Enzymes PDF

UNIT: METABOLISM LEVEL: STANDARD ENZYMES Content Statements: C1.1.1 Enzymes as catalysts C1.1.2 Role of enzymes in metabolism C1.1.3 Anabolic and catabolic reac:ons C1....

UNIT: METABOLISM LEVEL: STANDARD ENZYMES Content Statements: C1.1.1 Enzymes as catalysts C1.1.2 Role of enzymes in metabolism C1.1.3 Anabolic and catabolic reac:ons C1.1.4 Enzymes as globular proteins with an ac:ve site for catalysis C1.1.5 Interac:ons between substrate and ac:ve site to allow induced-fit binding C1.1.6 Role of molecular mo:on and substrate-ac:ve site collisions in enzyme catalysis C1.1.7 Rela:onships between ac:ve site structure, enzyme-substrate specificity and denatura:on C1.1.8 Effects of temperature, pH and substrate concentra:on on the rate of enzyme ac:vity C1.1.9 Measurements in enzyme-catalysed reac:ons C1.1.10 Effect of enzymes on ac:va:on energy METABOLISM Metabolism describes the totality of chemical processes that occur within a cell in order to maintain life. These reactions provide a source of energy and enable the synthesis and assimilation of cellular materials. Metabolic reactions are catalysed by enzymes and can be described as being either anabolic or catabolic: Anabolism: H2O Smaller compounds are combined to form larger compounds In the case of organic compounds, this involves condensation Water is released as a by-product of condensation reactions Monomers Polymer Catabolism: H2O Large compounds are broken down into smaller compounds In the case of organic compounds, this involves hydrolysis Water is required as an input for hydrolysis reactions Polymer Monomers METABOLIC REACTIONS Photosynthesis is an example of an anabolic reaction. It is an endergonic process that uses light energy to synthesise organic compounds from inorganic sources. Conversely, cell respiration is a catabolic reaction. It is an exergonic process that releases chemical energy (ATP) from the breakdown of organic compounds. LIGHT CO2 CO2 CHEMICAL ENERGY ENERGY H2O PHOTO- CARBOHYDRATES AEROBIC H2O SYNTHESIS (E.G. GLUCOSE) RESPIRATION O2 O2 ENZYMES Enzymes control the metabolism of a cell. An enzyme is a globular protein that acts as a biological catalyst. It speeds up the rate of a chemical reac:on by lowering the acRvaRon energy threshold required for the reac:on to proceed. Enzymes are not changed or consumed by the reac:ons they catalyse and thus can be reused. Enzymes are commonly named a^er the molecules they react with (called the substrate) and end with the suffix ‘-ase’ (e.g. lipase is an enzyme that breaks down lipids, whereas proteases digest proteins). products enzyme substrate enzyme–substrate enzyme–product enzyme complex complex SPECIFICITY All enzymes possess an indenta:on or cavity to which a substrate can bind – this is called the acRve site. The shape and chemical proper:es of the ac:ve site are complementary to a par:cular substrate. Thus, an enzyme demonstrates specificity for a given substrate. Some enzymes are highly specific and have an ac:ve site that precisely fits one dis:nct substrate (the ‘lock and key’ model), while other enzymes may be broadly specific and recognise a class of related molecules (e.g. proteases can digest a variety of proteins). In these instances, the ac:ve site undergoes a conforma:onal change to improve bonding (the ‘induce fit’ model). This stresses the bonds in the substrate and increases reac:vity (lowers ac:va:on energy hurdle). MOLECULAR MOTION Enzyme reac:ons occur in aqueous solu:ons (such as the cytoplasm or inters::al fluid), with the substrate and enzyme moving randomly in Brownian mo:on. For enzyme reac:ons to occur, a substrate and enzyme must physically collide in the correct orienta:on to facilitate binding to the ac:ve site. The frequency of the successful collisions can be improved by increasing the molecular moRon of the par:cles or increasing the concentraRon of parRcles (either substrate or enzyme). Thermal energy can be introduced to increase the kine:c energy of the par:cles, while the enzyme or substrate may occasionally be fixed in a sta:c posi:on (e.g. membrane-bound) to localise reac:ons to par:cular sites and increase the likelihood of catalysis. DENATURATION The shape and chemical proper:es of the ac:ve site are highly dependent on the terRary structure of the enzyme. This structure can be modified by external factors such as temperature and pH. These factors may disrupt the chemical bonds needed to maintain the ter:ary structure, poten:ally leading to a change in the shape of the ac:ve site. This will result in denaturaRon (loss of biological Denature ac:vity) as the enzyme will no longer be able to interact with the substrate. In most cases, denatura:on results in an irreversible loss of biological ac:vity. However, some enzymes may be able to return to a func:onal state if restored to their na:ve condi:ons. In these instances, denatura:on is considered to be reversible. FACTORS AFFECTING ENZYME ACTIVITY The efficiency of an enzyme-catalysed reac:on will be influenced by two key factors: The frequency of successful enzyme-substrate collisions (due to more particles or more kinetic motion) The capacity for the enzyme and substrate to interact upon collision (will be impacted by denaturation) Factors that affect enzyme ac:vity include: temperature, pH, substrate concentra:on or enzyme levels TEMPERATURE Low temperatures result in insufficient thermal energy for the ac:va:on energy threshold to be reached. As Rate of Reac2on temperature increases, par:cles gain kineRc energy, resul:ng in more frequent enzyme-substrate collisions. At an op:mal temperature, enzyme ac:vity will peak, because higher temperatures will disrupt the bonding within the enzyme, causing a loss of ter:ary structure and a resul:ng loss of biological ac:vity (denatura:on). Temperature (°C) pH All enzymes have an op:mal pH, at which the ac:vity of the enzyme is at its highest. Outside of this op:mal Rate of Reac2on range, enzyme ac:vity will diminish. Amino acids are zwiUerions (have both a posi:ve and nega:ve charge), and changing the pH alters the charge of the enzyme (which in turn alters both solubility and overall shape). Enzymes will denature outside of an op:mal pH range, leading to a characteris:c bell-shaped ac:vity curve. pH SUBSTRATE CONCENTRATION Increasing substrate concentra:on will increase the ac:vity of a corresponding enzyme. Higher substrate Rate of Reac2on levels will result in increased frequency of collisions with the enzyme in a given period of :me. Above a certain substrate concentra:on, the enzyme ac:vity will plateau. This is because the environment is now saturated with substrate and all enzyme ac:ve sites are occupied (reac:on is now at maximum catalysis). Substrate Concentra2on ENZYME CONCENTRATION Increasing enzyme concentra:on will result in a linear increase in ac:vity. This is because the rate of reac:on will be propor:onal to the amount of enzyme available for reac:on (more enzymes will result in more frequent enzyme-substrate collisions, leading to a higher rate of ac:vity). Enzymes will typically exist in low concentra:ons within living organisms. This is because enzymes are not consumed by the reac:ons that they catalyse and can con:nually be reused. Enzymes are generally only used in higher amounts in industrialised sefngs (e.g. in certain household products or in the genera:on of chemicals or biofuels).