Enzymes Flashcards
What are enzymes
Proteins that act as biological catalysts for intra and extracellular reactions to determine structure and function
Therefore affect metabolism of cells and whole organism
Specific tertiary structure determines shape of active site, complementary to a specific substrate
Formation of enzyme-substrate complexes lowers activation energy of metabolic reactions
Give an example of an enzyme that catalyses intercellular reactions
Catalase: catalyses decomposition of hydrogen peroxide (which causes oxidative stress) into water and oxygen
Give two 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
Explain the induced fit model of enzyme action
Shape of active sit is not directly complementary to substrate and is flexible
Conformational change enable ES complexes to form when substrate absorbs
This puts strain on substrate bonds lowering activation energy. Bonds in enzyme-product complex are weak so product desorbs
Explain the lock and key model of enzyme action
Suggests that active site has rigid shape determined by tertiary structure so is only complementary to 1 substrate. Formation of ES complex lowers activation energy
Bonds in enzyme-product complex are weak , so product desorbs
Name 5 factors which affect the rate of enzyme controlled reactions
- Enzyme concentration
- Substrate concentration
- Concentration of inhibitors
- pH
- Temperature
How does substrate concentration affect rate of reaction?
Given that enzyme concentration is fixed, rate increases proportionally to substrate concentration
The rate levels off when maximum number of ES complexes form at any given time
How does enzyme concentration affect the rate of reaction
Given that substrate is in excess, rate increase proportionately to enzyme concentration
Rate levels off when maximum number of ES complexes form at any given time
How does temperature affect the rate of enzyme-controlled reactions
Rate increases as kinetic energy increases and peaks at optimum
Above optimum, ionic and H-Bonds in tertiary structure break = active site no longer complementary to substrate (denaturation)
What is the temperature coefficient
Q10 measures the change in rate of reaction per 10*C temperature increases
Q10 = R2/R1 (where R represents rate)
How does pH affect rate of reaction
Enzymes have a narrow optimum pH range
Outside range H+ / OH- ions interact with H-Bonds and ionic bonds in tertiary structure = denaturation
How do competitive inhibitors work
Bind to active site since they have similar shape to substrate. Temporarily prevent ES complexes forming until released.
Increasing substrate concentration decreases their effect
How do non-competitive inhibitors work
Bind at allosteric binding site
Trigger conformational change of active site
Increasing substrate concentration has no impact on their effect
What is end-product inhibition
One of the products of a reaction acts as a competitive or non-competitive inhibitor involved in the pathway
Prevents further formation of products
What are irreversible inhibitors
Permanently prevent formation of ES complexes
Heavy metal ions e.g. mercury, silver cause disulphide bonds in tertiary structure to break
Bind to enzymes by strong covalent bonds e.g. cyanide binds to cytochrome c
Define metabolic poison
Substance that damages cells by interfering with metabolic reactions
Usually an inhibitor
Give some examples of metabolic poisons
Respiratory inhibitors include:
- Cyanide: non-competitive, irreversible, inhibits cytochrome c oxidase
- Malonate: Competitive, inhibits succinate dehydrogenase
- Arsenic: Competitive, inhibits pyruvate dehydrogenase
What are inactive precursors in metabolic pathways
To prevent damage to cells, some enzymes in metabolic pathways are synthesised as inactive precursors e.g. proteases
One part of the precursor acts an inhibitor.
ES complexes form when it is removed
What are cofactors
Non-protein compounds required for enzyme activity:
- Coenzymes
- Inorganic cofactors
- Prosthetic groups
What are coenzymes
Organic cofactors. Do not bind permanently
Often transport molecules or electrons between enzymes
Frequently derived from water-soluble vitamins e.g. the hydrogen acceptor NAD is derived from niacin
What are inorganic cofactors? Give an examples
Facilitate temporary binding between substrate and enzyme. Often metal ions e.g. Cl- is the cofactor for amylase
What are prosthetic groups? Given an example
Tightly bound cofactors act as a permanent part of enzyme’s binding site e.g. Zn2+ for carbonic anhydrase
Suggest how a student could produce a desired concentration of solution from a stock solution
Volume of stock solution = required concentration x final volume needed / concentration of stock solution
Volume of distilled water = final volume needed - volume of stock solution
