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1、,Chapter 3 Enzymes,Molecular structure and function of enzyme Characteristic and Mechanism of Enzyme-Catalyzed Reaction Kinetics of Enzyme-Catalyzed Reaction Regulation of Enzyme Naming and Classification of Enzyme,What are enzymes?,Enzymes are biologic polymers that catalyze the chemical reactions.
2、 Biological catalysts,The vast majority of enzymes are proteins. RNA: ribozyme DNA: DNAzyme,Enzyme, E Substrate, S Product, P,1 Molecular structure and function of enzyme,Molecular component Active site,Simple enzymes: enzymes require no other chemical groups other than their amino acid residues for
3、 activity. trypsin, chymotrypsin, ribonuclease A Conjugated enzymes: enzymes contain chemical groups other than AA, the non-amino acid parts are usually called prosthetic groups (coenzymes & cofactors) and the protein part alone called apoprotein.,1. Molecular component enzyme,Prosthetic groups coul
4、d be inorganic metal ions (e.g., Fe2+, Mg2+, Zn2+) or complex organic molecules called coenzymes. A complete catalytically active enzyme (including its prosthetic group) is called a holoenzyme. holoenzyme = apoprotein + prosthetic group,Coenzymes often function as transient carriers of specific (fun
5、ctional) groups during catalysis. Many vitamins, organic nutrients required in small amounts in the diet, are precursors of coenzymes.,Coenzymes,Substrates are bound to a specific region of an enzyme called the active site. The active site is a three-dimensional conformation formed by groups that co
6、me from different parts of the linear amino acid sequence. Active site contains catalytic groups (participate in the catalytic processes) and binding groups (specificity of binding).,2. Active site,Active site,Catalytic groups,Binding groups,Substrate,Other essential groups,Ser: OH Cys: SH His: imid
7、azole,Active site is cleft or pocket with a generally nonpolar character. In many enzymes, the active site has shape complementary to those of their substrates only after the substrates are bound (the induced fit). For conjugated enzymes, the active site always contains coenzymes.,3. Isozymes,*Disti
8、nct forms that catalyze the same reaction, differ in molecular components and physical & chemical properties.,Two or more polypeptide chains Different polypeptide chains are products of different genes, differ in AA sequence. Are tissue specific,LDH,Lactate Dehydrogenase,LDH is a tetramer of two non
9、-identical subunits,Cellular/Plasma specific activities of various tissues,Tissue Heart Kidney RBC Brain WBC Skeletal Muscle Liver,Diagnostic Enzyme Analysis,2 The Characteristic and Mechanism of Enzyme-Catalyzed Reaction,Characteristics of enzymes Mechanism of Enzyme-Catalyzed Reaction,1. Character
10、istics of enzymes,The two most striking characteristics of enzymes High efficiency High specificity Be regulated,High efficiency,Urease is a good example: Catalyzed rate: 3x104/sec Uncatalyzed rate: 3x10 -10/sec Ratio is 1x1014 !,Most reactions in biological systems do not occur at perceptible rates
11、 in the absence of enzymes. Enzymes accelerate reactions by factors of at least a million. The rate enhancements (rate with enzyme catalysis divided by rate without enzyme catalysis) brought about by enzymes are often in the range of 107 to 1014,What Enzymes Do,Enzymes accelerate reactions by loweri
12、ng activation energy Enzymes do this by binding the transition state of the reaction better than the substrate,Enzymes, like all other catalysts, do not affect reaction equilibria, only accelerate reactions.,The rate constant of a reaction (k) is related to the free energy difference between the tra
13、nsition state and the ground state of the substrate (GF),Activation energy,Transition state is a fleeting molecular moment (not a chemical species with any significant stability) that has the highest free energy during a reaction.,An enzyme increases the rate constant of a reaction (k) by lowering i
14、ts GF.,The combination of a substrate and an enzyme creates a new reaction pathway whose transition state energy is lower than that of the reaction in the absence of enzyme.,High specificity,Enzymes are highly specific both in the reaction catalyzed and in their choice of substrates. An enzyme usual
15、ly catalyzes a single chemical reaction or a set of closely related reactions.,Low specificity: some peptidases, phosphatases, etc. Intermediate specificity: trypsin, chymotrypsin, etc Absolute or stereochemical specificity: urease, succinate dehydrogenase, etc stereochemical specificity: lactate de
16、hydrogenase,Enzymes exhibit various degrees of specificity,Weak interactions between substrates and enzymes may generate conformational changes on the enzyme.,2. Mechanism of Enzyme-Catalyzed Reaction,Amino acids in general acid-base catalysis,S E T pH Inhibitor Activator,3 Kinetics of Enzyme-Catalyzed Reaction,Vi was measured at the beginning of the enzyme-catalyzed reaction, when substrate concentration can be considered constant (S will decrease as th
