LE 8-10 Endergonic reaction: G is positive, reaction is not spontaneous Exergonic reaction: G is negative, reaction is spontaneous G = +3.4 kcal/mol G = –7.3 kcal/mol G = –3.9 kcal/mol NH 2 NH 3 Glu Glutamic acid Coupled reactions: Overall G is negative; together, reactions are spontaneous AmmoniaGlutamine ATP H2OH2O ADP P i + + +
LE 8-11 NH 2 Glu P i P i P i P i NH 3 P P P ATP ADP Motor protein Mechanical work: ATP phosphorylates motor proteins Protein moved Membrane protein Solute Transport work: ATP phosphorylates transport proteins Solute transported Chemical work: ATP phosphorylates key reactants Reactants: Glutamic acid and ammonia Product (glutamine) made + + +
LE 8-12 P i ADP Energy for cellular work (endergonic, energy- consuming processes) Energy from catabolism (exergonic, energy- yielding processes) ATP +
LE 8-14 Transition state CD A B EAEA Products CD A B G < O Progress of the reaction Reactants C D A B Free energy
LE 8-15 Course of reaction without enzyme E A without enzyme G is unaffected by enzyme Progress of the reaction Free energy E A with enzyme is lower Course of reaction with enzyme Reactants Products
LE 8-16 Substrate Active site Enzyme Enzyme-substrate complex
LE 8-17 Enzyme-substrate complex Substrates Enzyme Products Substrates enter active site; enzyme changes shape so its active site embraces the substrates (induced fit). Substrates held in active site by weak interactions, such as hydrogen bonds and ionic bonds. 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. Substrates are converted into products. Products are released. Active site is available for two new substrate molecules.
LE 8-18a Optimal temperature for typical human enzyme Optimal temperature for enzyme of thermophilic (heat-tolerant bacteria Temperature (°C) Optimal temperature for two enzymes 0 20 40 60 80100 Rate of reaction
LE 8-18b Optimal pH for pepsin (stomach enzyme) Optimal pH for trypsin (intestinal enzyme) pH Optimal pH for two enzymes 0 Rate of reaction 1 23 45 67 8 910
LE 8-19a Substrate Active site Enzyme Normal binding A substrate can bind normally to the active site of an enzyme.
LE 8-19b Competitive inhibitor Competitive inhibition A competitive inhibitor mimics the substrate, competing for the active site.
LE 8-19c Noncompetitive inhibitor Noncompetitive inhibition A noncompetitive inhibitor binds to the enzyme away from the active site, altering the conformation of the enzyme so that its active site no longer functions.
LE 8-20a Allosteric enzyme with four subunits Regulatory site (one of four) Active form Activator Stabilized active form Active site (one of four) Allosteric activator stabilizes active form. Non- functional active site Inactive form Inhibitor Stabilized inactive form Allosteric inhibitor stabilizes inactive form. Oscillation Allosteric activators and inhibitors
LE 8-20b Substrate Binding of one substrate molecule to active site of one subunit locks all subunits in active conformation. Cooperativity another type of allosteric activation Stabilized active form Inactive form
LE 8-UN141 Enzyme 1 AB Reaction 1 Enzyme 2 C Reaction 2 Enzyme 3 D Reaction 3 Product Starting molecule
LE 8-21 Active site available Initial substrate (threonine) Threonine in active site Enzyme 1 (threonine deaminase) Enzyme 2 Intermediate A Isoleucine used up by cell Feedback inhibition Active site of enzyme 1 can’t bind theonine pathway off Isoleucine binds to allosteric site Enzyme 3 Intermediate B Enzyme 4 Intermediate C Enzyme 5 Intermediate D End product (isoleucine)