Question

Indicate whether each of the following statements about the heme-binding site in myoglobin is true or false. If you think a statement is false, explain why.
a. The proximal histidine covalently binds iron.
b. The distal histidine covalently binds oxygen.
c. The distal histidine binds iron
d. Free heme binds CO with the Fe, C and O atoms in a linear array.
e. The iron in heme binds the oxygen atom of CO

Answer 1

Answer:

A. True

B. False

C. False

D. True

E. False

Explanation:

A. The proximal histidine covalently binds iron.

This statement is true because the proximal histidine is covalently bonded to the fifth coordination position of iron in myoglobin

B. The distal histidine covalently binds oxygen.

This statement is false because the distal histidine interacts with the oxygen covalently bonded to the sixty coordination position of iron by means of a hydrogen bond not a covalent bond.

C. The distal histidine binds iron

This statement is false because the distal histidine is not bonded to iron but to oxygen but stabilizes the oxygen bonded to iron

D. Free heme binds CO with the Fe, C and O atoms in a linear array.

This statement is true because free heme has more affinity for CO than O2 as it has the least steric hindrance when the Fe, C, and O atoms lie in a straight line. On the other hand, when O2 binds to free heme, the axis of the oxygen molecule is positioned at an angle to the Fe-O bond thereby producing significant steric hindrance.

E. The iron in heme binds the oxygen atom of CO.

This statement is false because the iron in heme binds to the carbon atom, C, of CO rather than to oxygen atom.