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3 active site and r groups of its amino acids can

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Unformatted text preview: te complex • The active site • Is the region on the enzyme where the substrate binds Substate Active site Enzyme Figure 8.16 (a) • Induced fit of a substrate • Brings chemical groups of the active site into positions that enhance their ability to catalyze the chemical reaction Enzyme- substrate complex Figure 8.16 (b) • The catalytic cycle of an enzyme 1 Substrates enter active site; enzyme changes shape so its active site embraces the substrates (induced fit). Substrates Enzyme-substrate complex Active site available for o new substrate ole. Enzyme 5 Products are Released. Figure 8.17 Products 2 Substrates held in active site by weak interactions, such as hydrogen bonds and ionic bonds. 3 Active site (and R groups of its amino acids) can lower E A and speed up a reaction by • acting as a template for substrate orientation, • stressing the substrates and stabilizing the transition state, • providing a favorable microenvironment, • participating directly in the catalytic reaction. 4 Substrates are Converted into Products. • The active site can lower an EA barrier by • • • • Orienting substrates correctly Straining substrate bonds Providing a favorable microenvironment Covalently bonding to the substrate Effects of Local Conditions on Enzyme Activity Activity • The activity of an enzyme • Is affected by general environmental factors Effects of Temperature and pH pH • Each enzyme • Has an optimal temperature in which it can function Optimal temperature for typical human enzyme Optimal temperature for enzyme of thermophilic R at e of r eac t i on (heat-tolerant) bacteria 0 20 40 Temperature (Cº) (a) Optimal temperature for two enzymes Figure 8.18 80 100 • Has an optimal pH in which it can function Optimal pH for pepsin (stomach enzyme) Rate of reaction Optimal pH for trypsin (intestinal enzyme) 3 4 0 2 1 (b) Optimal pH for two enzymes Figure 8.18 5 6 7 8 9 Cofactors Cofactors • Cofactors • Are nonprotein enzyme helpers • Coenzymes • Are organic cofactors Enzyme Inhibitors Enzyme • Competitive inhibitors • Bind to the active site of an enzyme, competing with A substrate can Substrate the substrate bind normally to the active site of an enzyme. Active site Enzyme (a) Normal binding A competitive inhibitor mimics the substrate, competing for the active site. Figure 8.19 (b) Competitive inhibition Competitive inhibitor • Noncompetitive inhibitors • Bind to another part of an enzyme, changing the function A noncompetitive inhibitor binds to the enzyme away from the active site, altering the conformation of the enzyme so that i...
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