Enzyme
-A biological catalyst that speeds up metabolic reactions without being changed, and with no unwanted by-products.
-It does this by lowering the activation energy required for two substrate molecules to join by drawing them close in the active site.
-Eg in the formation of collagen, cartilage, blood vessel walls and connective tissue.
-Only catalyse specific reactions.
Turnover
-Number of reactions an enzyme can catalyse per second.
Active site
-Where substrate molecules bind to the enzyme.
-Shape is complementary to that of the substrate.
-Each substrate only has one active site.
-Defined by the tertiary structure, only 6-10 amino acids.
-Some enzymes have more than one active site.
-Can be altered by changes in temperature and pH that affect bonds that hold the proteins in their tertiary structure.
Catabolic pathways
-Metabolites break down into smaller molecules and release energy.
Anabolic pathways
-Synthesise large molecules from smaller ones using energy.
Intracellular enzymes
-Made and kept inside the cell.
-Over 1000 different reactions, involved in metabolic pathways made up of several reaction.
-Reactants and products in these reaction are known as metabolites.
Catalase
-Four polypeptide chains, and contains an Fe haem group.
-Found in all organisms exposed to oxygen.
-Breakdown of hydrogen peroxide (product of many metabolic reactions) into water and oxygen, protecting cells from damage.
-Turnover of 6 million per second.
-2H2O2 -> O2 + 2H2O.
-Found in vesicles called peroxisomes.
-Optimum pH is 7 in humans.
-Optimum temperature is 45C.
Extracellular reactions
-Made in the cell but work outside of them.
-During digestion enzymes are secreted from the alimentary canal into the gut lumen to digest molecules in food..
-Amylase is produced in the salivary gland and breaks down polysaccharide starch into maltose. Same reaction occurs from pancreas and acts in the small intestine.
-Trypsin is produced in the pancreas and acts in the small intestine to break down proteins into smaller peptides by hydrolysing peptide bonds.
Lock and key model
-Old model for enzyme bonding.
-Shows a specific shaped enzyme on a complementary active site.
-Form an enzyme product complex while still in the active site.
Induced fit model
-Newer model.
-Active site changes shape slightly to fit the substrate.
-Still complementary, but chains of amino acids change structure slightly to fit it exactly.
-Non-covalent forces (ionic bonds, hydrogen bonds…) bind substrate to active site.
-Product molecules detached due to slightly different shape.
-Moves back after reaction.
Effect of temperature on enzyme activity
-Higher kinetic energy, so more successful collisions between active site and substrates and quicker rate of formation of enzyme-substrate complexes.
-Highest at the optimum temperature.
-Afterwards the energy of temperature causing vibration, which breaks weak bonds (tertiary) in enzymes, causing them to break and the enzyme to denature irreversibly.
-Active site can no longer bind with substrate as it has changed shape.
Temperature coefficient
-Q10 = rate at 10C higher temp/rate at a current temperature.
-Shows how much rate of reaction changes every 10C raised of temperature.
-Value of 2 means in doubles, while of 3 means it triples.
-Value drops at temps above optimum.
Effect of pH on enzyme activity
-Enzymes have optimum pH’s at which they work best.
-Too high or low a pH will lead to H+ ions binding to the polar bonds (hydrogen and ionic) in the tertiary structure and active site, upsetting it by altering structure and the charge.
-Therefore this interferes with the substrate binding to the active site as they are no longer complementary.
-Small changes slow RoR but hydrogen bonds can re-form and restore the shape of the active site.
-After a certain point the enzyme with denature.
pH buffer
-Something that resists a change in pH to keep it constant.
-Can be proteins such as haemoglobins can accept H+ ions from acids and so act as buffers.
Effect of substrate concentration on enzyme activity
-Few substrates mean many active sites are free and there is a low rate of reaction.
-An increase meaning more active sites engage and enzyme-substrate complexes are formed.
-Once all active sites are in use no further substrates can bind. The enzyme concentration is now the limiting factor.
Competitive inhibitors
-Have a similar shape to the substrates, bind to active site and block it. They are not changed by the enzyme.
-Means the substrate molecule cannot enter and no reaction is catalysed and no product molecule is formed.
-Affected by concentrations of inhibitors compared to substrates.
Inactivators
-A competitive inhibitor that is non-reversible.
-Most competitive inhibitors are not also inactivators.
Non-competitive inhibitors
-Bind to a site other than the active site (allosteric).
-Disrupt tertiary structure, means active site changes shape.
-Therefore substrates can no longer bind as they are no longer complementary.
-Most of the reactions are irreversible as the shape of the enzyme is altered.
-Maximum rate of reaction is decreased, increasing conc of substrates with not have an impact.
-Can occur in metabolic series, in which excess end product binds to the enzyme in the first reaction of the pathway, altering the shape of the active site. This is reversible when conc of end product decreases.
Reversible/non reversible bonds
-Reversible: weaker hydrogen or ionic. Inhibitor can be removed.
-Non reversible: strong covalent bonds. Cannot be removed easily.
Effect of enzyme mutations on the body
-Enzymes are made of proteins, and are coded for by genes.
-If a mutation occurs the tertiary structure is altered and it may not be able to bind to its specific substrate.
-If this occurs to an enzyme involved in metabolic reactions a metabolic disorder occurs.
-Similar can happen with enzymes involved in the formation of collagen in bone, blood vessels, and connective tissue.
Prosthetic group
-A cofactor (non-protein molecule) that is permanently attached by covalent bonds to an enzyme molecule.
-Eg zinc in carbonic anhydrase in erythrocytes, that catalyses conversion of H2O and CO2 into carbonic acid.
Non-permanent cofactors
-Can be present during formation of enzyme-substrate complex, as they temporarily bind to molecules and ease the formation of the complex.
-May act as a co-substrate, acting with the substrate to form the correct shape to bind to the active site.
-Can change distribution of charge on surfaces, and make temporary bonds easier to form.
-Eg chloride ions in amylase.
Coenzymes
-Non-protein molecules that binds temporarily to the active site of enzyme molecules at the time of the substrate binding.
-Chemically changed during the reaction.
-Most derived from vitamins such as B12, Folic acid, nicotinamide or thiamine.
-A deficiency can lead to diseases.
Enzyme degradation
-Cells are continuously degrading old enzyme molecules to their amino acids and synthesising new enzyme molecules from amino acids.
-This means abnormally shaped proteins, or any unneeded enzymes, are removed.
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Cell Structure55
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