progress curves and initial velocity (enzyme kinetics)

what is a progress curve?

continuous plot of reactant or product concentration against time

what are the two regions of a progress curve?

first order region

zero order region

draw a basic progress curve

what is initial velocity, v₀?

initial rate of reaction

• lots of substrate

• no limiting factors

• measured before substrate runs out/product slows reaction down

why does velocity/rate of reaction decrease over time?

• may run out of substrate

• product/feedback inhibition

• enzyme losing structure

• reverse reaction favoured

what is the first order region of a curve?

an increase in substrate will increase the rate proportionally

what is the zero order region of a curve

a constant

doesn’t depend on substrate concentration

what is the pre-steady state?

the initial transient phase of enzyme-catalysed reaction before the steady state is established

lasts a few milliseconds

first increase in [ES complex]

what is the steady state?

measures initial rate whilst substrate concentration is larger than concentration of enzyme

allows the determination of K_m and V_max

what assumptions are made when analysing the steady state

1. there is very little product therefore the reverse reaction is neglible

2. ES complex concentration is constant (ES complexes break down as quickly as they are formed)

3. initial velocity is determined by ES complex concentration

4. E+S ↔ ES is fast step, ES ↔ E + P is slow step (rate-limiting)

5. changes in substrate concentration are small, assume that [S] ≈ [S₀]

6. enzyme exists in only two forms: free enzyme and ES complex