T3 - Enzymes Flashcards
Explain the effects of enzymes on chemical reactions
- enzymes decrease the activation energy for a reaction
- enzymes therefore make reaction more likely to proceed
- more molecules will have an E greater (or equal to) the Ea of the reaction
- substrates bind to active site of enzyme
- forms enzyme-substrate complex
key features of active sites
- active site occupies a small part of the enzyme
- active site is formed by amino acids from different parts of the primary sequence (other parts of enzyme act as a scaffold to bring the active site regions into the right orientations)
- active site are clefts or crevices (to exclude substances like water, which is present in high concs and can often cause problems for chemical reactions)
- active sites are complimentary to the substrate
- substrates are bound to enzymes by multiple weak bonds (allows substrate to bind and product to be released)
what is the induced fit model
unlike lock and key model, IFM shows that enzymes are flexible structures
- the enzyme and substrate change confirmation until the substrate is completely bound to the enzyme
- that activates enzyme into performing catalytic function
describe how reaction rates vary as a function of enzyme and substrate concentration
rate of enzyme catalysed reaction (V) is proportional to enzyme/substrate concentration → however, depends on limiting factor (ie will plateau if all active sites are used up, and will plateau if not enough substrate)
what is Km and Vmax
these values are rates
Vmax is the theoretical maximum rate if all enzyme molecules are saturated with substrate
(can’t measure directly)
Km is the substrate concentration that gives half the maximal RoR.
- the lower the Km value, the higher affinity the enzyme has for its substrate (and vice versa)
- therefore, a lower Km value makes an enzyme more effective at lower substrate concentrations
what is V0?
initial rate of reaction
- the only time we know substrate concentration is when t=0
- take tangent to curve through 0
definition: activity (enzymes)
the catalytic effect exerted by an enzyme, expressed as units per milligram of enzyme (specific activity) or molecules of substrate transformed per minute per molecule of enzyme (molecular activity)
what is the international unit of enzyme activity
1 unit = the amount of enzyme that produces 1μmol of product (or 1μmol of substrate converted) per min under standard conditions
- often expressed as standardised rate
- eg per L of serum or per g of tissue
- to note that it is unlikely to be under standard conditions as physiological conditions vary
describe the effects of enzyme inhibitors on enzyme kinetics
also need to be able to distinguish the two from simple graphs
competitive inhibitors
- bind at active site of enzyme
- has similar shape and interactions as substrate
- reduces proportion of enzyme molecules bound to substrate
non-competitive inhibitors
- bind at free enzyme away from active site
- denatures enzyme active site
- decreases concentration of functional enzyme
… therefore both NC + C inhibitors decrease the RoR o enzyme-catalysed reactions
definition: active site
the region of an enzyme where substrate molecules bind and undergo a chemical reaction
definition: allosteric
to do with enzymes
allosteric regulation is the regulation of an enzyme by binding an effector molecule at a site other than the enzyme’s active site
definition: catalyst
a substance that increases the rate of a chemical reaction (by lowering the Ea) without itself undergoing any permanent chemical change.
definition: enzyme
a substance produced by a living organism which acts as a catalyst to bring about a specific biochemical reaction.
ie a biological catalyst
definition: substrate
in enzyme-catalysed reactions
any substance that reacts to the active site of an enzyme. A chemical bond is formed between the active site and the substrate, and then a product is released
definition: transition state
molecule is no longer a substrate but not yet a product
also the point where there is maximum value of energy
