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Enzymes

Enzymes. Course of reaction without enzyme. E A. without enzyme. E A with enzyme is lower. Reactants. Free energy. ∆ G is unaffected by enzyme. Course of reaction with enzyme. Products. Progress of the reaction. Enzymes are protein catalysts.

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Enzymes

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  1. Enzymes

  2. Course of reaction without enzyme EA without enzyme EA with enzyme is lower Reactants Free energy ∆G is unaffected by enzyme Course of reaction with enzyme Products Progress of the reaction Enzymes are protein catalysts • Change the rates of a reaction (x 106-1012) without being consumed • Lower energy barriers  initial energy investment or activation energy(EA) • Cells can carry out necessary metabolic reactions even at moderate temperatures

  3. Substrate CH2OH CH2OH CH2OH CH2OH O O O O H H H H H H H Sucrase H OH H HO OH H HO H2O O + H H OH O HO CH2OH CH2OH Active site OH H H H OH H OH OH Sucrose Glucose Fructose C12H22O11 C6H12O6 C6H12O6 Enzyme Enzymes are substrate specific • Quaternary structure accounts for specificity • Active site – pocket or groove on surface of protein • Substrates have to be compatible with the active site  lock and key model • Substrates can cause the enzyme’s shape to change slightly to enhance activity “induced fit” • Can act on 1000 substrate molecules per second Enzyme-substrate complex

  4. 1 Substrates enter active site; enzyme changes shape so its active site embraces the substrates (induced fit). 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 EA 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 Enzyme-substrate complex 6 Active site Is available for two new substrate Molecules. Enzyme 5 Products are Released. 4 Substrates are Converted into Products. Products The catalytic cycle of an enzyme

  5. Enzyme action is affected by conditions within the cell • temperature • pH • presence of cofactors/coenzymes • presence of enzyme inhibitors • competitive inhibitors • noncompetitive inhibitors

  6. A substrate can bind normally to the active site of an enzyme. Substrate Active site 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. Enzyme (a) Normal binding A competitive inhibitor mimics the substrate, competing for the active site. Competitive inhibitor Noncompetitive inhibitor (c) Noncompetitive inhibition (b) Competitive inhibition Enzyme inhibitors

  7. Metabolic control depends on allosteric regulation • Regulatory molecules activate/inhibit enzyme shape and function by binding to allosteric sites • Protein action at active site is affected by binding at a different site

  8. Feedback inhibition • Metabolic pathways make use of more than one enzyme • End product is an allosteric inhibitor that switches off enzyme action • e.g. synthesis of isoleucine from threonine

  9. Postlab: Factors affecting enzyme action • Temperature • increase in effective collisions • optimum temperature – most rapid enzyme action • rapid fall – enzymes denatured above a given threshold temperature

  10. Postlab: Factors affecting enzyme action 2. pH • narrow range • activity falls of rapidly on both sides of optimum • structure is affected by degrees of acidity and alkanity

  11. Postlab: Factors affecting enzyme action 3. Substrate concentration [S] • E + S  ES  E + P • below saturation point –  [S]   E that can bind to S • point of saturation onwards -  [S]  no further increase in reaction rate

  12. Postlab: Factors affecting enzyme action 4. Enzyme concentration [E] • If excess substrate is available, rate  [E] • [P]  [E], c.p.

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