Enzyme-substrate interactions

Name the factors that affect enzyme activity:

• pH

• Denaturing reagents

• Temperature

• Enzyme concentration

• Substrate concentration

• Inhibitors

pH

Affects ionisation of side chains and active site; extreme pH causes denaturation.

Denaturing reagents disrupt protein structure. Name some denaturing reagents:

• Detergents

• Urea

• Guanidinium hydrochloride

Temperature

Increases kinetic energy and collision rate; high temperatures cause protein unfolding.

Enzyme concentration

More enzyme → more active sites → higher maximum rate.

Substrate concentration

Rate increases with substrate until enzymes are saturated.

Inhibitors

Reduce enzyme activity by blocking or altering the active site.

What 4 things do simple models assume?

• Only one molecule of one substrate binds to the enzyme

• Enzyme and substrate form a [ES] complex

• Enzyme converts substrate to product and product release is fast

• Products bind weakly to enzymes

What does the Michaelis–Menten curve show?

The relationship between reaction rate (v) and substrate concentration [S], showing saturation at high [S].

What’s the Michaelis-Menten equation?

What is v?

v = rate

What is [S]?

[S] = substrate concentration

What is Vmax?

Vmax = rate when all enzyme active sites are occupied

What is Km?

Km = [S] at which v = ½ Vmax.

The substrate concentration when half of all enzyme active sites are occupied.

What does it mean for “enzyme reactions to saturate”?

All enzyme active sites are occupied, the rate has reached Vmax, and increasing substrate concentration no longer increases the reaction rate.

Why do enzyme reactions saturate at high substrate concentrations?

Because all enzyme active sites become occupied, so adding more substrate cannot increase the rate further.

How can Km and Vmax be measured directly?

By plotting v against [S] (Michaelis–Menten plot).

Michaelis-Menten Plot

• Plots v against [S]

• Can be difficult to decide when V_max is reached

• Usually requires a computer programme

Lineweaver-Burk Plot

• Plots 1/v against 1/[S]

• Y-intercept = 1/V_max

• X-intercept = -1/K_m

• Higher precision;

• Lower accuracy;

• Errors are not equal at all points (least squares regression is not appropriate).

Direct Linear Plot

• x axis = Km

• y axis = Vmax

• Each data point gives a straight line; all lines intersect near true Km and Vmax

• Gives median value

• Makes no assumptions about the errors

• Accurate

What is kcat?

kcat = Vmax ÷ amount of enzyme.

It is the first-order rate constant for conversion of substrate to product.

What does kcat/Km represent?

A measure of enzyme efficiency (specificity constant) that allows comparison between different enzymes.

What are 6 assumptions of the steady state model (Michealis-Menten)?

• Substrate is in large excess over enzyme ([E] « [S])

• [S] remains constant

• Measured rate is initial rate (linear start of curve)

• [ES] remains constant

• The product binds weakly to enzyme

• The backwards reaction is neglible

If V_max = 0.5 what does that mean?

• The maximum rate of the enzyme catalysed reaction is 0.5 units per second. 

• When the enzyme is working as fast as it possibly can (all enzyme active sites are full of substrate), it can make 0.5 units of product per unit time.

What does Km tell us?

It tells us how strongly an enzyme binds to its substrate.

What does a low Km mean?

A low Km means the enzyme binds tightly (it works well even at low substrate levels).

What does a high Km mean?

A high Km means the enzyme binds weakly (it needs more substrate to work efficiently).

What does Kcat  show?

Kcat shows how fast the enzyme can convert substrate to product once the substrate is bound.

Exam Question:

In an enzyme kinetic experiment, the value for 1/V_max was 0.102 min/nmol and -1/K_m was -3.45 mM^-1 from a Lineweaver-Burk (double reciprocal) plot.

Each assay contained 0.14 mg of enzyme with a molecular weight (M_W) of 47 146 Da.

Showing your workings, calculate values for V_max, K_m, k_cat, k_cat/K_m, assuming one active site per monomer (hint: k_cat = V_max divided by amount of enzyme) [10 marks].

• 1/V_max = 0.102 min/nmol

• -1/K_m = -3.45 mM^-1

• Lineweaver-Burk Plot

• Each assay contained 0.14 mg of enzyme with a molecular weight (M_W) of 47 146 Da.

Calculate values for V_max, K_m, k_cat, k_cat/K_m