4. Enzymes

What is the role of inhibitors?

some metabolic poisons are enzyme inhibitors

-potassium cyanide- inhibitor of a vital enzyme in aerobic respiration called cytochrome coxidase

Some medicinal drugs work by inhibiting the activity of enzymes

-aspirin-competitive inhibitor of COX enzymes which synthesise prostaglandins which cause pain + inflammation

What are non-competitive inhibitors?

Bind to an allosteric site (a part of the enzyme other than the active site)

causes the tertiary structure of the enzyme to change meaning the active site changes shape

active site no longer complementary

often bind permanently to the enzyme and their action is therefore irreversible

inhibition is unaffected by substrate conc

What are competitive inhibitors?

similar shape and structure to the substrate

complementary in shape to the active site and compete with the substrate to bind with the active site

enzyme-inhibitor complex is formed, blocking the active site and this prevents the enzyme from catalysing a reaction

can be overcome by increasing substrate conc.

What are enzyme inhibitors?

any substance or molecule that slows down or stops an enzyme-controlled reaction

there are two types of inhibitor: competitive and non-competitive

What is it called when the cofactor is added and the enzyme is activated?

holoenzyme

What is a precursor called?

apoenzyme

What are precursor groups?

cofactors that are required by certain enzymes to carry out their catalytic function

tightly bound and form a permanent feature of the protein

where are inorganic cofactors obtained?

minerals

where are coenzymes obtained?

vitamens e.g. B3

What is a coenzyme?

when a cofactor is an organic molecule

What is a cofactor?

non-protein ‘helper’ component that helps to carry out their functions

may transfer atoms or groups from one reaction to another in a multi-step pathway or they may actually form part of the active site of the enzyme

What is renaturation?

if the pH change isn’t extreme and is able to return to the optimum pH the protein will return to its normal shape and be a catalyst again

Key words to use when describing factors affecting enzyme activity

• collision frequency

• denaturation

• precise tertiary structure

• complimentary

• frequency of successful collisions

• excess

• limiting factor

What is the Course of enzyme-catalysed reactions?

-Initially there is a high concentration of substrate molecules but no products

-There are therefore many successful collisions between the substrate and the enzyme's active site

-All the active sites are filled and the substrate is rapidly broken down

-As the reaction proceed the concentration of substrate molecules decreases as they are broken down and the concentration of products increases

-It then becomes less likely that substrate molecules will come into contact with enzyme active sites because there are fewer substrate molecules to come into contact with the enzyme active sites.

-Therefore it takes longer for remaining substrate molecules to be catalysed by the enzyme so the rate of formation of product is slowed

-The graphs flatten off/plateau when all of the substrate molecules are used up and so no new products are made

What is Vmax?

the maximum reaction rate of the enzyme (when the enzyme is saturated with substrate)

All the enzyme's active sites are bound to a substrate

Number of ESC= number of enzymes present

What is Q10?

Q10 is the temperature coefficient of a reaction

It is the factor by which the rate of reaction increased with a 10 degree rise in temperature

For most enzyme-controlled reactions, Q10=2

What factors affect enzyme activity?

• no. of collisions

• shape of the active site