CELLS @ WORK Cell Metabolism and ATP • ATP is the energy molecule of the cell. • Billions are used and reassembled every second!! • Endergonic Rxn – Requires energy (ie. Photosynthesis) • Exgergonic Rxn – releases energy (ie.Cellular Respiration) • Glucose = 1 dollar • ATP = 1 penny Enzymes Our “Key” to Biochemical Reactions ENZYMES • Proteins that accelerate chemical reactions • Almost all processes in the cell need enzymes in order to occur – Cellular Respiration, Photosynthesis, food digestion etc. • Are extremely selective – very specific to certain reactions ENZYMES • For Example: Lysozyme digests bacterial cell walls, and is found in human tears, egg-white, etc ENZYMES • Enzymes are known to catalyze about 4,000 reactions in the human body • Named according to the reaction they catalyze … “ase” is added to the name of the substrate • Ex: Lactase breaks down lactose HOW ENZYMES WORK: • By providing a lower activation energy for a reaction and dramatically accelerating its rate • For example… (Do not copy) – the reaction catalysed by orotidinephosphate decarboxylase will consume half of its substrate in 78 million years if no enzyme is present. However, when the decarboxylase is added, the same process takes just 25 milliseconds HOW ENZYMES WORK: Enzyme SUBSTRATE PRODUCT(S) Eg: Sucrase Sucrose + Water Glucose + Fructose Enzymes help a reaction to occur … without being directly involved!!! Check these out! • http://www.yellowtang.org/animations/ enzymes.swf • http://www.yellowtang.org/animations/ enyme_action_final.swf Energy levels of molecules Enzymes lower the activation energy of a reaction Initial energy state of substrates Activation energy of enzyme catalysed reaction Activation energy of uncatalysed reactions Final energy state of products Progress of reaction (time) Enzymes lower activation energy by forming an enzyme/substrate complex Substrate + Enzyme Enzyme/substrate complex Enzyme/product complex Product + Enzyme In anabolic reactions enzymes bring the substrate molecules together. In catabolic reactions the enzyme active site affects the bonds in substrates so they are easier to break HOW ENZYMES WORK: • “Lock and Key” Model: HOW ENZYMES WORK: Lock-and-key hypothesis assumes the active site of an enzyme is rigid in its shape How ever crystallographic studies indicate proteins are flexible. The Induced-fit hypothesis suggests the active site is flexible and only assumes its catalytic conformation after the substrate molecules bind to the site. When the product leaves the enzyme the active site reverts to its inactive state. Enzyme Reaction Rates Rates of enzymes • Rate of enzyme action is dependent on number of substrate molecules present Rate of Reaction (M) Vmax = maximum rate of reaction Vmax approached as all active sites become filled Some active sites free at lower substrate concentrations Substrate concentration Temperature -Enzymes denature at 60oC Rate of reaction Optimum temperature Enzyme denaturing and losing catalytic abilities Rate doubles every 10oC Temperature Some thermophilic bacteria have enzymes with optimum temperatures of 85oC pH - affects the formation of hydrogen bonds and sulphur bridges in proteins and so affects shape. trypsin cholinesterase Rate of Reaction (M) pepsin 2 4 6 pH 8 10 In Summary... • Work at optimal concentrations, temp. & pH • If it is too hot, an enzyme can become DENATURED… and will no longer function properly Define the following terms: 1. Anabolic reactions: Reactions that build up molecules 2. Catabolic reactions: Reactions that break down molecules 3. Metabolism: Combination of anabolic and catabolic reactions 4. Catalyst: A substance that speeds up reactions without changing the produced substances 5. Metabolic pathway: Sequence of enzyme controlled reactions 6. Specificity: Only able to catalyse specific reactions 7. Substrate: The molecule(s) the enzyme works on 8. Product: Molecule(s) produced by enzymes Competitive Inhibitors • A molecule similar in shape to the substrate bonds with the enzyme’s active site and inhibit its function. • Can be reversible or irreversible • Poisons: cyanide and arsenic bind to key enzymes in this manner…death may result! Non-Competitive Inhibitors • Attach to a binding site on the enzyme other than the active site...causing the shape to change. • The enzyme loses affinity for substrate Allosteric Regulation • Inhibits or stimulates enzyme activity • Enzyme can be turned on or off...therefore cellular reactions are controlled through feedback. The switch: Allosteric inhibition Allosteric means “other site” Active site E Allosteric site Switching off • These enzymes have two receptor sites • One site fits the substrate like other enzymes • The other site fits an inhibitor molecule Substrate cannot fit into the active site Inhibitor molecule Inhibitor fits into allosteric site The allosteric site the enzyme “on-off” switch Active site Substrate fits into the active site E Allosteric site empty The inhibitor molecule is absent Conformational change Substrate cannot fit into the active site E Inhibitor molecule is present Inhibitor fits into allosteric site Negative Feedback Inhibition This example demonstrates how an end product can inhibit the first step in its production. 1. Isoleucine binds to the allosteric site of threonine deaminase and prevents threonine from binding to the active site because the shape of the active site is altered. 2. When the level of isoleucine drops in the cell’s cytoplasm, the isoleucine is removed from the allosteric site on the enzyme, the active site resumes the activated shape and the pathway is “cut back on” and isoleucine begins to be produced. Example! • http://www.yellowtang.org/animations/f eedback_inhibition.swf The End!!!
