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Biology 2.1.3
AS OCR biology - enzymes
| Question | Answer |
|---|---|
| What is the structure of enzymes? | Globular proteins with specific tertiary structure, water soluble as hydrophilic R groups face outwards. Specifically shaped active site depends on R groups. |
| What is the function of enzymes? | Biological catalysts, speed up the rate of a metabolic reaction, unchanged at the end of the reaction. Enzyme action may be intracellular or extracellular |
| How are enzymes better than inorganic catalysts? | They don't produce a range of unwanted by-products |
| Enzymes are highly __?__ for their substrate - this means: | Specific - they only bind to one type of substrate |
| What are the two types of enzyme action? | Intracellular (e.g. respiration in mitochondria) or extracellular (e.g. digestion) |
| What are cofactors + types? | Substances that cause enzyme-controlled reactions to take place at the appropriate rate e.g. coenzymes, inorganic ion cofactors, prosthetic groups |
| What are coenzymes + function? | Organic non-protein molecules that bind to the active site at the same time as the substrate for a short time - carry chemical groups between enzymes to allow an enzyme cascade reaction |
| What are prosthetic groups? | Coenzymes that are permanent parts of the quaternary structure of the protein |
| What is the function inorganic ion cofactors? | Increase the rate of reaction - bind to enzyme or substrate, affect charge distribution so enzyme-substrate complex forms more easily |
| What is activation energy and how do enzymes reduce it and why? | Activation energy is the amount of energy required for a reaction to proceed. Enzymes reduce the amount of activation energy required because the active site fits the substrate perfectly, lowering the amount of energy needed to start a reaction |
| Describe how an enzyme catalyses a reaction | 1 The substrate binds to the active site due to bonds between oppositely-charged groups, forming an enzyme substrate complex. 2 Bonds are made or broken producing an enzyme product complex. 3 Product leaves active site and enzyme is available for reuse |
| What is the lock-and-key hypothesis? | The enzyme’s active site and the substrate molecule have complementary shapes |
| What is the induced-fit hypothesis? | Substrate collides with active site, enzyme molecule changes shape slightly so active site fits more closely around substrate.Shape change puts strain on/destabilises substrate so faster rate of reaction. Product doesnt fit in active site so leaves |
| What are oxidoreductases? | Enzymes that catalyse the transfer of electrons between different substrates |
| How do you keep temperature constant in an experiment of enzyme activity? | By carrying out a reaction in a water bath with a thermostat. Equilibration - enzyme and substrate must be placed separately in a water bath so they reach the required temperature before the investigation begins |
| How do you keep enzyme concentration constant in an experiment of enzyme activity? | By accurately measuring surface area and mass of enzyme, and volume of enzyme in solution |
| How do you keep pH constant in an experiment of enzyme activity? | pH buffers keep pH at a set level by maintaining a constant H+ concentration |
| How + why does enzyme activity change with temperature? | Low temp - very slow reaction, low KE, few successful collisions. Optimum temp - highest ror, high KE, many collisions between enzyme and substrate, many enzyme-substrate complexes form. High temp - denaturing, high KE, vibration, weak bonds break |
| What is pH a measure of? | pH is a measure of the H+ ion concentration, high concentration means acidic |
| How + why does enzyme activity change with pH? | Curve= symmetrical bell-shaped curve, effective in narrow pH ranges. Optimum pH - conc of H+ ions in solution holds tertiary structure of active site in best shape complementary to substrate. |
| Continued | Too high/low - H+ ions interfere with hydrogen and ionic bonds, affecting tertiary structure of active site so it isn’t complementary to substrate and reduces the formation of enzyme-substrate complexes. Enzymes denature at extreme pHs so activity stops |
| How + why does substrate concentration affect enzyme activity? | Higher chance of collision of substrate & active site so more enzyme-substrate complexes form, more product. V-max is max rate of reaction where all active sites are occupied at a time, so enzyme conc is limiting factor, higher substrate conc has no effec |
| How + why does enzyme concentration affect enzyme activity? | Higher number of active sites + chance of collision of substrate and active site, so more enzyme-substrate complexes form. Max rate of reaction-where all substrate is being catalysed, so enzyme conc is limiting factor + higher substrate conc has no effect |
| What is a limiting factor? | A factor that in higher concentration would increase the rate of reaction, if all other conditions are kept constant |
| What are inhibitors, types + uses? | Inhibitors reduce the reaction rate in an enzyme-controlled reaction by affecting the enzyme molecule. Competitive inhibitors = active-site directed. Non-competitive inhibitors = non-active site directed. Poisons + medicinal drugs. |
| How do competitive inhibitors work? | the inhibitor is a similar shape to the true substrate and occupies and blocks the active site for a short while, reversible |
| How do non-competitive inhibitors work? | the inhibitor attaches to the enzyme (not active site) and disrupts tertiary structure of active site so it isn’t complementary to the substrate so no enzyme-substrate complexes can form, many are irreversible, e.g. poison |
| What is a potassium cyanide and how does it work? | Poison- non-competitive inhibitor for a respiratory enzyme called cytochrome oxidase found in mitochondria which prevents ATP formation - organism can’t respire aerobically and anaerobic respiration causes lactic acid to build up in the blood |
| How is infection by virus treated? | Infection by viruses (e.g. HIV) can be treated using competitive inhibitors that inhibit protease activity, which is an enzyme needed by viruses to build virus coats, preventing viruses from replicating |
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