Enzymes

Review of Reaction Terms  G = (Free Energy of Products) - (Free Energy of Reactants)

Types of Reactions Exergonic –Energy is released –  G is negative (more energy in reactants than products) –Spontaneous reaction

Types of Reactions Endergonic –Absorbs energy (energy must be put into the reaction) –  G is positive (more energy in the products than the reactants) –Nonspontaneous reaction

Enzyme Function Enzymes are proteins that speed up reactions –Typically end in “ase” (ex. lactase) Highly specific –Substrate = the molecules an enzyme acts on Can be regulated Return to original structure/shape after reaction –reusable

Enzyme Function Immense catalytic power Enzymes speed up a reaction by lowering the activation energy (E A ) Enzymes do NOT change the energy in the reactants or the products.

Enzymes & Substrates Enzymes bring substrates together in favorable ways in an Enzyme-Substrate Complex –Proximity and Orientation –Altered Environment (esp. a hydrophobic pocket) –Hydrogen Transfer Move H from one molecule to another (to increase reactivity) Enzyme binds the substrate in the active site substrates Enzyme- Substrate Complex products

Active Sites Takes up relatively small part of enzyme 3-D region that is put together by different parts of the linear structure Clefts or crevices Active site

Active Sites Substrate bound to active site by: –Ionic attractions –Van der waals forces –Hydrophobic interactions –Hydrogen bonds

Induced Fit Enzymes and substrates do not fit together like a lock and key Active site is more flexible– molds around the substrate(s) –Like a glove around a hand

Coenzymes & Cofactors Non-protein molecules that are needed for an enzyme to function When a coenzyme or cofactor binds  allows it to function properly (activates it) Ex. Metal ions, certain vitamins Image taken without permission from time/Weber/Microbiology%20Majors/Chpater5/chapter5sub/chapter5sub_print.htm l

Inhibition Competitive Inhibitor –Binds to the active site –Competes with the substrate –Can be overcome by adding more substrate Noncompetitive Inhibitor –Binds to enzyme in an area other than the active site –Changes the shape of the active site to prevent substrate binding

Phosphorylation Addition of a phosphate group to an enzyme This changes its structure –Can either activate or inactivate the enzyme Phosphate group

Feedback Inhibition Molecules can bind to enzymes at a “regulatory” site to inhibit its activity (like a noncompetitive inhibitor) –Molecule is usually end- product of chain of reactions –Stops entire chain of reactions

Allosteric Inhibition When a protein’s function is affected by the binding of a molecule at a location other than the active site Both allosteric inhibition and activation is possible.

Cooperativity Occurs in enzymes with multiple subunits (chains) Binding a substrate to one subunit makes it easier for all of the rest of the subunits to bind substrates

Other methods of Regulation Some enzymes are synthesized in an inactive form and activated later by cleavage Why would removing part of the enzyme changes its function? –Removing amino acids changes its structure which then changes its function

Effects of Temperature on Enzymes Each enzyme has a particular temperature that it functions best at At non-ideal temperatures: –Too high: Extra energy causes molecules to move around too much  bonds can’t be maintained –Too low: Active site becomes less flexible  can’t catalyze reactions as well.

Effect of pH on Enzymes Each enzyme also has a pH that it functions best at At non-ideal pHs excess H+ and OH- ions interfere with ionic attractions between + and – charged amino acids.