6 Enzymes

• Enzymes are made up of long chains of amino acids that fold into complex three-dimensional structures.
• Enzymes are biological catalysts which speed up the rate of reactions.
• Many enzymes are made up of proteins.
• The mode of action of enzymes in terms of their catalytic action via their active sites is examined in this topic.
• Enzyme-substrate complexes are formed through the proposed models of enzyme-substrate interaction.
• The first intermediate in glycolysis, glucose-6-phosphate, inhibits the very enzyme that produces it, hexokinase.
• Various factors such as temperature, pH, enzyme concentration and substrate concentration affect enzyme-catalysed reactions.
• Enzymes can also be influenced by inhibitors and activators which can regulate the rates of enzymatic reactions.
• The mode of action of enzymes in terms of an active site, enzyme-substrate complex, lowering of activation energy and enzyme specificity can be explained using the lock-and-key and induced-fit hypotheses.
• Enzymes lower activation energy barriers.
• The effects of temperature, pH, enzyme concentration and substrate concentration on the rate of an enzyme-catalysed reaction can be investigated by measuring rates of formation of products (e.g measuring gas produced using catalase) or rate of disappearance of substrate (e.g using amylase, starch and iodine).
• The 'induced fit' hypothesis states that when the substrate binds to the enzyme, it induces a change in the conformation of the enzyme and its active site, allowing the active site to be moulded into a more precise fit for the substrate, enabling the enzyme to perform its catalytic function more effectively.
• The 'lock & key' hypothesis explains why enzymes are highly specific by stating that the enzyme's active site has a specific surface conformation and charge produced by the 3-dimensional folding of the polypeptide chain.
• Enzymes lower the activation energy barrier of a chemical reaction, allowing more reactant molecules to reach the transition state at moderate temperatures.
• Enzymes lower activation energy barriers by various mechanisms, including lowering the energy of the transition state, increasing the frequency of collision of reactants, and increasing intramolecular vibrations in molecules, making bonds more likely to break.
• All chemical transformations pass through an unstable structure called the transition state, which is poised between the chemical structures of the substrates and products.
• An enzyme consists of four different categories of amino acid residues: contact residues, catalytic residues, structural residues, and non-essential residues.
• The structure of competitive and non-competitive inhibitors with reference to the binding sites of the inhibitor can be described.
• The end product alters the conformation of the specific enzyme active site, thus substrate cannot bind to active site in the correct orientation, so rate of reaction is decreased.
• In end-product inhibition, a metabolic pathway is inhibited by the binding of the end product of a biochemical pathway to an enzyme that acts early in the pathway.
• Most enzymes are protein in nature.
• When inhibitor saturation is reached, the rate of reaction will be almost zero.
• The rate of reaction will continue to decrease with increasing inhibitor concentration.
• The binding of substrates in allosteric enzymes exhibit cooperativity, where the binding of a substrate to the first subunit changes the conformation of the other subunits, such that it becomes easier to accept subsequent substrates.
• Allosteric enzymes usually consist of two or more subunits where each subunit has its own active site that binds substrates and allosteric site that binds activators or inhibitors.
• Phosphofructokinase which functions in glycolysis is an allosteric enzyme.
• Allosteric enzymes can be regulated by allosteric inhibitors and activators.
• Covalent modification can alter the function of an enzyme.
• In the presence of an allosteric inhibitor, the same V max can be reached at higher substrate concentrations.
• The binding of an allosteric inhibitor on the rate of reaction is opposite to that of increasing substrate concentration.
• The effects of competitive and non-competitive inhibitors (including allosteric inhibitors) on the rate of enzyme activity can be explained.
• An activator molecule can bind to an enzyme and change its overall shape, enabling the enzyme to better bind with its substrate.
• Shape or conformational changes can also act as an on/off switch, for example, when inhibitor molecules bind to a site on an enzyme distinct from the substrate site, they can make the enzyme assume an inactive conformation, thereby preventing it from catalyzing a reaction.
• Enzymes are proteins that can change shape and therefore become active or inactive.
• About 30% of proteins in eukaryotic cells are phosphorylated.
• Conversely, the binding of activator molecules can make an enzyme assume an active conformation, essentially turning it on.
• Many of the molecular transformations that occur within cells require multiple steps to accomplish.
• Another family of enzymes, called protein phosphatases, reverses the effects of protein kinases by catalysing the removal of phosphate group attached to the protein, a process known as dephosphorylation.
• For example, phosphorylation of glycogen phosphorylase increases the enzymatic activity, whereas phosphorylation of glycogen synthase decreases the enzymatic activity.
• Phosphorylation is a post-translational modification that effectively regulates proteins, including enzymes, where a phosphate group is added to an amino acid side chain of a protein.