Chapter 8, Sections 4

and 5Andrew Pidhorodeckyj

Annie Sell

8.4: Enzymes speed up metabolic reactions by lowering energy barriers

Enzymes are biological catalysts

Catalysts are agents that speed the rate of a reaction but are unchanged by the reaction.

The Activation Energy Barrier

Chemical reactions involve both breaking and forming of chemical bonds.

Energy must be absorbed to make binds unstable enough that they will break.

Energy is released when bonds form and molecules return to stable, lower energy states.

Activation Energy- is the energy that must be absorbed by reactants to reach the unstable transition state, in which the bonds are likely to break, and from which the reaction can proceed.

Is also called free energy of activation Ea

Graph of Exergonic Reaction

How Enzymes Lower the Ea Barrier

The Ea Barrier is essential to life.

It prevents the energy-rich macromolecules of the cell from decomposing spontaneously.

For metabolism to proceed in a cell, Ea must be reached.

Heat, a possible source of activation energy in reactions, would be harmful to the cell and speed metabolic reactions.

Enzymes are able to lower activation energy for specific reaction so that metabolism can proceed at cellular temperatures.

Enzymes do not change the free energy change for a reaction.

Substrate Specificity of Enzymes

Protein enzyme are macromolecules with characteristic three-dimensional shapes.

This shape determines the specificity of an enzyme for a particular substrate.

The substrate attaches at the enzyme’s active site.

Active site- a pocket or groove found on the surface of the enzyme that has a shape complementary to the substrate.

The substrate is temporarily bound to its enzyme, forming an enzyme-substrate complex.

Interaction between substrate and active site cause enzymes to change shape slightly, creating what is called an induced fit.

The induced fit enhances the ability of the enzyme to catalyze the chemical reaction.

Catalysis in the Enzyme’s Active Site

The substrate is often held in the active site by hydrogen or ionic bonds.

The side chains (R groups) of some of the surrounding amino acids in the active site facilitate the conversion of substrate to product.

The product the leaves the active site, and the catalytic cycle repeats.

Whether an enzyme catalyzes the forward or backward reaction is influenced by the relative concentrations of reactants and products and the free energy change of the reactants.

Enzymes catalyze reactions moving toward the equilibrium.

Enzymes catalyze reactions involving the joining of two reactants by binding the substrates closely together and properly oriented.

An induced fit can stretch bonds in a substrate molecule and make them easier to break.

An active site may provide a microenvironment that would be necessary for a particular reaction.

The rate of a reaction will increase with increasing substrate concentration up to the point at which all enzyme molecules are saturate with substrate molecules and working at full speed.

Only adding more enzyme molecules will increase the rate o the reaction at that point.

Effects of Local Conditions on Enzyme Activity

The velocity and temperature of an enzyme-catalyzed reaction may increase so much that the hydrogen and ionic bonds are disrupted.

Other interactions that stabilize protein conformation may be disrupted as well.

Each enzyme has an optimal temperature and pH where it is most active.

If the pH is changed, the protein shape and enzyme function may be affected as well.

Effects of Local Conditions on Enzyme Activity (cont.)

Cofactors- small molecules that bind either permanently or reversibly with enzymes and are necessary for enzyme catalytic function.

Can be: Inorganic (ex. various metal ions) Organic, called coenzymes

Most vitamins are coenzymes.

Effects of Local Conditions on Enzyme Activity (cont.)

Enzyme inhibitors disrupt the actions of enzymes either: Reversibly by binding to the enzyme with

weak bonds. Irreversibly by attaching to covalent bonds.

Effects of Local Conditions on Enzyme Activity (cont.)

Competitive inhibitors- compete with substrate for the active site of the enzyme.

Too many substrate molecules will cause this type of inhibition to be overcome.

Noncompetitive inhibitors- do not bind to the active site of the enzyme, therefore changing the enzyme’s conformation and disrupting enzyme action.

8.5 Regulation of enzyme activity

helps control metabolism

Allosteric Regulation of Enzymes

Allosteric regulation- molecules may inhibit or activate enzyme activity when they bind to a site separate from the active site.

An enzyme may be made of two or more polypeptide bonds, each having their own active site.

Where the two subunits join, these enzymes may have binding (allosteric) sites.

The entire unit may waver between two conformational states.

Allosteric Regulation of Enzymes (cont.)

The catalytically active conformation is stabilized by the binding of an activator.

The inactive form of the enzyme is reinforced by the binding of an inhibitor.

Allosteric Regulation of Enzymes (cont.)

Cooperativity- when the induced-fit binding of a substrate molecule to one subunit can change the conformation, making the active sites of all subunits more active.

Feedback inhibition- when the product of a pathway acts as an allosteric inhibitor of an enzyme early in the pathway.

Feedback inhibition commonly regulates metabolic pathways.

Specific Localization of Enzymes within the Cell

To facilitate the sequence of reactions, enzymes may be associated in a multienzyme complex for some steps of a metabolic pathway.

High concentrations of the enzymes and substrates needed for a particular pathway may be contained in specialized cellular compartments.

Specific Localization of Enzymes within the Cell (cont.)

Enzymes often are incorporated into the membranes of cellular components because the cell’s complex internal structures promote metabolic order.