B7: Enzymes

By Monika Visnevska

What?

• Enzymes are biological catalysts.

• Enzymes increase rate of reaction by providing an alternate pathway with lower activation energy.

• They are responsible for thousands of methabolic processes in living orgnisms.

B.7.1 Describe the characteristics ofbiological catalysts (enzymes).

• Enzymes are proteins (mostly)• Highly specific for their substrates• Work in narrow pH range and temperature• Enzyme activity depends on their tertiary and

quaternary structure.• Work by either lock and key mechanism or by

induced fit model (enzymes change shape to fit shape of substrate)

B.7.2 Compare inorganic catalysts andbiological catalysts (enzymes).

Inorganic catalysts Biological catalysts (enzymes)

Mineral ions/simple inorganic molecules Globular proteins (complex organic molecules)

Can catalyze several different reactions Catalyze specific typesof reactions (substrate specific)

Wider range, less sensitive to pH or temperature changes

Narrow pH and temperature range

Increase rates by fractions Increase rates by 10^3-10^6

Are not regulated by other molecules Are regulated by specific molecules (cofactors)

Are not synthesized in living cells Are synthesized in living cells by ribosomes

Increase rate of reaction by lowering activation energy

B.7.3 Describe the relationship betweensubstrate concentration and enzyme

activity.

• At low [S] (substrate conc.) rate of reaction is proportional to [S], 1st order,as there are active sites available for binding.

• As [S] further increases, rate increase slows down and eventually stops, due to occupation of active sites.

• At very high [S], rate is constant (max rate), 0th reaction order (unaffected), because all enzymes are saturated with substrates.

B.7.4 Determine Vmax and the value of the Michaelis constant (Km) by graphical means and explain its significance.

• Vmax: maximum rate of reaction, when enzyme is saturated with substrate

• Km: is the concentration of substrate which permits the enzyme to achieve half Vmax. (experimentally determined)

The higher Km, the lower enzyme's activity.

Importance of determining Km

• The Km of an enzyme, relative to the concentration of its substrate under normal conditions permits prediction of whether or not the rate of formation of product will be affected by the availability of substrate.

An enzyme with a low Km relative to the physiological concentration of substrate, is normally saturated with substrate, and will act at a more or less constant rate, regardless of variations in the concentration of substrate within the physiological range.

An enzyme with a high Km relative to the physiological concentration of substrate, as shown above, is not normally saturated with substrate and its activity will vary as the concentration of substrate varies, so that the rate of formation of product will depend on the availability of substrate.

If two enzymes, in different pathways, compete for the same substrate, then knowing the values of Km and Vmax for both enzymes permits prediction of the metabolic fate of the substrate and the relative amount that will flow through

each pathway under various conditions.

B.7.5 Describe the mechanism of enzyme action, including enzyme substrate complex, active site and induced fit model.

GENERAL PRINCIPLE

1. Substrate binds to active site of enzyme

2. Enzyme-substrate complex forms

3. Products + unchanged enzyme

Induced fit model

• Specific substrate interacts with the active site of enzyme, and both slightly change their shape to fit together.

B.7.6 Compare competitive inhibition andnon-competitive inhibition.

• Inhibitor: chemical that prevents substrate binding to enzyme; deactivates enzyme.

• Competitve: when a inhibitor chemically resembles substrate and therefore can bind to enzyme's active site instead of substrate.

• Non-competitive: compound binds to other site than active site what alters the shape of active site thus preventing the binding between substrate and enzyme.

B.7.7 State and explain the effects of heavymetalions, temperature changes and pH changes on enzyme activity.

Heavy metal ions:• React irreversibly with -SH group by replacing H

atom and forming a covalent bond between S atom and the heavy metal ion

• Ion poisons enzyme by binding to active site • Ion disrupts folding of enzyme, structure

alterations ->> Lead to stuctural changes, reduction in activity

Heavy metal examples: mercury, chromium, thalium, lead, zink, nickel.

Temperature changes• Increacing temperature increases rate of

enzymatic reaction, i.e. product formation, until a certain point. According to collision theory, increased temperatures result in more frequent collisions between enzyme and substrate.

• Rate of reacion drops sharply when certain denaturation temperature is reached, i.e. enzyme disintegrates, since hydrogen and other non-covalent bonds are broken.

pH changes

• At narrow pH range, enzyme is active.

• Deviations from optimum pH cause denaturation of enzyme.

• https://www.youtube.com/watch?v=vTQybDgweiE