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What is an enzyme?

Biological catalyst. Speeds up chemical reactions by lowering activation energy. They are not used up in the reaction/remain unchanged. Intracellular - act within the cell. Extracellular - act outside the cell. Specific to one substrate(s)/reaction.


Give an example and function of an intracellular enzyme.

Catalase - breaks down hydrogen peroxide, formed during metabolic processes, into water and oxygen.
Four polypeptide chains and a haem group.
Held in vesicles in white blood cells, breaks down ingested pathogens.


Give an example and function of an extracellular enzyme.

Amylase - produced in salivary glands, hydrolysis amylose to maltose.
Trypsin - made in pancreas, released into lumen of small intestine; hydrolysis peptide bonds.


What is a cofactor?

A substance that must be present to ensure enzyme reaction takes places. Prosthetic group or coenzyme.


What is a prosthetic group?

Molecule permanently bound to enzyme molecule via covalent bond. Carbonic anhydrase has zinc ion bound to active site.


Define a cofactor.

Ion or organic coenzyme not permanently bound to active site. Temporary bonds form between enzyme or substrate to ease formation of enzyme - substrate complex. Co-substrates bind to substrate to enable substrate to form correct complement to active site. Cofactor may change distribution of charges on surface of substrate.


What is a coenzyme?

Small organic, non-protein molecule. Binds temporarily to active site of enzyme. Coenzyme altered by reaction and is recycled to original state.


What is the active site?

Cleft or pocket on surface of enzyme. Complementary to shape of substrate.


What is an enzyme substrate complex?

Enzyme with substrate(s) held in active site. Joined by non-covalent forces.


What is an enzyme product complex?

Enzyme with product(s) held in active site. Joined by non-covalent forces.


Outline the lock and key hypothesis.

Tertiary structure of enzyme gives active site a shape that is exactly complementary to specific substrate. Substrate fits exactly into active site. Temporary hydrogen bonds form ESC. Substrate broken into product, products leaves active site.
Note: if more than one substrate enters active site to form larger product, an enzyme product complex forms as an intermediary stage before product is released.


Outline the induced fit hypothesis.

Active site has shape that complements the substrate but is not an exact fit when substrate not bound. On binding of substrate, interactions between R groups lining active site and surface of substrate cause conformational change. Enzyme active site molds around substrate. ESC forms. Hydrogen bonds, ionic interactions and hydrophobic interactions bind substrate to active site. Substrate converted to product, product remains in active site, enzyme product forms. Product has different conformation to substrate, active site relaxes. Product released.


Describe the relationship between heat and kinetic energy and enzyme catalysed reactions.

An increase in heat leads to an increase in kinetic energy. Enzyme and substrate gain kinetic energy. An increase in successful collisions leads to an increase in ESC. More product is released. Until an optimum temperature is reached.


Describe what happens to the rate of reaction as the temperature rises above the optimum.

Molecules vibrate as kinetic energy increases. Hydrogen bonds break and tertiary structure of enzyme active site alters. Substrate no longer fits into active site and rate of reaction decreases. As temperature rises further ionic interactions and disulphide bonds break and enzyme active site is irreversibly altered. The enzyme is denatured. Heat does not break the peptide binds that hold the primary structure.


What is the temperature coefficient?

For most enzyme catalysed reactions - between 0°C and 40°C the rate of reaction will double for every 10°C increase. Above the optimum temperature the coefficient drops as the enzyme active site denatures.
Q10 = rate of reaction at (x + 10) °C divided by rate of reaction at x °C.


What does optimum temperature mean?

The temperature at which an enzyme reaches its maximum rate of reaction.
Note that optimum temperatures vary according to the conditions in which an organism lives.


Define pH.

A measure of acidity/alkalinity. Acids dissociate into protons, H+ and a negative ion. Alkalis dissociate into hydroxide, OH- and a positive ion.


What is a buffer?

A chemical or molecule that resists changes in pH. They do this by accepting or donating protons or hydroxide ions.


Describe the effect of changes in pH on bonds within enzymes.

Protons carry a positive charge and are attracted to negative R groups. Hydroxide ion carry a negative charge and are attracted to positive R groups. Protons and hydroxide ions disrupt hydrogen bonds that hold secondary structures in place. Protons and hydroxide ions disrupt hydrogen bonds and ionic bonds that hold tertiary structures in place. Disruption of secondary and tertiary structures may alter the shape of the active site so that it no longer compliments the substrate. Protons and hydroxide ions may cluster around positive/negative R groups that line the active site and interfere with interactions between the active site and the substrate molecule.


Describe the effect of small changes in pH on enzyme activity.

Small changes either side of the optimum alter the shape of the active site. If it is no longer complementary to the shape of the substrate the rate of reaction reduces. If the optimum pH is restored, hydrogen bonds reform and the shape of the active site is restored. The rate of reaction is restored.


Describe the effect of extreme changes in pH on enzyme activity.

Large changes in pH away from the optimum permanently denature the enzyme active site. The rate of reaction stops.
Not all enzymes have the same optimum pH and the range at which they are active also varies depending on where it is found.


Define concentration.

The number of molecules per unit volume.


Describe the effect of changing substrate concentration on rate of reaction.

At zero concentration of substrate there is no reaction.
As substrate concentration increase there are more successful collisions between enzyme active sites and substrates.
More enzyme substrate complexes form, more product is released.
The rate of reaction increases until it reaches its maximum tare. This is the turnover number.
Beyond this concentration the rate does not increase.
All enzyme active sites are occupied.
Enzyme concentration is now the limiting factor.


State the factors which determine cellular enzyme concentration.

Enzyme synthesis, regulated by transcription and translation.
Enzyme degradation, removes surplus enzymes and abnormally formed enzymes.


Describe the effect of increasing enzyme concentration on rate of reaction.

At zero concentration there is no rate of reaction.
As concentration increased more active sites are available.
There is an increase in successful collisions between enzyme active sites and substrates.
More enzyme substrate complexes form per unit time.


Describe the effect of increasing enzyme concentration with fixed/limited substrate concentration on rate of reaction.

Increasing enzyme concentration further will of increase rate of reaction.
Substrate concentration becomes a limiting factor.


Why is the initial rate of reaction fastest?

Greatest chance of successful collisions between substrates and enzyme active sites, more ESC form.
As reaction proceeds substrate is use up, fewer molecules left to react.
Fewer successful collisions, rate decrease.
As product accumulates it impedes successful collisions between enzyme active sites and substrates.
Rate decreases/stops.


How do you calculate initial rate of reaction?

Plot a graph - this is usually time, s, on x axis against volume of gas, cm3, on y axis.
Draw tangent to line at steepest point.
Rate is change in y axis divided by change in x axis.
In this example the unit of rate is cm3/s.


What is competitive inhibition?

Molecule has a shape that is similar to the substrate.
Inhibitor fits into active site and forms enzyme inhibitor complex.
Substrate cannot enter active site.
Rate of reaction decreases.
Increasing substrate concentration overcomes the effect of a competitive inhibitor.


Describe the effect of substrate/inhibitor concentration on rates of reaction.

As inhibitor concentration increases there are more collisions between the inhibitor and the enzyme active site.
Inhibition increases, rate decreases.
As substrate concentration increases there are more successful collisions between enzyme active site and substrate.
Inhibition reduces, rate increases.