7.6 Enzymes (AHL) Pp 210 - 213 Pp 69 -71

Metabolic pathways Metabolic reactions often occurs in small step reactions with a number of intermediate products in-between Metabolic reactions includes: Catabolic reactions such as respiration & Anabolic reactions such as photosynthesis Each step in a metabolic pathway each is unique & requires its own enzyme to allow finer control of metabolic pathway

A Biochemical Pathway

Induced-fit model lock & Key hypothesis does not explain the broad specificity of some enzymes the molecular shape of active sites is not always complementary to that of the substrate induced-fit model attempts to over come these difficulties when substrate binds to the enzyme, the shape of active site changes to accommodate the substrate bonds in substrate weaken & activation energy decreases the structure of the enzyme allows for a certain amount of adaptation to the substrate, this explains broad specificity of some enzymes such as proteases

Effect of enzymes on activation energy Enzymes lower the activation energy of the chemical reaction that they catalyse - Ea In the activated complex or transition state energy is put into the substrate to weaken the structure. This allow the reaction to occur with a minimal amount of additional energy required. Normal activation energy would denature the proteins of the cell. Thus reduced activation energy make these reactions possible in a cell. After the product is formed energy is released – ΔG Exergonic reactions release more energy than the activation energy

Competitive and non-competitive inhibition Inhibitors are substances that reduce or completely stop the action of an enzyme Inhibitor can attach on the active site (competitive inhibitor) or on another region of the enzyme molecule other than the active site(non-competitive inhibitor)

Comparing competitive and non-competitive inhibition of enzymes both types of inhibitor reduce enzyme activity both types of inhibitor bind to the enzyme both types of inhibitor prevent the substrate from binding to the active site

Difference between competitive and non-competitive inhibition example: succinate dehydrogenase is inhibited by malonate substrate and inhibitor are chemically similar & have same shape inhibitor binds to active site inhibitor does not change the shape of the active site increases in substrate concentration reduce the inhibition example: pyruvate kinase is inhibited by alanine substrate and inhibitor are chemically not similar & have different shape inhibitor binds away from the active site i.e. at allosteric site inhibitor changes the shape of the active site increases in substrate concentration do not affect the inhibition

Control of metabolic pathways by end-product inhibition end-product inhibition is an example of negative feedback it controls rate of product synthesis i.e. amount of product produced when there is too much of end-products, the product inhibits the enzyme that catalyses the 1st reaction (allosteric enzyme) the product binds to the enzyme at an allosteric site i.e. site other than the active site enzyme changes shape & the substrate cannot bind to the active site binding of inhibitor to an allosteric site is reversible i.e. when the inhibitor detaches, the active site returns to the original shape & substrates can bind again end-product inhibition controls the metabolic pathway by switching on & off the initial stage of the metabolic pathway based on the concentration of end-product e.g. ATP inhibition of phosphofructokinase, in glycolysis

Feedback Inhibition of Biochemical Pathways

Revision Questions Describe the induced-fit model of enzymes. Outline how enzymes lower the activation energy of the chemical reactions that they catalyse. With reference to one example of each, distinguish between competitive and non-competitive inhibition. Explain the control of metabolic pathways by end-product inhibition.