Enzymes Regulatory 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 Enzymes In 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 specificity Endoenzymes (intracellular)/exoenzymes (extracellular)

Energy Requirements of a Chemical Reaction Figure 5.2

Enzyme Components Biological catalysts Apoenzyme: Protein Specific for a chemical reaction; not used up in that reaction Apoenzyme: Protein Cofactor: Nonprotein component Coenzyme: Organic cofactor Holoenzyme: Apoenzyme plus cofactor

The Parts of an Enzyme

Properties of Coenzymes and Cofactors Many 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

Components of a Holoenzyme Figure 5.3

Important Coenzymes NAD+ NADP+ FAD Coenzyme A

mechanism Substrate binding Formation enzyme substrate complex Production formation and dissociation Enzyme recovery

The Mechanism of Enzymatic Action Figure 5.4a

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 Classification Oxidoreductase: Oxidation-reduction reactions Transferase: Transfer functional groups Hydrolase: Hydrolysis Lyase: Removal of atoms without hydrolysis Isomerase: Rearrangement of atoms Ligase: Joining of molecules, uses ATP

Factors Influencing Enzyme Activity Temperature pH Substrate concentration Inhibitors

Temperature and pH Enzymes 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

Effect of Temperature on Enzyme Activity Figure 5.5a

Effect of pH on Enzyme Activity Figure 5.5b

Effect of Substrate Concentration on Enzyme Activity Figure 5.5c

Enzyme Inhibition Competitive 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

Enzyme Inhibitors: Competitive Inhibition Figure 5.7a–b

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

Enzyme Inhibitors: Noncompetitive Inhibition Figure 5.7a, c

Noncompetitive (allosteric) inhibition of enzymes

modification enzymes 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

Enzyme Inhibitors: Feedback Inhibition Figure 5.8