• Enzymes are?biological catalysts
  • ‘Biological’ because they function in?living systems
  • ‘Catalysts’ because they?speed up the rate of chemical reactions?without being used up or changed themselves
  • Enzymes have an active site to which specific substrates bind
• Enzymes are also?globular proteins
• Critical to the enzyme's function is the?active site?where the?substrate?binds
• Enzymes are?specific?to the substrate
  • The shapes of the enzyme and substrate and their?chemical properties?are?complementary, to allow the substrate to fit into the active site, like two jigsaw pieces fitting together
  • This is called?enzyme-substrate specificity
• Due to this specificity, thousands of enzymes are needed throughout an organism, to carry out?individual chemical reactions

• Enzyme catalysis involves molecular motion and the collision of substrates with the active site
• For an enzyme-catalysed reaction to take place, substrates?collide at random?with the enzyme's active site
• This must happen at the correct?orientation?and?speed?in order for a reaction to occur
  • Unsuccessful collisions?can occur when the molecules are not correctly aligned with each other at the moment of collision
  • The molecules 'bounce' off each other and?no reaction?takes place
• Some enzymes have?two substrates?that must each collide with a separate active site?at the same time
• Substrates bind to enzymes,?forming a temporary?enzyme-substrate complex
• The?active site?of an enzyme has a?specific shape?and?chemical properties?to bind with a specific substrate
• The reaction occurs within the enzyme-substrate complex which leads to changes in the?chemical structure of the substrate
• Products?are formed, which?detach?and?move away?from the active site, which can be re-used

The active site of an enzyme has a specific shape to fit a specific substrate (when the substrate binds an enzyme-substrate complex is formed)

• The?specificity?of an enzyme is a result of the?complementary nature?between the shape of the active site on the enzyme and its substrate(s)
• The?shape?of the active site (and therefore the specificity of the enzyme) is determined by the?complex 3-D shape?of the protein that makes up the enzyme
  • Proteins are formed from?chains of amino acids?held together by peptide bonds
  • The order of amino acids in this chain determines the shape of an enzyme
  • If the order is altered, the resulting three-dimensional shape changes

An example of enzyme specificity – the enzyme catalase can bind to its substrate hydrogen peroxide as they are complementary in shape, whereas DNA polymerase is not

The temporary formation of an enzyme-substrate complex

• Enzyme reactions can either be?catabolic?or?anabolic
• Catabolic?reactions involve the?breakdown?of complex molecules into simpler products, which happens when a single substrate is drawn into the active site and broken apart into two or more distinct molecules
• Examples of catabolic reactions include?cellular respiration?and?hydrolysis?reactions

A catabolic reaction

• Anabolic?reactions involve the?building?of more complex molecules from simpler ones when two or more substrates are?held in the active site, forming bonds between them and releasing a?single product
• Examples of anabolic reactions include?protein synthesis?and?photosynthesis

An anabolic reaction

• Temperature, pH and substrate concentration affect the rate of activity of enzymes
• Enzymes have a?specific optimum temperature?– the temperature at which they catalyse a reaction at the?maximum rate
• Lower temperatures?either?prevent?reactions from proceeding or?slow them down:
  • Molecules move relatively?slowly
  • Lower frequency of successful collisions?between a substrate molecule and the active site of enzyme
  • Less frequent enzyme-substrate complex formation
  • Substrate and enzyme collide with?less energy, making it less likely for bonds to be formed or broken (stopping the reaction from occurring)
• Higher temperatures speed up reactions:
  • Molecules move more?quickly
  • Higher frequency successful collisions?between a substrate molecule and the active site of enzyme
  • More frequent enzyme-substrate complex formation
  • Substrate and enzyme collide with?more energy, making it more likely for bonds to be formed or broken (allowing the reaction to occur)
• However, as temperatures continue to increase, the rate at which an enzyme catalyses a reaction?drops sharply, as the enzyme begins to?denature

The effect of temperature on the rate of an enzyme-catalysed reaction

Changes in pH

• pH is a result of the?hydrogen ion concentration?in a solution
• A?low pH?is?acid?and has a?high?hydrogen ion concentration
• A?high pH?is?alkaline?and has a?low?hydrogen ion concentration
• A?10× increase?in hydrogen ion concentration lowers the pH by?1 unit
  • pH is therefore measured on a?logarithmic scale?of hydrogen ion concentration,?not a linear scale
• Water has a pH of 7, regarded as?neutral
• Extremes of pH?can also alter hydrogen bonding within an enzyme's structure and cause irreversible?denaturation
• Each enzyme has an?optimum?pH
• Not all enzymes have an optimum pH near to neutral. For example
  • The?stomach enzyme?pepsin is adapted to work best at?pH 2
  • Certain bacterial enzymes work at?pH 9-10, in line with the pH of the bacteria's main habitat

The effect of pH on three enzymes' rates of reaction

Changes in substrate concentration

• The more substrate molecules are present in a solution, this?increases the frequency of collisions?with the enzyme's active site
• Active sites are?occupied?or 'blocked' by substrates whilst the reaction is taking place
• The more active sites are occupied,?the fewer are available to catalyse other substrate?molecules
• As substrate concentration rises, the slower the rise in the rate of the enzyme-catalysed reaction
• The active sites have become?saturated
• At the?point of active site saturation, increasing the substrate concentration will cause?no further increase?in the rate of reaction
• At the point of active site saturation, a method of increasing the rate of reaction would be to make more active sites available by?increasing the enzyme concentration

The effect of substrate concentration on enzyme activity

• Enzymes can be denatured
• High temperatures?and?extremes of pH?cause denaturation
• Bonds?(eg. hydrogen bonds) holding the enzyme molecule in its precise 3D shape start to?break
• This causes the?3-dimensional shape?of the protein (ie. the enzyme) to?change
• This permanently?damages?the?active site, preventing the?substrate?from?binding
• Denaturation?has occurred if the?substrate can no longer bind
• The reaction that was previously catalysed?now no longer takes place
• Denaturation often causes the enzyme to?become insoluble?and form a?precipitate
• Very few human enzymes can function at temperatures?above 50°C
  • This is because humans maintain a body temperature of about 37°C, therefore even temperatures exceeding 40°C will cause the denaturation of enzymes
  • High temperatures cause increased vibrations in the bonds and the?hydrogen bonds between amino acids?start to break, changing the conformation of the enzyme