Enzymes: Organic Catalysts IB Biology HL E. McIntyre

What are Enzymes? Enzymes are proteins. Enzymes allow many chemical reactions to occur within the homeostasis constraints of a living system. Enzymes function as organic catalysts. A catalyst is a chemical involved in, but not changed by, a chemical reaction. Many enzymes function by lowering the activation energy of reactions. By bringing the reactants closer together, chemical bonds may be weakened and reactions will proceed faster than without the catalyst. The use of enzymes can lower the activation energy of a reaction (Ea).

Name of Enzymes End in –ase Identifies a reacting substance sucrase – reacts with sucrose lipase - reacts with lipid Describes function of enzyme oxidase – catalyzes oxidation hydrolase – catalyzes hydrolysis Common names of digestion enzymes still use –in pepsin, trypsin

Enzyme Action: Lock and Key Model An enzyme binds a substrate in a region called the active site Only certain substrates can fit the active site Amino acid R groups in the active site help the substrate to bind Enzyme-substrate complex forms Substrate reacts to form product Product is released (OR an enzyme may be used to hydrolyse -split- a substrate molecule)

Enzyme Action: Induced Fit Model Enzyme structure flexible, not rigid Enzyme and active site adjust shape to bind substrate Increases range of substrate specificity Shape changes also improve catalysis during reaction

Lock and Key vs Induced Fit Model

Learning Check E2 The active site is (1) the enzyme (2) a section of the enzyme (3) the substrate B. In the induced fit model, the shape of the enzyme when substrate binds (1) Stays the same (2) adapts to the shape of the substrate

Solution E2 The active site is (2) a section of the enzyme B. In the induced fit model, the shape of the enzyme when substrate binds (2) adapts to the shape of the substrate

Enzyme Inhibition Inhibitors cause a loss of catalytic activity Change the protein structure of an enzyme May be competitive or noncompetitive Some effects are irreversible

Competitive Inhibition A competitive inhibitor Has a structure similar to substrate Occupies active site Competes with substrate for active site Has effect reversed by increasing substrate concentration

Noncompetitive Inhibition A noncompetitive inhibitor Does not have a structure like substrate Binds to the enzyme but not active site Changes the shape of enzyme and active site Substrate cannot fit altered active site No reaction occurs Effect is not reversed by adding substrate

Learning Check E4 Identify each statement as describing an inhibitor that is (1) Competitive (2) Noncompetitive A. Increasing substrate reverses inhibition B. Binds to enzyme, not active site C. Structure is similar to substrate D. Inhibition is not reversed with substrate

Solution E4 Identify each statement as describing an inhibitor that is (1) Competitive (2) Noncompetitive A. 1 Increasing substrate reverses inhibition B. 2 Binds to enzyme, not active site C. 1 Structure is similar to substrate D. 2 Inhibition is not reversed with substrate

Factors that Affect Enzyme Efficacy Temperature Increases in temperature  increase in molecular movement  will speed up the rate of nonenzyme mediated reactions, and so temperature increase speeds up enzyme mediated reactions, but only to a point. When heated too much, enzymes (since they are proteins dependent on their shape) become denatured. When the temperature drops, the enzyme regains its shape. Thermolabile enzymes, such as those responsible for the color distribution in Siamese cats and color camouflage of the Arctic fox. Siamese cats: coat colours on the tips of the nose, ears and paws are darker colours at the tips of their bodies, where the temperature is slightly cooler, and the enzymes will thus only be active in these regions. Arctic Fox: in summer, the enzymes for white coat colour are denatured, then as the weather gets colder, the enzymes begin working and produce a white coat.

…Factors that Affect Enzyme Efficacy Concentration of substrate and product also control the rate of reaction, providing a biofeedback mechanism.

…Factors that Affect Enzyme Efficacy Changes in pH will also denature the enzyme by changing the shape of the enzyme. Remember: the shape of a protein (in this case, an enzyme determines its function). Enzymes are also adapted to operate at a specific pH or pH range.

Enzyme Inhibition There are a number of substances that can inhibit enzyme activity. Competitive Inhibitors are so similar to the enzyme’s substrate that they are able to enter the enzyme’s active site and block the normal substrate from binding. A drug, sulfanilamide resembles PABA, a chemical essential to a bacteria that infects animals. sulfanilamide competes with PABA, shutting down an essential bacterial (but not animal) pathways. enzymes