EnzymesRegulatory enzymes are usually the enzymes

that are the rate-limiting, or committed step, in a pathway, meaning that after this step a particular reaction pathway will go to completion

There are five primary forms of enzyme regulation: substrate availability, allosteric, post-translational modification, interaction with control proteins

Properties of EnzymesIn general, chemical reactions that release

energy can occur without input of energy

The oxidation of glucose releases energy, but the reaction does not occur without an input of energy

Activation energy: the energy required to start such a reaction

Enzymes lower the activation energy so reactions can occur at mild temperatures in living cells

Enzymes

Provide a surface on which reactions take place

Active site: the area on the enzyme surface where the enzyme forms a loose association with the substrate

Substrate: the substance on which the enzyme acts

Enzyme-substrate complex: formed when the substrate molecule collides with the active site of its enzyme

Enzymes generally have a high degree of specificityEndoenzymes (intracellular)/exoenzymes

(extracellular)

Figure 5.2

Energy Requirements of a Chemical Reaction

Enzyme ComponentsBiological catalysts

Specific for a chemical reaction; not used up in that reaction

Apoenzyme: ProteinCofactor: Nonprotein component

Coenzyme: Organic cofactorHoloenzyme: Apoenzyme plus cofactor

The Parts of an Enzyme

Properties of Coenzymes and CofactorsMany enzymes can catalyze a reaction only if

substances called coenzymes, or cofactors are present

Apoenzyme: protein portion of such enzymes

Holoenzyme: nonprotein coenzyme or cofactor that is active when combined with apoenzyme

Coenzyme: nonprotein organic molecule bound to or loosely associated with an enzyme

Cofactor: an inorganic ion (e.g. magnesium, zinc) that often improve the fit of an enzyme with its substrate

Figure 5.3

Components of a Holoenzyme

Important CoenzymesNAD+

NADP+

FADCoenzyme A

mechanism1. Substrate binding2. Formation enzyme substrate complex3. Production formation and dissociation 4. Enzyme recovery

Figure 5.4a

The Mechanism of Enzymatic Action

Each substrate binds to an active site, producing an enzyme-substrate complex. The enzyme helps a chemical reaction occur, and one or more products are formed

Enzyme ClassificationOxidoreductase: Oxidation-reduction

reactionsTransferase: Transfer functional groupsHydrolase: HydrolysisLyase: Removal of atoms without hydrolysisIsomerase: Rearrangement of atomsLigase: Joining of molecules, uses ATP

Factors Influencing Enzyme ActivityTemperaturepHSubstrate concentrationInhibitors

Temperature and pHEnzymes are affected by heat and extremes

of pH

Even small pH changes can alter the electrical charges on various chemical groups in enzyme molecules, thereby altering the enzyme’s ability to bind its substrate and catalyze a reaction

Most enzymes have an optimum temperature, near normal body temperature, and an optimum pH, near neutral, at which they catalyze a reaction most rapidly

The rate at which an enzyme catalyzes a reaction increases with temperature up to the optimum T

Figure 5.5a

Effect of Temperature on Enzyme Activity

Figure 5.5b

Effect of pH on Enzyme Activity

Figure 5.5c

Effect of Substrate Concentration on Enzyme Activity

Enzyme InhibitionCompetitive inhibitor: A molecule similar in

structure to a substrate can bind to an enzyme’s active site and compete with substrate

Noncompetitive inhibitors: attach to the enzyme at an allosteric site, which is a site other than the active site

noncompetitive inhibitors: distort the tertiary protein structure and alter the shape of the active site

Feedback inhibition: regulates the rate of many metabolic pathways when an end product of a pathway accumulates and binds to and inactivates the first enzyme in the metabolic pathway

Figure 5.7a–b

Enzyme Inhibitors: Competitive Inhibition

Competitive inhibition of enzymes

Allosteric regulation of enzyme activity Allosteric regulation = the activation or

inhibition of an enzyme’s activity due to binding of an effectors molecule at a regulatory site that is distinct from the active site of the enzyme

Allosteric regulators generally act by increasing or decreasing the enzyme’s affinity for the substrate

Figure 5.7a, c

Enzyme Inhibitors: Noncompetitive Inhibition

Noncompetitive (allosteric) inhibition of enzymes

modification enzymesEnzyme

Modifyinggroup

EnzymeModifyinggroup

Inactive Enzyme Active Enzyme

Can either activate it or inhibit it by altering the conformation of the enzyme or by serving as a functional group in the active site

denaturation

denaturation

Figure 5.8

Enzyme Inhibitors: Feedback Inhibition