Enzymes

Biochemical Pathways

are turned on/turned off by a feedback inhibition - a common control mechanism (also referred to as a negative feedback)

Factors Affecting Enzyme Activity III Inhibitor - Feedback -

substance that binds to an enzyme and decreases its activity •Competitive inhibitors - bind to the active site of an enzyme and compete with the substrate for the same active site •Noncompetitive inhibitors - bind to the enzyme in a location other than the active site and changing the function •Allosteric sites- specific binding sites acting as on/off switches End product of metabolic pathway can act as pathway inhibitor

Factors Affecting Enzyme Activity II Activators -

substances that bind to and keep the enzymes in their active configurations - increases enzyme activity Cofactors: - chemical components that facilitate enzyme activity - nonprotein (inorganic) •Metal ions such as Cu, Mg, Mn, Fe, Zn... •Alter the shape of the enzyme's active site Coenzymes: - organic molecules that function as cofactors - organic cofactors •Most vitamins are coenzymes, (vit. C & B, folic acid, biotin) •Assist with moving atoms during a reaction

The Catalytic Cycle Of An Enzyme

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 EA and speed up a reaction by • acting as a template for substrate orientation • stressing the substrate bonds 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 molecules.

Factors Affecting Enzyme Activity I

Temperature-Each enzyme has an optimal temperature in which it can function pH (and ionic interactions)- Each enzyme has an optimal pH in which it can function (and ionic interactions also hold enzymes together)

Substrate Specificity of Enzymes

• Almost all enzymes are globular proteins with one or more active sites on their surface. • The substrate is the reactant an enzyme acts on • Reactants bind to the active site to form an enzyme-substrate complex. • The 3-D shape of the active site and the substrates must match, like a lock and key • Binding of the substrates causes the enzyme to adjust its shape slightly, leading to a better induced fit. • Induced fit of a substrate brings chemical groups of the active site into positions that enhance their ability to catalyze the chemical reaction • When this happens, the substrates are brought close together and existing bonds are stressed. This reduces the amount of energy needed to reach the transition state.

Cooperatively

• Is a form of allosteric regulation, the inactive form oscillates back and forth with the active form when the active form is not stabilized by substrate.

Two ways to increase reaction rates

•Add Energy (Heat) - molecules move faster so they collide more frequently and with greater force. •Add a catalyst - a catalyst (Proteins that function as catalysts are called enzymes) reduces the energy needed to reach the transition state, without being changed itself. •An enzyme catalyzes reactions by lowering the activation energy needed to start the reaction (see diagram)

Activation Energy

•All reactions, both endergonic and exergonic, require an input of energy to get started. The EA... • Is the initial amount of energy needed to start a chemical reaction • Activation energy is needed to bring the reactants close together • weaken existing bonds to initiate a chemical reaction. • Can be in the form of heat (higher temperature).

Regulation Of Enzyme Activity Helps Control Metabolism

•Enzymes change shape when regulatory molecules bind to specific sites, affecting function •Allosteric regulation is the term used to describe when a protein's function at one site is affected by binding of a regulatory molecule at another site

Allosteric Activation and Inhibition

•Enzymes have active and inactive forms •The binding of an activator stabilizes the active form of the enzyme •The binding of an inhibitor stabilizes the inactive form of the enzyme • Allosteric regulation may either inhibit or stimulate an enzyme's activity

Enzymes

•are biological catalysts that increase reaction rates •by lowering activation energy threshold to initiate a reaction •are not consumed by the rx •are proteins •have primary, secondary, tertiary and quaternary structures •contain an active site where a substrate binds and is converted to products •show specificity

4 important points to remember about enzymes-catalyst

•enzymes are proteins •enzymes are highly specific (each enzyme typically speeds up only one or a few chemical reactions). •an enzyme's unique shape holds substrates in position to react (lowers EA). •enzymes are not consumed in the reactions they catalyze

Reaction Rates

•molecules do not have enough kinetic energy to reach the transition state when they collide. •most collisions are non-productive, and the reaction proceeds very slowly