清华大学：《生物化学》课程PPT教学课件（英文版）Enzymes Basic Concepts Kinetics

Biochemistry Chapter 8: Enzymes Yongzhang luo 罗永章 The Protein Chemistry lab 蛋白质化学实验室 Department of Biological Sciences biotechnology Tsinghua University 清华大学生物科学与生物技术系

Biochemistry Chapter 8: Enzymes Yongzhang Luo 罗 永 章 The Protein Chemistry Lab 蛋白质化学实验室 Department of Biological Sciences & Biotechnology Tsinghua University 清华大学生物科学与生物技术系

Enzymes Basic Concepts Kinetics

Enzymes Basic Concepts & Kinetics

FORMATION OF AN ENZYME-SUBSTRATE COMPLEX IS THE FIRST STEP IN ENZYMATIC CATALYSIS Much of the catalytic power of enzymes comes from their bringing sub- strates together in favorable orientations in enzyme-substrate(Es) com lexes. The substrates are bound to a specific region of the enzyme called the active site. Most enzymes are highly selective in their binding of sub- strates. Indeed the catalytic specificity of enzymes depends in part on the specificity of binding. Furthermore, the activities of some enzymes are controlled at this stage

The existence of es complexes has been shown in a variety of ways At a constant concentration of enzyme, the reaction rate increases with increasing substrate concentration until a maximal velocity is reached (Figure 8-8). In contrast, uncatalyzed reactions do not show this saturation effect. In 1913. Leonor Michaelis interpreted the maximal veloc ity of an enzyme-calalyzed reaction in terms of the formation of a discrete Es complex. At a sufficiently high substrate concentration, the catalytic sites are filled and so the reaction rate reaches a maximum. Though indirect this is the most general evidence for the existence of ES complexes

Chapter 8 189 ENZYMES Maximal velocity Non-catalyzed Substrate concentration Figure 8-8 Velocity of an enzyme-catalyzed reac- tion as a function of the substrate concentration

Non-catalyzed

2. ES complexes have been directly visualized by electron microscopy, as in the micrograph of DNA polymerase I bound to its DNA template(Figure 8-9). X-ray crystallography has provided high-resolution images of sub- Figure 8-9 Electron micrograph of DNA polymerase I mole- cules(white spheres bound to a threadlike synthetic DNA template. [ Courtesy of Dr Jack Griffith strates and substrate analogs bound to the active sites of many enzymes

Nicholas R. Cozzarelli Professor of Biochemistry Molecular Biology

3. The spectroscopic characteristics of many enzymes and substrates change upon formation of an Es complex These changes are particularly striking if the enzyme contains a colored prosthetic group. Tryptophan synthetase, a bacterial enzyme that contains a pyridoxal phosphate prosthetic group, affords a nice illustra- tion c subunit Indole-3-glycerol phosphate indole glyceraldehyde 3-phosphate Indole +serine Ba subunit, tryptophan+H2O