Homocystinuria is an inherited autosomal recessive disorder that affects L-homocysteine metabolism. Treatment includes supplementation with pyridoxine and/or betaine and dietary L-methionine restriction. Refinement of therapeutic regimes will be enabled by an understanding of the distinct effects of homocystinuria-associated mutations. This study: (1) probed the role of active site residues D319, S323 and Y324 in the positioning of PLP in the model yeast CBS enzyme and (2) investigated a set of 11 homocystinuria-associated mutations in the context of human CBS. In the first study, D319A
eliminated activity, the S323A and Y324F variants had negligible effect on enzymatic function. Residue G148 provides flexibility to the mobile loop. Residues E176, K384 and G307 are involved in maintaining active site architecture. Residues T257, G259, G305 and S349 aid PLP positioning while T257 and G259 functions in the bi-directional communication between both cofactors. Gaining insight into CBS structure-function relationships can help refine current homocystinuric treatment.