Yeast cystathionine beta-synthase is a pyridoxal phosphate enzyme but, unlike the human enzyme, is not a heme protein

Our studies of cystathionine beta-synthase from Saccharomyces cerevisiae (y east) are aimed at (1) clarifying the cofactor dependence and catalytic mec hanism and (2) obtaining a system for future investigations of the effects of mutations that cause human disease (homocystinuria or coronary heart dis ease). We report methods that yielded high expression of the yeast gene in Escherichia coli and of purified yeast cystathionine beta-synthase. The abs orption and circular dichroism spectra of the homogeneous enzyme were chara cteristic of a pyridoxal phosphate enzyme and showed the absence of heme, w hich is found in human and rat cystathionine beta-synthase. The absence of heme in the yeast enzyme facilitates spectroscopic studies to probe the cat alytic mechanism. The reaction of the enzyme with L-serine in the absence o f L-homocysteine produced the aldimine of aminoacrylate, which absorbed at 460 nm and had a strong negative circular dichroism band at 460 nm. The for mation of this intermediate from the product, L-cystathionine, demonstrates the partial reversibility of the reaction. Our results establish the overa ll catalytic mechanism of yeast cystathionine beta-synthase and provide a u seful system for future studies of structure and function. The absence of h eme in the functional yeast enzyme suggests that heme does not play an esse ntial catalytic role in the rat and human enzymes. The results are consiste nt with the absence of heme in the closely related enzymes O-acetylserine s ulfhydrylase, threonine deaminase, and tryptophan synthase.