Enzymes- biological catalysts

• Enzymes are proteins, eg. amylase, lipase, protease

• Activity depends on tertiary and quaternary structure and the specificity of enzyme action

Tertiary structure- intermolecular forces (strong and weak)

•van der Waals forces•Ionic bonds•Hydrogen bonds•Disulphide bridges

Amylase- degrades starch

Protease- degrades protein (HIV)

Lipase- degrades lipids

Efficiency• Very effective: Can speed up a reaction 108

to 1020 times• An enzyme can catalyse a specific reaction,

e.g. degrade starch to maltose•  Enzymes operates under mild conditions

(temp, pH etc)- or they get denatured• Inorganic catalysts often works under

harder condition, often on solid surfaces etc.

Catalyst

• Speed up the rate of a thermodynamically unfavourable reaction

• Doesn’t change or isn’t consumed in the process

• Doesn’t alter the H or G in the reaction• Doesn’t change the position of an

equilibrium or the equilibrium constant• The reaction takes another path

A catalyst lowers the activation energy (Ea)- the energy needed to start a reaction.If the Ea is reduced the reaction will go faster

Enzymes lower the Ea

Reactant + Enzyme ES Product + EnzymeSubstrate, S Enzyme-Substrate complex 

The enzyme can now catalyse a new substrate molecule

Induced-fit: the tertiary structure of the enzyme changes

Inhibition of enzymes

• The presence of other molecules, inhibitors, decrease the enzymes activity

• Irreversible competition- the inhibitor binds covalently to the enzyme- it is POISONED

• Reversible competition- the inhibitor binds loseley and can let go again

Competitive inhibition• A competitive inhibitor binds to the active site,

but can’t change it into products• Often chemically similar to the substate

Non-competitive inhibition

• A non-competitive inhibitor does not bind to the active site, but changes the active site

• Often heavy metal ions

Enzyme kinetics- Michaelis-Menten equations

Substrate + Enzyme SE-complex Product + Enzyme

If we have no substrate present, then there will be no reaction, so at [S] = 0, the rate will be 0.

• Adding substrate [S] to the vessel will increase the amount of substrate binding to the enzyme and the reaction rate will increase. If we keep adding substrate however, there will come a point when all the enzymes are working as fast as they can and are essentially saturated, therefore, the rate will not be able to increase regardless of how much extra substrate is added.

• Thus, the plot of increasing substrate will be a curve that eventually flattens out reaching a maximum reaction rate (known as Vmax).

www.mymcat.com/wiki/Enzyme_Inhibition

Substrate + Enzyme SE-complex Product + Enzyme

substrate

From the Michaelis-Menten equations, a constant (Km) is used which relates the equilibrium constants of the forward and reverse reactions all together. This point, where [S] = Km, has a rate exactly equal to Vmax/2. A high Km value implies the enzyme is not very good at holding onto its substrate whereas a low Km implies the enzyme has a very high affinity for its substrate.

Competetive inhibitor

Increasing the concentration of the inhibitor will decrease the reaction rate, however, Vmax can still be achieved if enough substrate is added to out compete the inhibitor.

The right graph will shift where the x-intercept (1/Km) is, but the y-intercept (1/Vmax) will stay the same.

Non-competetive inhibitor

When noncompetitive inhibitors are added to a reaction, Vmax reduces as the enzyme can no longer function as well. The Km for the reaction does not change because while the enzyme isn't catalyzing the reaction as well, it can still bind the substrate just as well as it did before. Km is a measure of the affinity of the substrate to the enzyme.In the plot to the right the y-intercept increases (because Vmax in 1/Vmax is decreasing) but the 1/Km position stays the same.