Enzymes I Flashcards
1) Define enzymes
Biological catalysts which speed up the rate of reaction without altering the final equilibrium between reactants and products
- composed of one or more polypeptide chains folded into a complex 3D structure which is stabilised by weak H bonds, electrostatic salt links and hydrophobic interactions
- Active sites contain functional groups that stabilise the transition state of the reaction
2) Describe the concept of enzyme catalysis
- Enzymes usually work on one type of reaction and work on a few related molecules -> ‘specificity’
- Some only act on one substrate
- If there are D and L isomers, the enzyme usually acts on only one
- Specificity is determined by the shape of the active site
3) What are the 4 stages of enzyme catalysis?
- Substrate binds to enzyme active site
- Enzyme-substrate complex is formed and bonds within substrate are weakened so reaction can take place
- Enzyme-product complex is formed
- Product desorbs off the enzyme, leaving the active site free for the next substrate
4) Describe how the enzyme complementary to transition state works (induced fit model)
- Enzyme changes shape to allow the substrate to bind to its active site, in a position where it is more likely to from products (this is where binding between enzyme and substrate is strongest)
[e.g. hexokinase changes shape, on binding on substrate]
5) What is a complex, co-ordinated metabolic pathway and what is an advantage of this?
- group of enzymes in one compartment of a cell which work together in a chain, starting with a reactant and finishing with a product
- advantage is a more controlled reaction -> if there is a high conc of product, there is a feedback regulatory loop to prevent the formation of more product by inhibiting an enzyme in the chain
6) State the 6 different groups of enzymes and their functions
- oxidoreductase - adds O2 or removes H2 (lactate dehydrogenase)
- transferase - transfer of functional groups (alanine amino transferase)
- hydrolase - hydrolytic reactions (trypsin)
- lyase - adds groups to C=C bonds (ATP citrate lyase)
- isomerase - transfer of functional groups within one molecule (phosphoglucose isomerase)
- ligase - form C-C or C-N bonds with ATP cleavage (DNA ligase)
7) What is the modified lock and key model?
Concept that the enzyme constrains the substrates to interact by pushing them closer together so they are able to react more easily
8) What are the effects of pH and temperature on an enzyme catalysed reaction?
- pH: varies for each enzyme, each has its own optimum pH level where the reaction velocity peaks on a graph on reaction velocity against pH
[peak is where active site is most stable] - temp: graph peaks at optimum temperature, where reaction velocity is highest, and above this the velocity decreases due to denaturation of the enzyme as the active site loses its specific shape
9) Name organic elements which are enzyme cofactors (presence essential for enzyme activity)
- Cu2+ ; cytochrome oxidase
- Fe2+/3+ : cytochrome oxidase, catalase, peroxidase
- K+ : pyruvate kinase
- Mg2+ : hexokinase, G6Pase, pyruvate kinase
- Ni2+ : urease
- Zn2+ : carbonic anhydrase
10) Give 2 examples of iron containing centres
- Haem group -> Fe coordinated between nitrogen groups
- Flavin enzymes -> iron-sulfur centre where Fe is coordinated with the sulfur of 4 cysteine residues in the polypeptide chain
11) Define isoenzymes
- enzymes with different protein structures that catalyse the same reaction > coded for by different genes, often found in different cellular compartments
- distinct biochemical roles - hexokinase and glucokinase
12) Define enzyme kinetics and reaction rate
Enzyme kinetics: study of rate of enzyme catalysed reaction and how this rate varies with different substrate concentrations, amounts of inhibitors, metal ions and cofactors, and pH
- Reaction rate: decrease in amount of substrate or increase in amount of product, over time
13) Describe what saturation kinetics means
- at a low [substrate] , reaction is 1st order as rate is directly proportional to [S]
- at a high [substrate] , reaction is 0 order, as enzyme sites are fully saturated so rate is independant of [S]
14) What is the Michaelis-Menton reaction model?
k1 k2
E+S ES ——–> E + P
k-1 (reverse)
[ k1, k2 and k-1 are rate constants]
15) State the 3 assumptions of the Michaelis-Menton model
- [S]»_space; [E] : amount of substrate bound to enzyme at any one time is small (sites are fully saturated)
- [ES] does not change with time (steady state), formation of ES = breakdown of ES
- initial velocities are used, the concentration of product is small and the reverse reaction of P to S is ignored
