Enzymes are proteins that act as biological catalysts for intra & extracellular reactions to determine structure & function
Enzymes affect the metabolism of cells & whole organisms
The specific tertiary structure of enzymes determines the shape of the active site, which is complementary to a specific substrate
Formation of enzyme-substrate (ES) complexes lowers the activation energy of metabolic reactions
Example of an enzyme that catalyses intracellular reactions: Catalase catalyses the decomposition of hydrogen peroxide into water + oxygen
Examples of enzymes that catalyse extracellular reactions:
Amylase: carbohydrase catalyses digestion of starch to maltose in saliva/small intestine lumen
Trypsin: pancreatic endopeptidase catalyses hydrolysis of peptide bonds in small intestine lumen
Induced fit model of enzyme action:
Shape of active site is not directly complementary to substrate & is flexible
Conformational change enables ES complexes to form when substrate adsorbs
This puts strain on substrate bonds, lowering activation energy
Bonds in enzyme-product complex are weak, so product desorbs
Lock and key model of enzyme action:
Active site has a rigid shape determined by tertiary structure, complementary to 1 substrate
Formation of ES complex lowers activation energy
Bonds in enzyme-product complex are weak, so product desorbs
Factors affecting the rate of enzyme-controlled reactions:
Enzyme concentration
Substrate concentration
Concentration of inhibitors
pH
Temperature
How substrate concentration affects rate of reaction:
Rate increases proportionally to substrate concentration until the maximum number of ES complexes form
How enzyme concentration affects rate of reaction:
Rate increases proportionally to enzyme concentration until the maximum number of ES complexes form
How temperature affects the rate of enzyme-controlled reactions:
Rate increases as kinetic energy increases & peaks at optimum temperature
Above optimum, denaturation occurs as ionic & H-bonds in 3° structure break
Temperature coefficient (Q10):
Measures the change in rate of reaction per 10°C temperature increase
Q10 = R2 / R1 (where R represents rate)
How pH affects rate of reaction:
Enzymes have a narrow optimum pH range
Outside the range, denaturation occurs as H+ / OH- ions interact with H-bonds & ionic bonds in 3° structure
Competitive inhibitors:
Bind to the active site, temporarily preventing ES complexes from forming
Increasing substrate concentration decreases their effect
Non-competitive inhibitors:
Bind at allosteric binding site, triggering a conformational change of the active site
Increasing substrate concentration has no impact on their effect
End-product inhibition:
One of the products of a reaction acts as a competitive or non-competitive inhibitor for an enzyme involved in the pathway, preventing further formation of products
Irreversible inhibitors:
Permanently prevent formation of ES complexes
Heavy metal ions e.g. mercury, silver cause disulphide bonds in tertiary structure to break
Reversible inhibitors:
May be competitive or non-competitive
Bind to enzyme temporarily, allowing ES complexes to form after the inhibitor is released
Metabolic poison:
Substance that damages cells by interfering with metabolic reactions, usually an inhibitor
Examples of metabolic poisons:
Cyanide: non-competitive, irreversible, inhibits cytochrome c oxidase