• biological catalysts
  • globular proteins
  • interact with the substrate to make them react much faster
  • within the tertiary structure has an active site
  • the active site is complementary to the substrate
1 of 23

role of enzymes in reactions

  • all anabolic (building up) reactions are catalyzed by enzymes
  • catabolic (breaking down) reactions also involve enzymes
  • energy for reactions is obtained from food during digestion which is also catalysed by enzymes
  • enzymes increase the rate of reaction up to a point called the Vmax
2 of 23

mechanism of enzyme action

  • molecules move and collide randomly
  • for reaction molecules must collide in the right orientation
  • high temperatures and pressures increase the speed of molecules and so increase successful collisions and so the rate of reaction
  • the specificity of the enzyme - an enzyme only catalyses one biological reaction
  • activation energy - the energy needed to start a reaction
  • enzymes help molecules collide successfully and reduce the activation energy
3 of 23

lock and key hypothesis

  • only a specific substrate will fit exactly into the active site
  • substrate + enzyme = enzyme-substrate complex
  • once the reaction has taken place, the enzyme-product complex is formed
  • the product(s) leave the enzyme
  • the enzyme is unchanged and can take part in other reaction
  • the substrate is held by temporary bonds between enzymes r-group and substrate
  • these put a strain on bonds in the substrate which helps reaction
4 of 23

induced fit hypothesis

  • active site changes slightly as the substrate enters
  • initial interaction between  enzyme and substrate is weak
  • weak interactions rapidly induce changes in enzymes tertiary structure
  • binding is strengthened
  • stronger binding between substrate and enzyme puts pressure on the substrate
  • some bonds are weakened and hence lower activation energy
5 of 23

intracellular enzymes

  • enzymes that act within cells
  • example: catalase ensures hydrogen peroxide is broken down to oxygen and water preventing accumulation
6 of 23

extracellular enzymes

  • all reactions inside cells need substrates
  • nutrients are often present in diet or environment
  • tend to be large molecules that cannot enter the cell
  • must be broken into smaller components first
  • enzymes are released from cells to break down large molecules in digestion
  • these enzymes work outside the cell
  • used by both single-celled and multicellular organisms
  • single-celled organisms release enzymes into the environment
7 of 23

digestion of starch

  • amylase made in salivary glands (and pancreas into pancreatic juice) is released in saliva into the mouth
  • amylase breaks starch into maltose
  • maltase present in the small intestine breaks maltose into glucose
  • glucose is small enough to be absorbed into the bloodstream
8 of 23

digestion of proteins

  • trypsin is a protease
  • produced in pancreas
  • released into the small intestine in pancreatic juice
  • breaks protein down into smaller peptides
  • amino acids produced from proteases will absorb into the bloodstream
9 of 23

temperature on enzyme activity

  • increase in temperature increases the kinetic energy of particles
  • particles move faster and collide more frequently
  • more frequent successful collisions between enzyme and substrate
  • The Q10 temperature coefficient is ( how much rate of reaction changes with 10 degrees in temp) usually 2 for enzyme reactions (double)
10 of 23

denaturalisation by temperature

  • protein structure of enzymes is affected by temperature
  • temperature increase causes bonds holding the proteins together to vibrate
  • eventually, bonds strain and break 
  • the shape of the tertiary structure of the protein is denatured
  • the active site is no longer complementary to the substrate
11 of 23

optimum temperature

  • when the enzymes have their highest rate or activity
  • most enzymes in the human body are around 40 degrees
  • thermophilic bacteria, in hot springs, are closer to 70 degrees
  • psychrophilic organisms, in the arctic regions, are closer to 5 degrees
  • once enzymes have denatured above their optimum temperature, the decrease in rate is rapid
  • all enzyme molecules denature at about the same temperature
  • the decrease in temperature, below optimum, is less rapid as the enzymes have not denatured
12 of 23

temperature extremes

  • enzymes that work in cold temperatures have more flexible structures, particularly their active sites
  • less stable than enzymes working at higher temperatures
  • small temperature changes denature them
  • enzymes that work in hot temperatures have an increased number of bonds, especially hydrogen bonds and sulphur bridges
  • more table than enzymes that work at lower temperatures
  • more resistant to temperature change
13 of 23

