Control of Metabolic Pathways (2) Higher Human Biology Unit 1 - Human Cells

Control by regulation of enzyme action Some metabolic pathways are required to operate continuously Their genes that code for their enzymes need to be switched on continuously These metabolic pathways are controlled by regulating the action of their enzymes

Signal molecules Some enzyme activity is controlled by signal molecules E.g. Epinephrine (Adrenaline) Triggers liver cells to activate an enzyme that converts glycogen to glucose. Intracellular - molecules that originate in the cell itself Extracellular - molecules that originate from the cells environment

Effect of inhibitors Inhibitors decrease the rate of an enzyme-controlled reaction There are 2 types Competitive inhibitors Non-competitive inhibitors

Competitive Inhibitor These compete with molecules of substrate for the active sites on an enzyme Molecular structure similar to the substrate Rate of reaction is reduced

Competitive Inhibition

Effect of increasing substrate concentration

Non-competitive inhibitors Does not combine directly with an enzyme’s active site Attaches to a non active (allosteric) site Changes the shape of the enzyme Active site becomes altered indirectly Prevents it combining with the substrate

Activators and Inhibitors Some enzyme molecules are composed of several subunits. Several active sites Several allosteric sites These may be active or inactive

Feedback inhibition by end product As the concentration of an end product builds up, some of it binds to an enzyme in the pathway. This slows down the conversion of intermediate metabolites Which in turn lowers the concentration of the product. When the concentration of the product drops, fewer molecules of the enzyme are affected More end product is produced and so on…….. Negative feedback control Prevents wasteful conversion and accumulation of intermediates and final products

Metabolite W Enzyme 1 Metabolite X Some of end product (Z) binds to enzyme 1 and prevents conversion of W to X Enzyme 2 Metabolite Y Enzyme 3 Metabolite Z