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When answering questions on concentration of substrate or enzyme, what should you link it to?
More concentration leads to more enzyme-substrate complex being formed. So more product formed.
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Why is the highest reaction rate known as the initial reaction rate?
- Because it is when enzyme and substrate are mixed for first time is the point where rate is greatest.
- This is because the concentration of reactants (substrates) are highest and concentration of products lowest, so increase collision rate, and more enzyme-substrate complex formed.
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What is a limiting factor?
A factor in a situation where, if all other conditions are kept constant, increasing the conc of that factor alone will increase reaction rate.
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Why are enzyme concentrations in cells quite low?
- Becuase they can be reused.
- Also means enzyme activity can be more easily controlled (by changing conc of enzyme or substrate or changing conditions)
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What is an enzyme inhibitor?
Any substance or molecule that slows down rate of an enzyme-controlled reaction by affecting the enzyme molecule in some way.
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How does a competitive enzyme inhibitor work?
- Inhibitor molecules have similar shape to that of the substrate molecules.
- Competes for the active site and occupies it, forming enzyme-inhibitor complexes. (products not formed because inhibitor is not the same as the substrate).
- It reduces rate of reaction because it reduces the number of substrates which can enter the active site and hence produce enzyme-substrate complexes.
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How does the conc of substrate affect the level of inhibition?
- Level of inhibition depends on conc of inhibitor and substrate.
- Where conc of substrate molecules increased, level of inhibition decreases.
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How does a non-competitive inhibitor work?
- Attach to enzyme in a region away from active site
- Distorts the tertiary structure - leads to change in shape of active site
- So enzyme-substrate complexes cannot form.
- Many (not all) permanently bind to the enzyme - therefore it is irreversible, enzyme effectively denatured.
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Level of inhibition in a non-competitive inhibitor depends on what?
- The number of inhibitor molecules present.
- Changing the substrate concentration will have no effect on this form of inhibition.
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Suggest a method of determining whether inhibition of an enzyme-controlled reaction is competitive or non-competitive.
- Test with various substrate concentrations.
- If the rate of reaction increases to the point where it is the same as a reaction with no inhibitor, then it is a competitive inhibitor.
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Why is inhibition of enzyme activity important in controlling metabolic processes?
- Cells must be able to control the concentration of various molecules in the cell.
- If they are products of enzyme-controlled reactions, it is important to regulate enzyme activity in order to achieve and maintain optimum level of the product.
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What is a cofactor?
- Any substance that must be present to ensure enzyme-controlled reactions take place at an appropriate rate.
- Some are part of the enzyme (prosthetic groups), others affect enzyme on temporary basis (coenzymes and inorganic ion cofactors)
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What are coenzymes and what are their roles usually?
- Small, organic, non-protein molecules that bind for a short period to the active site.
- In many reactions, they take part in reaction and change in some way. (But unlike substrates, they are recycled back to be used again).
- Role is often to carry chemical groups between enzymes so to link together enzyme-controlled reactions that need to take place in sequence.
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What is a prosthetic group?
- A coenzyme that is a permanent part of an enzyme molecule.
- Vital to function of enzyme and other molecule (eg. haem group in haemoglobin). They contribute to shape and other properties like charges.
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What are the roles of inorganic ion cofactors?
- Presence of certain ions increase reaction rate
- Affects the charge distribution and sometimes the shape of enzyme-substrate complex.
- Thus the enzyme-substrate complex forms more easily.
- (eg. amylase catalyse breakdown of starch to maltose only if chloride ions are present)
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Prosthetic groups are bound to enzymes as it is being made. Suggest where in the cell this processing takes place.
The Golgi apparatus
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Give an example of a poison and how it affects the body.
- Potassium cyanide
- Non-competitive inhibitor for a vital respiratory enzyme called cytochrome, found in mitochondria.
- Inhibition of this enzyme decreases use of oxygen, so ATP cannot be made, & more lactic acid builds up in the blood because organism respires anaerobically.
- Metabolic poisons can often be enzyme inhibitors
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Why is alcohol a treatment for thylene glycol poisoning?
Because it acts as a competitive inhibitor on the enzyme Alcohol dehydrogenase.
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How does penicillin work to inhibit growth of microorganisms?
- It inhibits the enzymes responsible for building the cell wall. Because penicillin is similar in shape to one of the substrates used in wall building.
- But penicillin is different from the substrate, therefore makes the wall weak and fall apart.
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How do you work out the initial reaction rate from a graph?
- Take a tangent to the steepest portion of graph and measure the gradient of the tangent, and this will be your intial rate of reaction.
- eg. x-axis - time(s); y-axis - volume of O2
- then take steepest tangent and do
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Examiner's tip!
Exam questions often give experimental procedures and ask for comments on how they might be improved. Read information about how variables were controlled, whether repeat readings were taken, and whether a control was used.
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Metabolic processes where products of one enzyme-controlled reaction is the substrate of another, and so on in a chain of enzyme reactions is called what?
A metabolic pathway.
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Many diseases are caused by the lack of a functioning specific enzyme in a metabolic sequence because of a faulty gene for example. These are called?
Inborn errors of metabolism.
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What is end-product inhibition?
- Where end product can attach to one of the enzymes early in the sequence and act as a non-competitive inhibitor.
- This controls the amount of end-product produced, and ensures it doesn't build up.
- eg. ATP is end-product inhibitor of one enzyme that catalyses an early reaction in sequence of respiration.
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