pH on enzyme activity

  • both hydrogen bonds and ionic bonds hold proteins in their precise shapes
  • change in pH refers to change in hydrogen ion concentration
  • the active site will only be the right shape at a certain hydrogen concentration
  • changes in pH will change the shape of the active site
  • a return to the optimum pH will allow a protein to resume their normal shape and catalyse reactions again
  • when the pH is altered significantly, the structure of the enzyme will be irreversibly damaged
  • changing the hydrogen ion concentration will change to interaction with polar and charged r-groups
  • the more hydrogen there is, the fewer r-groups will interact which leads to bonds breaking
14 of 23

substrate-enzyme concentration

  • increasing the concentration of the substrate increases the collision rate, which increases the rate of reaction (also true when the concentration of the enzymes increases)
  • the rate of reaction will only increase to Vmax, at which point all active sites will be occupied by substrates
  • the only way to increase the rate then would be to increase the enzyme concentration so a higher Vmax can be reached
15 of 23

enzyme inhibitors

  • inactivate enzymes
16 of 23

competitive inhibition

  • the molecule has a similar shape to the substrate
  • the molecule fits in the active site of the enzyme
  • the molecule blocks the substrate from entering the enzymes active site
  • the enzyme cannot carry out its function, so it's inhibited
  • most only bind temporarily
  • it reduces the rate of reaction
  • if the substrate concentration is increased enough, Vmax can still be reached
  • example: statins are competitive inhibitors, they inhibit the enzyme that is used in the synthesis of cholesterol
17 of 23

non-competitive inhibitor

  • the inhibitor binds to the allosteric site, not the active site
  • the inhibitor binding causes changes in the enzyme's tertiary structure
  • the active site changes shape
  • may be reversible or non-reversible
  • the active site is no longer complementary to the substrate
  • the substrate cannot bind to the enzyme
  • the enzyme is inhibited
  • increasing the concentration of the enzyme or the substrate will not overcome the effect of a non-competitive inhibitor
  • example: irreversible inhibitors cannot be removed, they are often toxic, proton pump inhibitors block the enzyme system responsible for the secretion of hydrogen ions into the stomach, they are used to treat long term indigestion
18 of 23

end product inhibition

  • when the product of the reaction acts as the inhibitor of the enzyme of that reaction
  • negative feedback
  • excess products aren't made
  • resources aren't wasted
  • example: breakdown of glucose catalysed by phosphofructokinase (PFK), competitively inhibited by ATP, ATP regulates its own production, high levels of ATP means more ATP binds to PFK's allosteric site, glucose is not broken down and ATP is not produced at the same rate, when ATP is used up less binds to PFK, PFK is able to catalyse breakdown of glucose, more ATP is produced
19 of 23


  • a non-protein helper
  • may transfer atoms from one reaction to another
  • may form part of the active site
  • inorganic cofactors are obtained via diet as minerals
20 of 23


  • organic cofactor
  • many coenzymes are derived from vitamins
  • example: vitamin B3 is used to synthesise NAD - a coenzyme used in respiration
21 of 23

prosthetic group

  • a cofactor
  • required by some enzymes to carry out catalyses
  • tightly bound to enzymes
  • a permanent feature
22 of 23

precursor activation

  • some enzymes are produced in an inactive form
  • for enzymes that cause damage
  • for enzymes whose actions need to be controlled
  • they will often have to undergo a change in their tertiary structure, particularly in their active site
  • this can happen by a cofactor
  • before a cofactor is added, enzymes are apoenzymes
  • after enzymes are activated they're holoenzymes
  • sometimes a change in structure is brought out by another enzyme
  • changes in conditions (pH or temperature) can result in a change of structure, thus activating the enzyme
  • these precursor enzymes are proenzymes or zymogens
23 of 23


No comments have yet been made

Similar Biology resources:

See all Biology resources »See all Biological molecules resources »