Biochemical reactions 2

Question 1

How is enzyme activity controlled/regulated (5)

Answer 1

1. Enzymes sometimes need to be controlled. An organism can create its own molecules to slow down and stop the activity of enzymes and proteins.
2. Activators: Makes enzymes work faster. Your body/food can then create activators. Hormones (e.g. adrenaline) can trigger responses that activate enzymes (i.e. glycogen phosphorylase).
3. Inhibitors: Opposite of activators - slow down or stop the activity of an enzyme. Bond to the protein, changing the overall shape of the enzyme - when the shape changes, the enzyme will not work the same way.
4. Temperature: Proteins change shape as temperatures change. High enough temperatures will cause the enzyme to denature and have its structure start to break up.
5. pH Levels: The acidity of the environment changes the shape of proteins in the same way that temperature does. Also, affects the ionisation of amino acids.

Question 2

What is the effect of pH on enzyme activity (3)

Answer 2

1. Usually effective in a narrow pH range – denature beyond this – disruption of intermolecular interactions
2. Optimal pH may be due to the exact nature of the chemical reaction it performs.
3. Chymotrypsin pH optimum: 7.8 (pH 6.0: about 35% of maximal activity, pH 9.3: 40% of maximal activity)

Question 3

How is enzyme activity regulated via activators and inhibitors (4)

Answer 3

1. An allosteric regulator binds reversibly to an allosteric binding site.
2. Intermolecular bonds are formed.
3. Induced fit alters the shape of the enzyme.
4. The active site is distorted and is/is not recognised by the substrate.

Question 4

How are cofactors/coenzymes used as ‘reagents’ for biochemical reactions (6)

Answer 4

1. Non-protein chemical compounds or “helper molecules” are required for the enzyme’s biological activity.
2. Subdivided into either inorganic ions or a complex organic or metalloorganic molecule called a coenzyme;
3. Most are derived from vitamins and from required organic nutrients in small amounts.
4. A cofactor that is tightly or even covalently bound is termed a prosthetic group.
5. Sometimes, “cofactor” is limited to inorganic substances.
6. An inactive enzyme without a cofactor is called an apoenzyme - a complete enzyme with a cofactor called a holoenzyme.

Question 5

How do Apoenzymes work (2)

Answer 5

1. Apoenzyme (inactive) → Haloenzyme via coenzyme.
2. Haloenzyme binds with the substrate.

Question 6

How is NAD+/NADH used as enzyme cofactors (2)

Answer 6

1. Niacin (Vitamin B3) is oxidised to form NAD+.
2. NAD+ is reduced to form NADH.

Question 7

How does Lactate dehydrogenase (LDH) use NADH to make lactate (4)

Answer 7

1. LDH catalyses the conversion of pyruvate to lactate and back, as it converts NADH to NAD+ and back.
2. A dehydrogenase is an enzyme that transfers a hydride from one molecule to another.
3. LDH is of medical significance because it is found extensively in body tissues, such as blood cells and heart muscle.
4. LDH has been of medical significance because it is found extensively in body tissues, such as blood cells and heart muscle. Because it is released during tissue damage, it is a marker of common injuries and diseases such as heart failure.