Enzyme kinetics Flashcards
what is the rate of reaction referred to?
Velocity - V = micromole/min
what are the reasons for tailing off ‘plateau’ in reaction?
- product inhibition
- reverse reaction
- substrate depletion
what is the specific activity of an enzyme expressed in?
micromole of substrate converted per minute per mg of protein - umol/min/mg protein
why do enzymes reach maximum velocity?
enzyme running out of substrate
enzyme instability
enzyme containing a limited number of sites for the substrate to bind
what is the initial velocity of reaction dependant on?
the [substrate]
what is Vmax?
occurs when all of the active sites of an enzyme are saturated and this occur when [S] is much greater than kM
aka maximal rate of an enzyme reaction in micromole/min
what is Km?
Michaelis constant - Vmax/2
what happens at high [S] and low [S]
high [S] - Velocity is independent of [S]
Low [S] - velocity is linearly proportional to [S]
give Michaelis menten equation
V = Vmax x [S] / Km x [S]
what is the line weaver burke plot?
1/V = kM/Vmax x 1/[S] + 1/Vmax
what does Vmax reveal?
the turnover number of an enzyme - how many enzyme substrates can turn into product in 1 second
Define turnover number
The number of molecules converted to product per enzyme molecule in unit time when enzyme is saturated
Equation for the turnover number of an enzyme (Kcat / K2)
Vmax / [ET]
(thus = Vmax = kCat x [ET]
what is catalytic efficiency / equation
how fast an enzyme can go -
Kcat / Km = conc per second
what does a higher value mean in catalytic efficiency?
the higher the value the better the enzyme works on that particular substrate
what is the physiological significance of Km?
Isozymes - isoforms of an enzyme which perform identical reactions. e.g Hexokinase isozymes.
Km values for hexokinase I, II, III and IV
I, II, III - 0.04 mM
Iv - 10 mM
what is the role of hexokianse IV?
when glucose is abundant, hexokinase IV is active. The liver contains hexokinase IV and more glucose-6-phosphate is produced which is used in synthesis of glycogen
how does pH affect enzymes?
enzymes are dependant on pH due to ionisation of side chains of amino acids. These form the active site and bind substrates - if they change chemically then they won’t bind to substrate and will stop working.
Some have optimal pH 7 e.g cytoplasm, mitochondria, cell surface enzymes
Some have more acidic optimal pH e.g in stomach - compartmentalised enzymes = lysosomes, endoscopes
how does temperature affect enzymes?
rate increases as temperature increases. BUT, after the optimal temperature if reached (37) then the rate will decrease as the enzyme has denatured - bonds are irreversibly broken or changed.
