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.
Highlights
“Activated” forms of L-homoserine have been shown to be more readily utilized as substrates than homoserine for the homocysteine synthetases of Neurospora crassu, yeast, and Escherichia coli KB
Since E. coli and S. typhimurium are metabolically very similar organisms, the work we present here corroborates the bacterial synthesis of homocysteine by this route
It may be observed that the specific activity of the Neurospora enzyme with 0-acetylhomoserine is some 40 times that obtained with homoserine and some 25 times that obtained with O-succinylhomoserine
Summary
“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 crassu, yeast, and Escherichia coli KB. This substrate preference is shown by the increased specific activities of the enzyme reactions. 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. The observation that the specific activity of the enzyme preparation was much higher for the latter reaction than the former suggested
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
