EnzymesEnzymes

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 (EA)

• 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 http://www2.raritanval.edu/departments/Science/full-time/Weber/Microbiology%20Majors/Chpater5/chapter5sub/chapter5sub_print.html

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+ ATP + ADP

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

Cleavage by Enterokinase

Inactive trypsinogen Active trypsin

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.