Chapter

If all Exergonic Reactions happen spontaneously… …then how come all of them haven’t already happened?

Activation Energy! We need catalysts! Biological Catalysts Are Enzymes

Activation Energy Energy C 6 H 12 O 6 +O 2 CO 2 + H 2 0 Reaction Time Energy Activation Energy

Enzymes are Proteins Organic catalysts - increase the rate of chemical reactions in cells. Hold reactant molecules close together for reaction to occur- uses an active site. The active site is used to bind the reactant molecules-substrate.

The active site: –Is the region on the enzyme where the substrate binds. Figure 8.16 Substate Active site Enzyme (a)

Induced fit of a substrate: –Brings chemical groups of the active site into positions that enhance their ability to catalyze the chemical reaction Figure 8.16 (b) Enzyme- substrate complex

The catalytic cycle of an enzyme Substrates Products Enzyme Enzyme-substrate complex 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 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. 5 Products are Released. 6 Active site Is available for two new substrate Mole. Figure 8.17

Conditions that Affect Protein Shape Can disrupt H bonds by: High Temperature pH Changes (Acidic or Basic) Salt (or Ion) Concentration Binding of Regulatory Molecules Disrupting the 2°, 3°, 4° structure is called denaturation.

Enzymes Enzymes have a temperature optimum. Too Cold - H bonds and van der Waals forces aren’t flexible enough to allow the induced fit for catalysis. Too Hot - they are too weak to maintain the enzymes shape and break apart.

Rate of Reaction Rxn Temperature, °C Most Human Enzyme Temp. Optimums Male Reprod. Enzymes

Siamese Cats and Pigment Enzymes 3 wks old 4 yrs. old

Pigment Enzymes in Harp Seal Pups

Rate of Reaction Rxn pH Most Human Enzyme pH Optimums 8 TrypsinPepsin

Inhibitors and Activators Inhibitors – are molecules (usually proteins) that bind to an enzyme and decrease its activity. Two Kinds: –A. Competitive Inhibitors – compete for the same active site as the substrate; displaces some of the substrate. –B. Noncompetitive Inhibitors – bind to the enzyme in some other location other than its active site., changing its shape.

Enzyme Inhibitors Competitive inhibitors –Bind to the active site of an enzyme, competing with the substrate Figure 8.19 (b) Competitive inhibition A competitive inhibitor mimics the substrate, competing for the active site. Competitive inhibitor A substrate can bind normally to the active site of an enzyme. Substrate Active site Enzyme (a) Normal binding

Competitive Inhibition

Noncompetitive inhibitors –Bind to another part of an enzyme, changing the function Figure 8.19 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. Noncompetitive inhibitor (c) Noncompetitive inhibition

Noncompetitive Inhibitors These inhibitors bind to a site other than the active site called the allosteric site (“other form”). Chemical “on/off” switches. Called Allosteric Inhibitor. If a molecule binds to an allosteric site and keeps the enzyme in its active shape – Allosteric Activator.

Enzymes change shape when regulatory molecules bind to specific sites, affecting function Stabilized inactive form Allosteric activater stabilizes active from Allosteric enyzme with four subunits Active site (one of four) Regulatory site (one of four) Active form Activator Stabilized active form Allosteric activater stabilizes active form Inhibitor Inactive form Non- functional active site (a) Allosteric activators and inhibitors. In the cell, activators and inhibitors dissociate when at low concentrations. The enzyme can then oscillate again. Oscillation Figure 8.20

Another Look at Allosteric Inhibitors

Allosteric Activation

Things that help Enyzmes Molecules that bind to the active site and HELP the enzyme’s function – Cofactor. Often metal ions, vitamins, etc. If cofactor is a nonprotein organic molecule- Coenzyme. Coenzymes often serve as electron acceptors to help break bonds. Coenzymes then transfer the electrons to other compounds. (Remember NAD + and FAD + ?

Enzymes Work with Co-Enzymes Captures H +

An Example of a Biochemical Pathway

Feedback Inhibition E1 Substrate A Substrate B E2 Substrate C Allosteric Inhibitor

Feedback inhibition Active site available Isoleucine used up by cell Feedback inhibition Isoleucine binds to allosteric site Active site of enzyme 1 no longer binds threonine; pathway is switched off Initial substrate (threonine) Threonine in active site Enzyme 1 (threonine deaminase) Intermediate A Intermediate B Intermediate C Intermediate D Enzyme 2 Enzyme 3 Enzyme 4 Enzyme 5 End product (isoleucine) Figure 8.21