Enzymes

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  • An enzyme is a globular protein which acts as a biological catalyst by speeding up the rate of a chemical reaction
  • Enzymes are not changed or consumed by the reactions they catalyse, and thus can be reused
  • Enzymes are typically named after the thing they react with (called a substrate) and end with the suffix 'ase' (e.g. lipase breaks down lipids)


Enzyme-Substrate Interaction

  • All enzymes possess an indentation or cavity to which the substrate can bind with high specificity - this is known as the active site
  • The active site and substrate complement each other in terms of both shape and chemical properties (e.g. opposite charges)
  • Binding to the active site brings the substrate into close physical proximity, creating an enzyme-substrate complex
  • The enzyme catalyses the conversion of the substrate into a product (or products), creating an enzyme-product complex
  • As the enzyme is not consumed in the reaction, it can continue to work once the product dissociates (hence only low concentrations are needed)
  • The enzyme and substrate share specificity - meaning each enzyme will only bind and react with a specific substrate


Models of Enzyme Activity

Lock and Key Model

  • A given enzyme will only interact with a small number of specific substrates that complement the active site (they share specificity)
  • This explanation of enzyme-substrate interaction is described as the 'lock and key' model (a lock only opens in response to a specific key)


The Lock and Key Model


Induced Fit Model

  • When enzymes and substrates bind, the active site is not completely rigid and may undergo a conformational change in shape to better fit the substrate
  • This conformational change may increase the reactivity of the substrate and be necessary for the enzyme's catalytic activity
  • The induced fit model explains how an enzyme may be able to bind to, and catalyse, several different substrates (broad specificity)


The Induced Fit Model


Mechanism of Enzyme Action

  • Every reaction requires a certain amount of energy to proceed - this is the activation energy (Ea)
  • Enzymes speed up the rate of a biochemical reaction by lowering the activation energy
  • If more energy is in the products than the reactants, energy is lost from the system (endergonic)
    • These reactions are usually anabolic (building things up), as the energy is being used up in bond formation between two substrate molecules
  • If more energy is in the reactants than the products, excess energy is released into the system (exergonic) 
    • These reactions are usually catabolic (breaking things down), as the energy is released from the broken bonds within molecules 


Reaction Pathway of a Typical Exergonic / Exothermic Reaction


Factors Affecting Enzyme Activity

Temperature

  • Low temperatures result in insufficient thermal energy for the activation of a given enzyme-catalysed reaction to be achieved
  • Increasing the temperature will increase the speed and motion of both enzyme and substrate, resulting in higher enzyme activity 
  • This is because a higher kinetic energy will result in more frequent collisions between enzyme and substrate
  • At an optimal temperature (may differ for different enzymes), the rate of enzyme activity will be at its peak
  • Higher temperatures will cause enzyme stability to decrease, as the thermal energy disrupts the hydrogen bonds holding the enzyme together
  • This causes the enzyme (particularly the active site) to lose its shape, resulting in a loss of enzyme activity (denaturation)


pH

  • Changing the pH will alter the charge of the enzyme, which in turn will protein solubility and may change the shape of the molecule
  • Changing the shape or charge of the active site will diminish its ability to bind to the substrate, abrogating enzyme function
  • Enzymes have an optimum pH (may differ between enzymes) and moving outside of this range will always result in a diminished rate of reaction


Substrate Concentration

  • Increasing substrate concentration will increase the activity of a particular enzyme
  • More substrate means there is an increased likelihood of enzyme and substrate colliding and reacting, such that more reactions will occur and more products will be formed in a given time period
  • After a certain point, the rate of reaction will cease to rise regardless of further increases to substrate concentration, as the environment has become saturated with substrate and all enzymes are bound and reacting (Vmax)


Factors Affecting Enzyme Activity


Denaturation

Denaturation is a structural change in a protein that results in the loss (usually permanent) of its biological properties

  • Heat and pH are two agents which may cause denaturation of an enzyme


Denaturation


Activation of Enzymes

  • Some enzymes are not initially synthesised in an active form and require further modification in order to become functional
  • Zymogens are inactive enzyme precursors that need to be cleaved in order to assume an active form (e.g. pepsinogen is converted into pepsin)
  • Other enzymes require the addition of prosthetic groups in order to become active
    • Cofactors are inorganic prosthetic groups (e.g. nickel in urease)
    • Coenzymes are organic prosthetic groups (e.g. pantothenic acid in coenzyme A)


Enzyme Inhibition

Competitive Inhibition

  • A molecule (inhibitor) which is structurally / chemically similar to the substrate and binds to the active site of the enzyme
  • This serves to block the active site and thus prevent substrate binding (competes for the active site)
  • Its effect can be reduced by increasing substrate concentration

Example:  Relenza is a competitive inhibitor of neuraminidase (influenza virus enzyme), preventing the release of virions from infected cells


Non-competitive Inhibition

  • A molecule (inhibitor) which is not structurally or chemically similar to the substrate and binds to a site other than the active site (allosteric site)
  • This causes a conformational change in the active site, meaning the substrate cannot bind 
  • Its effect cannot be reduced by increasing substrate concentration as it is not competing for the active site

Example:  Cyanide (CN-) inhibits enzymes (cytochrome oxidase) in the electron transport chain by breaking disulphide bonds within the enzyme


Competitive versus Non-competitive Inhibition


End Product Inhibition

  • End-product inhibition is a form of negative feedback in which increased levels of product decrease the rate of product formation 
  • Because metabolic pathways usually consist of chains (e.g. glycolysis) or cycles (e.g. Krebs cycle), the product can regulate the rate of its own production by inhibiting an earlier enzyme in the metabolic pathway
  • The product binds to an allosteric site of an enzyme, causing a conformational change in the active site (non-competitive inhibition)
  • As the enzyme can not currently function, the rate of product formation will decrease (and with less product there is less enzyme inhibition)


End-Product Inhibition


An example of end-product inhibition is the regulation of ATP formation by phosphofructokinase (an enzyme in glycolysis)

  • ATP inhibits phosphofructokinase, so that when ATP levels are high, glucose is not broken down (but instead can be stored as glycogen)
  • When ATP levels are low, phosphofructokinase is activated and glucose is broken down to make more ATP