1.4 Enzymes

What is an Enzyme? Enzymes are specialized proteins that act as catalysts; they speed up chemical reactions by lowering the activation energy required. Enzymes are NOT used up during a reaction. Enzymes are specific to a particular substrate (reactant). Although temperature increases the rate of most reactions, proteins are denatured at high temperatures and lose their function…this is problematic for living cells…therefore cells cannot rely on high temperatures as a source of activation energy…instead, catalysts allow reactions to proceed at suitable rates at moderate temperatures by reducing the activation energy required. Hydrolysis is the addition of water in a reaction to split molecules into simpler forms.

What is an Enzyme? Exothermic reaction (net energy output)

All reactions require an Activation Energy (EA) that must be overcome in order for the rxn to occur. Catalysts reduce the EA, but does not affect the free energy change ΔG. Catalysts can only speed up a reaction that would occur normally.

Substrate: the reactant that an enzyme works on Substrate: the reactant that an enzyme works on. It binds to a particular site (active site) on the enzyme. Enzyme name usually ends in – ase. Ex. amylase Amylose + H2O maltose maltase Maltose + H2O α-glucose · The catalyst cannot change an endergonic reaction into an exergonic reaction, it can only decrease the potential energy level of the transition state.

Enzyme-Substrate Complex Animation: Enzyme Action and the Hydrolysis of Sucrose Substrates bind to a depression on the surface of enzymes known as the active site, to form an enzyme-substrate complex. The active site undergoes a slight conformation change to better accommodate the substrate (induced fit). Paperclip Demo…page 71

Enzyme Activity Active Site: Where the substrate binds to the enzyme. (Lock and Key model) Induced Fit: Protein changes shape to accommodate substrate. Caused by interaction of functional groups.  Enzyme-Substrate Complex: Substrate attached to enzyme. Factors that can affect the rate of enzyme activity include: ·       Temperature ·       pH ·       Concentration Enzyme activity

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Factors that Affect Enzyme Activity There are only a limited number of specific enzyme molecules in a cell…it takes time for an enzyme to catalyze a reaction.

Factors that Affect Enzyme Activity Every enzyme has an optimal temperature and pH at which it works best. Most human enzymes work best around 37ºC.

Cofactors and Coenzymes (required by some enzymes to function) Cofactors: inorganic substances that aid in enzyme activity. (Ex. Zn2+, Mn2+)   Coenzymes: organic substances that aid in enzyme activity. (Ex. NAD+) Both bind to the active site of enzymes. NAD+ is a derivative of vitamin B3…this acts as an electron carrier in cellular respiration. Deficiencies in some vitamins may limit enzyme function. Animations

Enzyme Inhibition Competitive Inhibitors Compete with substrate for enzymes active site. Block active site Reversible, overcome by increasing substrate concentration E.g. Drugs: CO, Cyanide Non-competitive Inhibitors bind to the enzyme at an allosteric site (not the active site) and cause a conformation change in the enzyme, preventing the normal substrate from binding. loss of enzyme activity (e.g. DDT) With competitive inhibitors, if you raise the concentration of the substrate, then there is a greater chance that the proper substrate will bind, rather than the inhibitor. DDT acts as a competitive inhibitor that results in a change in the enzymes active site…the enzymes of the nervous system… Not all enzyme inhibitors are poisons…some are used by the cell to control enzyme activity.

Mouse Party

Drug Design Acetylcholine is the neurotransmitter produced by neurons referred to as cholinergic neurons. In the peripheral nervous system acetylcholine plays a role in skeletal muscle movement, as well as in the regulation of smooth muscle and cardiac muscle. In the central nervous system acetylcholine is believed to be involved in learning, memory, and mood. Acetylcholinesterase: This enzyme hydrolyzes acetylcholine into acetic acid and choline. If acetylcholinesterase activity is inhibited, the synaptic concentration of acetylcholine will remain higher than normal. If this inhibition is irreversible, as in the case of exposure to many nerve gases and some pesticides, sweating, bronchial constriction, convulsions, paralysis, and possibly death can occur. Although irreversible inhibition is dangerous, beneficial effects may be derived from transient (reversible) inhibition. Drugs that inhibit acetylcholinesterase in a reversible manner have been shown to improve memory in some people with Alzheimer's disease.

Allosteric Regulation Cells control enzyme activity to coordinate cellular activities. Allosteric Sites: Receptor sites that bind substances that inhibit or stimulate enzyme activity. Activators may bind to allosterically controlled enzymes to stabilize its shape and keep all active sites available. Allosteric inhibitors: Binds to allosteric site and stabilizes inactive form of the enzyme. Allosteric Regulation To reduce enzyme function, cells can restrict the production of a particular enzyme or inhibit the action of an enzyme that is already produced. Allosteric enzymes are typically composed of several subunits, each with its own active site

Tutorial 6.1 Enzyme Catalysis Tutorial 6.2 Allosteric Regulation of Enzymesl Essential Biochemistry - Enzyme Inhibition

Feedback Inhibition A method used by cells to control metabolic pathways involving a series of reactions. A product formed later in a sequence of reactions allosterically inhibits an enzyme that catalyzes the reaction earlier on.

Feedback Inhibition of Biochemical Pathways