amino acids essential for th e catalytic activity of the enzyme and that Asp562, Gln601, Arg604, and Trp 605 found in con1 together with Asp441 were probable catalytic sites of the enzyme. Here we report that another region, con2, in the C-terminal half o f Chs2p is also conserved exclusively in chitin synthases that resemble S. cerevisiae Chs1p and Chs2p. Alanine substitutions for the conserved amino a cids in con2 identified five amino acids, Asn797, His799, Asp800, Trp803, a nd Thr805, the mutation of which severely diminished enzymatic activity and the enzyme's ability to rescue the yeast chs2 Delta chs3 Delta null mutant strain. Although the activities of some of the mutant enzymes were too low to measure enzyme kinetics, most of the alanine mutations in con2 affected the k(cat) values rather than the K-m values. Whereas a conservative mutat ion of Asn797 restored the activity, those of His799, Asp800, Trp803, and T hr805 did not. A fine alignment of the amino acid sequences of con2 and Chs 3p revealed that Asp800, Trp803 and Thr805 are completely conserved near th e C-terminal ends of Chs3p and its homologs in other fungi. On the basis of these findings, we propose that Asp800, Trp803, and Thr805 in con2 are add itional residues involved in catalysis, and hypothesise that Asp800 togethe r with the previously identified Asp441 and Asp562 serve as polar residues necessary for the acid-based catalytic reaction of chitin synthase.