Enzymes

• Enzymes

  • Enzymes are a globular protein that speed up or metabolism and specific chemical reactions in the body.
  • Can either build up substances into a larger one, or break down large substances into many smaller ones.
  • All living organisms have enzymes, with the body naturally producing them.
  • All used in manufacturing and production of foods.
• Enzymes

  • During enzyme-catalysed reactions, reactants (substrates) are chemically converted to new substances called products.
  • This occurs by breaking chemical bonds in the reactant(s) and forming new bonds in the product(s).
  • Once the product is formed, it is released by the enzyme which can then be used again and again.
• Enzymes

  • Enzymes generally catalyse reactions as they put the substrates into a certain orientation which puts pressure on the chemical bonds, which makes it easier to break the bonds with energy.
• Types of Enzymes

  • Digestive enzymes breaks down molecules
  • Synthesis enzymes builds molecules
  • Enzymes can break apart a substrate, or combine/build two substrates into one product
• Active Site

  • Substrates bind to the active site on the surface of an enzyme.
  • This is referred to as enzyme-substrate binding.
  • Unique active site on enzymes which usually only one substrate can bind to (can have multiple in some cases).
  • Shapes of enzymes and substrates that react, are called complementary.
• Enzyme Specificity

  • Complementary binding of one substrate to the active site of one enzyme, is known as enzyme specificity.
  • This means only one substrate can be complementary (or recognised) by the enzyme.
• Cell Metabolism

  • There are many thousands of biochemical processes that take place in eukaryotic cells.
  • Cell metabolism refers to all the biochemical reactions occurring in an organism.
  • These reactions are critical to the survival of cells, and organisms as a whole.
  • Overall rate of all these reactions is called the metabolic rate.
• Metabolic Pathway

  • Most reactions occur in metabolic pathways, where a substrate is converted to a product by an enzyme. There may be follow up reactions on this product, which turns into a substrate and is then acted upon by other enzymes.
• Induced Fit Model

  • Substrates bind to active sites by weak bonds which often change the shape of both the enzyme and substrate.
  • Original “lock and key” model suggested active site and substrate fit and don’t change shape.
  • Induced-fit model suggests as substrate and enzyme bind, both shapes are modified.
• Activation Energy

  • Energy is required to break chemical bonds in all reactant molecules to form products.
  • The energy required to do this is known as activation energy.
  • The lower the activation energy is = quicker the reaction.
  • This is due to less energy being required to break chemical bonds.
• Factors Influencing Enzyme Activity

  • Temperature
  • pH
  • Concentration of reactions (substrates)
  • Concentration of enzyme
  • Concentration of products
  • Inhibitors
• Temperature

  • At low temps, substrates/enzymes have less energy so are less likely to collide with the required energy to bind.
  • Rate of reaction increases as temp increases, until optimum temperature.
  • Any temp above optimum temp = alters structure/shape of enzyme (denature). Substrate & active site no longer complementary.
  • Optimum temperature in humans is about 37 degrees celcius
• pH

  • Optimum pH = maximum enzyme activity.
  • pH’s above or below this denatures enzymes (alters shape) making enzyme-substrate not complementary.
  • Optimum pH of enzymes differs.
  • E.g. enzyme in blood = pH 7.
  • enzyme in stomach = pH 2.
  • Chemical buffers resist pH change in the body = stabilises pH.
• Concentration of Substrate/Enzyme

  • Low concentrations = less substrates to bind to enzyme/less enzymes for substrates to bind to.
  • As concentration increases, there are more substrates/enzymes that are able to bind, increasing reaction rate, due to increase collision rate at the active site.
  • Higher concentrations = less free enzymes so increase is smaller.
  • Plateau occurs at optimum substrate/enzyme concentration due to all enzymes being occupied (saturated).
  • If all enzymes are occupied, adding more substrates will not increase rate of reaction, or there are no more enzymes left.
• Concentration of Products

  • Some substances bind to the allosteric site.
  • Some end products of metabolic pathways bind to this site to inhibit (stop/decrease) enzyme activity. This is a reversible change.
  • This is referred to as end-product inhibition, which occurs in high end product concentrations.
• Inhibitors

  • Inhibitors can reduce the rate of enzyme catalysed reactions.
  • This will lead to an accumulation (gathering/collection) of substrates as they can’t bind to enzymes.
  • There are two types of inhibitors:
  • Competitive inhibitors – molecules that mimic substrates and bind to active site.
  • Non-competitive inhibitors – bind to allosteric site, changing shape or structure of active site.
• Four properties of enzymes

  • Biological catalysts
  • not used up in the reaction
  • Bind to the substrate at the active site
  • Lowers the activation and energy for a reaction to proceed
• Explain why enzymes are specific to one particular substrate

  Specificity exists between one enzyme and one substrate due to the complementary binding between an area on the substrate and the active site on the enzyme. This interaction is possible due to the special 3D grooves and clefts on both molecules.
• Name the two models to explain enzyme activity

  • lock and key model and induced fit model
  • The original lock and key model was simplistic in its nature and proposed that the substrate and enzyme's active site had perfect complementary shapes that fit together like a lock and a key. The induced fit model builds on this model, highlighting that the substrate and active site of enzyme are complementary in nature but upon binding, they induce a slight moderation in each other's structure, increasing their complementary fit and increasing stress on the bonds of the substrate.
• Explain what it means to say that enzymes are biological catalysts.

  Enzymes are biological catalysts because they speed up chemical reaction inside cells and living organisms.
• Name one physical and one chemical factor that changes the activity of enzymes and explain how they do this.

  • Physical factor (temperature): at optimum temperature the substrate and enzymes have kinetic energy enabling them to collide more frequently and bind together; however, temperatures above the enzymes optimum, progressively denature the enzymes decreasing reaction rate.
  • Chemical factor (pH): a pH either side of its optimum may alter/denature the enzyme's binding site and thus reduce enzyme substrate binding
• Active site

  The region of the enzyme catalase, where complementary binding occurs between the substrate
• Explain why catalase cannot catalyse another reaction such as bacterial cell wall synthesis.

  Catalase is specific to its substrate. Other substrates will not be complementary to its active site and therefore catalase cannot catalyse other reactions.
• Name three external factors that could affect the activity of the enzyme catalase.

  pH, temperature, inhibitor
• Explain how enzymes lower the activation energy requires for a reaction to proceed.

  Activation energy is the energy that chemical reactions require before they can proceed i.e. they require an initial input of energy. Enzymes lower this energy and make it easier for the reaction to start i.e. they catalyse the reaction. They do this by brining substrates into correct orientation and applying stress on bonds of substrate through induced fit model.