Biology Chapter 4

• biological catalysts
• globular proteins
• interact with substrate molecules

The role of enzymes in reactions

• living organisms need to be built and maintained so require synthesis of large polymer-based components
• chemical reactions required for growth are anabolic and are catalysed by enzymes
• energy is released from from breaking down of large organic molecules (e.g. glucose) which are catabolic reactions and also require enzymes
• digestion is catalysed by enzymes
• enzymes can only increase rate of reaction up to the Vmax

Intracellular enzymes

• enzymes that act within cells (e.g. synthesis of polymers to monomers)
• hydrogen peroxide is a toxic product of metabolism, catalase ensures it is broken down into oxygen and water

Extracellular enzymes

• all reactions in cells need a constant supply of substrates, supplied by nutrients
• too large to enter the cells so enzymes are released to break them down into smaller molecules
• they work outside the cell so are called extracellular
• single-celled organisms release into immediate environment
• examples of extracellular enzymes in digestion are amylase and tryspin

Digestion of starch

• starch -> maltose, disaccharide (by amylase, produced by salivary glands and pancreas)
• maltose -> glucose, monosaccharide (by maltase, present in small intestines)

Digestion of proteins

• Trypsin is a protease (enzymes that break down proteins -> smaller peptides)
• produced by the pancreas into small intestine
• amino acids produced can be absorbed into bloodstream

Temperature

• increasing temperature, increases kinetic energy (particles move faster so collide more frequesntly) = increase in rate of reaction
• temperature coeffiecient, Q10, is how much the rate of reaction increases with a 10 degree rise
• bonds holding the protein together vibrate more at higher temperatures until they break, changing tertiary structure, enzyme is denatured
• optimum temperature is the temperature at which the enzyme has highest rate of activity, once enzymes have denatured the decrease in rate of reaction is rapid, Q10 no longer applys as it is denatured
• enzymes adapted to the cold have more flexible structures (mainly active site) so less stable than enzymes at higher temperatures, denatured by smaller temperature changes
• enzymes adapted to high temperatures are more stable as they have a higher number of bonds (hydrogen and sulfur) in tertiary structure, shape is more resistant to temperature changes

pH

• a change in pH = a change in hydrogen ion concentration
• the active site will only be in the right shape at a certatin hydrogen ion concentration (optimum pH)
• when the pH changes the shape of the enzyme is changed, but if the pH returns to optimum then it will return to normal (renaturation)
• significant changes result in permenant shape changes (denatured)
• hydrogen ions interact with polar and charged R-groups so changing concentration afffects interactions

Substrate and enzyme concentration

• increase of substrate leads to higher collision rate and formation of more enzyme-substrate complexes
• increase of enzyme means more active sites
• rate of reaction increases to the Vmax

• enzymes are activated with cofactors, or inactivated with inhibitors

Competitive inhibition

• molecule that has similar shape to substrate can fit in the active site of an enzyme
• blocks the substrate from entering so stops enzyme catalysing reaction
• enzyme cannot carry out fuction (inhibited)
• reduces rate of reaction but doesn't lower Vmax
• example: statin, inhibitor of enzyme used in cholesterol synthesis

Non-competitive inhibition

• inhibitor binds to enzyme at the allosteric site
• which causes tertiaty structure to change, so active site changes shape
• active site no longer complementary to substrate
• example: organophosphates used as insecticides irreversibly inhibit the enzyme acetyl cholinestrase

End-product inhibition

• when the product of a reaction acts as an inhibitor to the enzyme
• negative feedback control mechanism for reaction, excess products not made and resources not wasted

cofactor- non-protein components necessary for the effective functioning of an enzyme

if the cofactor is an organic molecule it is called a conenzyme.

prosthetic group- cofactors, required by some enzymes to carry out their catalytic function, always tightly bound to enzyme and form permenant part of the structure, e.g. zinc ions in carbonic anhydrase, enzyme necessary for metabolism of carbon dioxide.

• many enzymes are produced in an inactive form (inactive precursor enzymes)
• these need to undergo a change in shape to be activated
• can be achieved by adding a cofactor, before the cofactor it is called an apoenzyme and after it is called a holoenzyme
• sometimes the change in shape can be brought about by the action of another enzyme, such as protease, which cleaves certain bonds in the molecule
  
• in some cases, change in temp or pH can result in a change to the tertiary structure
• these type of precursor enzymes are called zymogens or proenzymes