tailieunhanh - Biochemistry, 4th Edition P44

Biochemistry, 4th Edition P44. 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 Equations Define the Kinetics of Enzyme-Catalyzed Reactions 393 can be derived from Vmax 2 so the two constants of the Michaelis-Menten equation can be obtained from plots of v versus S . Note however that actual estimation of Vmax and consequently Km is only approximate from such graphs. That is according to Equation to get v Vmax S must equal 99 Km a concentration that may be difficult to achieve in practice. From Equation when S Li Km then v Vmax. That is v is no longer dependent on S so the reaction is obeying zero-order kinetics. Also when S Km then v V m ix Km S . That is the rate v approximately follows a first-order rate equation v k A where k Vmax Km. Km and Vmax once known explicitly define the rate of the enzyme-catalyzed reaction provided 1. The reaction involves only one substrate or if the reaction is multisubstrate the concentration of only one substrate is varied while the concentrations of all other substrates are held constant. 2. The reaction ES E P is irreversible or the experiment is limited to observing only initial velocities where P 0. 3. S 0 Er and Er is held constant. 4. All other variables that might influence the rate of the reaction temperature pH ionic strength and so on are held constant. Turnover Number Defines the Activity of One Enzyme Molecule The turnover number of an enzyme kcat is a measure of its maximal catalytic activity. kcat is defined as the number of substrate molecules converted into product per enzyme molecule per unit time when the enzyme is saturated with substrate. The turnover number is also referred to as the molecular activity of the enzyme. For the simple Michaelis-Menten reaction Equation under conditions of initial velocity measurements k2 kcat. Provided the concentration of enzyme Er in the reaction mixture is known kcat can be determined from Vmax. At saturating S v Vmax k2 Er . Thus k2 x kcat Er The term kcat represents the kinetic efficiency of the enzyme. Table .