Acyl Derivatives of Homoserine as Substrates for Homocysteine Synthesis in Neurospora crassa, Yeast, and Escherichia coli

Abstract

Abstract Activated forms of l-homoserine have been shown to be more readily utilized as substrates than homoserine for the homocysteine synthetases (enzymes which catalyze the formation of homocysteine from homoserine and sulfide in the presence of pyridoxal phosphate) of Neurospora crassa, yeast, and Escherichia coli KB. This substrate preference is shown by the increased specific activities of the enzyme reactions. Specificity for the type of acylated derivative of homoserine is apparent. Enzymes from the fungi preferentially utilize O-acetyl-l-homoserine, and the enzyme from the bacterium requires O-succinyl-l-homoserine. Methionineless mutants of Neurospora contain homocysteine synthetases with specific activities nearly equivalent to those of the wild type when the substrate is O-acetylhomoserine. End product feedback inhibition by l-methionine on the homocysteine synthetases, when the substrates used were acyl derivatives of homoserine, is described for the enzymes from Neurospora and yeast, but was not demonstrated with the enzyme from E. coli. O-Acetyl-l-serine is a required substrate for cysteine synthesis by an enzyme from E. coli KB. Some preference for O-acetylserine was exhibited by the enzyme from yeast; however, cysteine synthetase from Neurospora showed no marked preference for this compound over serine.