Question

(i) Co-operativity and allostery are mechanisms by which an enzyme can be

regulated. Define both co-operativity and allostery and explain how regulation

occurs in each case.

(6 marks)

(ii) Hemoglobin shows co-operative behaviour, whereas myoglobin does not. What

structural differences between myoglobin and hemoglobin allow co-operativity?

(2 marks)

(iii) Hemoglobin undergoes a conformational change upon oxygen binding. Describe

this conformational change, and its role in haemoglobin’s co-operative behaviour.

(4 marks)

(iv) Fetal hemoglobin (hemoglobin F, HbF) is the major hemoglobin present during

gestation. It constitutes approximately 60 to 80 percent of total hemoglobin in the

full-term newborn. It is almost completely replaced by adult hemoglobin

(hemoglobin A, HbA) by approximately 6 to 12 months of age, and it amounts to

less than 1 percent of total hemoglobin in the adult. HbF has a higher affinity for

oxygen than HbA.

a. Show this on an oxygen binding curve (5 marks)

b. What is the adaptive advantage of this? (3 marks)

Answer 1

Ans (i)

Co-operativity

It operates with the enzyme having multiple sites for its substrates.

Co-operativity is as process of substrate enzyme interaction where binding of substrate to the one site of an enzyme increase or decrease the affinity of the other binding site towards its substrate.

Co-operativity may be positive or negative. In positive co-operativity, binding of substrate to one site of an enzyme increase the affinity of the other binding site of enzyme for its substrate whereas in negative co-operativity, binding of substrate decrease the affinity of other binding sites of enzyme for its substrate.

This can be better understood by understanding the binding of oxygen molecule with hemoglobin

Hemoglobin has four binding sites for oxygen

When one molecule of oxygen binds to the hemoglobin, it increases the affinity of hemoglobin to bind with other oxygen molecule enabling the easy binding of second and third oxygen molecule. This phenomenon is referred as positive co-operativity.

Allostery

In allostery, the effector molecule binds to the enzyme (not to active site, called allosteric site) and brings conformational changes in enzyme that influences the binding of enzyme with its substrate.

Ans (ii)

To exhibit the process of co-operativity, the macromolecule (enzyme/receptors) has to have multiple subunits.

As I mentioned above, hemoglobin has four subunits and each subunit has a heme group that allow binding of oxygen. On the contrary, myoglobin has only one subunit. This structural difference hemoglobin shows co-operativity whereas myoglobin does not show the co-operativity.

Ans (iii)

When hemoglobin is free of oxygen (prior to oxygen binding or deoxygenatated hemoglobin), the heme group exist as nonplanner moiety and iron atom exist out of porphyrin plane. In this state, hemoglobin acquired dome shape.

Upon binding with one oxygen to Fe molecule, the porphyrin ring acquire the planner configuration and entire hemoglobin molecule changes its shape that allow easier oxygenation of the remaining heme groups.

Suggestion (iv)

a)