INVESTIGATION OF THE CATALYTIC AND REGULATORY DOMAINS OF CYSTATHIONINE β-SYNTHASE

Abstract

Cysthationine β-Synthase (CBS) is a pyridoxal 5’-phosphate (PLP)-dependent enzyme that catalyzes the condensation of serine and homocysteine to form cysthathionine. Deficiency in CBS activity is the leading cause for the disorder homocystinuria in humans. The focus of the studies described in this thesis is to investigate the mechanism of homocystinuria-associated mutations in the active site of yeast CBS, the potential of native tryptophan residues as probes of conformational change and the binding of SAM to the regulatory domain. Site-directed replacement variants of 9 residues located in the active-site of the truncated form of yeast CBS (residues 1-353) were characterized. The results suggest that the hydrogen bonding network comprising residues K112, E111, K327, E244 and T326 is a determinant of active-site architecture and dynamics. Residues G245, I246 and G247, situated adjacent to the cofactor, play a role in maintaining PLP in a catalytically productive orientation. With the goal of developing probes of conformational change within the catatlytic domain and of communication between the catalytic and regulatory domain of CBS, single and triple-substitution, site-directed variants of the four tryptophan residues of yeast CBS (W132, W263, W333 and W340) were characterized in the truncated and full-length enzyme forms. The results demonstrate that residue W263 is the main contributor for fluorescence resonance energy transfer to the PLP cofactor. Substitution of W333 and W340 causes a change in the degree of solvent exposure in the microenvironment of these residues. This is due to the presence of the regulatory domain in the full-length enzyme. Binding of SAM to the regulatory domain of human CBS leads to enzyme activation. In contrast, the Drosophila melanogaster and yeast CBS enzymes are not activated by SAM. The binding of SAM to the regulatory domains of yeast and human CBS was investigated via fluorescence spectroscopy and mass spectrometry. The results indicate that the full-length yeast enzyme does not bind to SAM, suggesting that, similar to the drosophila enzyme, yeast CBS may locked in a conformation that does not allow SAM-binding to its regulatory domain.