Biochemistry, 4th Edition P45. Continuing Garrett and Grisham's innovative conceptual and organizing framework, "Essential Questions," BIOCHEMISTRY guides students through course concepts in a way that reveals the beauty and usefulness of biochemistry in the everyday world. Streamlined for increased clarity and readability, this edition also includes new photos and illustrations that show the subject matter consistently throughout the text. New end-of-chapter problems, MCAT practice questions, and the unparalleled text/media integration with the power of CengageNOW round out this exceptional package, giving you the tools you need to both master course concepts and develop critical problem-solving skills you can draw upon. | What Is the Kinetic Behavior of Enzymes Catalyzing Bimolecular Reactions 403 Penicillin A Suicide Substrate Several drugs in current medical use are mechanism-based enzyme inactivators. For example the antibiotic penicillin exerts its effects by covalently reacting with an essential serine residue in the active site of glycopeptide transpeptidase an enzyme that acts to crosslink the peptidoglycan chains during synthesis of bacterial cell walls Figure . Penicillin consists of a thia-zolidine ring fused to a ß-lactam ring to which a variable R group is attached. A reactive peptide bond in the ß-lactam ring covalently attaches to a serine residue in the active site of the glycopeptide transpeptidase. The conformation of penicillin around its reactive peptide bond resembles the transition state of the normal glycopeptide transpeptidase substrate. The penicillinoyl-enzyme complex is catalytically inactive. Once cell wall synthesis is blocked the bacterial cells are very susceptible to rupture by osmotic lysis and bacterial growth is halted. What Is the Kinetic Behavior of Enzymes Catalyzing Bimolecular Reactions Thus far we have considered only the simple case of enzymes that act upon a single substrate S. This situation is not common. Usually enzymes catalyze reactions in which two or even more substrates take part. Consider the case of an enzyme catalyzing a reaction involving two substrates A and B and yielding the products P and Q enzyme A B -. P Q Such a reaction is termed a bisubstrate reaction. In general bisubstrate reactions proceed by one of two possible routes 1. Both A and B are bound to the enzyme and then reaction occurs to give P Q E A B------- AEB---- PEQ----- E P Q Reactions of this type are defined as sequential or single-displacement reactions. They can be either of two distinct classes a. random where either A or B may bind to the enzyme first followed by the other substrate or b. ordered where A designated the leading .
