Bioinorganic Chemistry Questions and Answers

Answer: A

In oxy-hemoglobin, the metal is low-spin in the +3 oxidation state while dioxygen is in the O₂^- form.

Deoxy-hemoglobin has high-spin Fe(II), too large to sit in the porphyrin plane. When O₂ binds, one electron is largely transferred from iron to oxygen (the Weiss description): iron becomes low-spin Fe(III), small enough to move into the plane, and the bound dioxygen behaves as superoxide. The evidence is the O-O stretching frequency, near 1105 cm^-1, close to that of the free superoxide ion and far below that of O₂ (1555 cm^-1). The two unpaired electrons, one on Fe(III) and one on O₂^-, are antiferromagnetically coupled, which makes oxy-hemoglobin diamagnetic.

High-spin Fe(III) would not fit in the porphyrin plane and would leave the complex paramagnetic. The neutral-dioxygen options describe the older Pauling picture, which does not match the O-O stretch.

Answer: A

The O-O stretching frequency is nearly 1100 cm^-1.

On binding to the iron in oxy-hemoglobin, O₂ takes on superoxide character (often described as Fe(III)-O₂^-). Adding an electron to a π* orbital lowers the O-O bond order from 2 to about 1.5, and the stretching frequency falls from 1555 cm^-1 in free O₂ to about 1105 cm^-1, close to that of the free superoxide ion.

1550 is free dioxygen, which would mean no electron transfer at all. About 850 is typical of peroxide (bond order 1), as in oxy-hemerythrin, not hemoglobin. 1950 is far above any O-O stretch.

Answer: A

The green pigment, chlorophyll, contains Mg(II) and a chlorin macrocycle.

Chlorophyll is a magnesium complex of a chlorin, a porphyrin-type ring in which one of the four pyrrole rings is partly reduced (one peripheral C=C is saturated). That reduction changes the ring's absorption so that it absorbs strongly in the red and blue and appears green.

The corrin ring, with one fewer bridging carbon, is the macrocycle of vitamin B₁₂, which holds cobalt. The fully unsaturated porphine (porphyrin) skeleton is the one in heme, which holds iron. Manganese is not the central metal of chlorophyll, although manganese is essential elsewhere in photosynthesis, in the oxygen-evolving complex.

Answer: B

The non-heme protein is hemocyanin.

Hemocyanin, the oxygen carrier of molluscs and arthropods, binds O₂ between two copper atoms, each held by histidine side chains. It contains no porphyrin, so it is not a heme protein; its oxygenated form is blue.

The other three all use iron in a heme (iron protoporphyrin IX) group: hemoglobin carries oxygen in blood, myoglobin stores it in muscle, and cytochrome P-450 is a heme enzyme that hydroxylates organic substrates, with a cysteine thiolate bound to the iron.

Answer: A

The frequencies are 1136 and 744 cm^-1 respectively.

In oxy-hemoglobin, O₂ binds end-on to one iron and is best described as Fe(III) with a bound superoxide, O₂^-, whose O-O stretch lies near 1100 to 1150 cm^-1; about 1136 cm^-1 is observed.

In oxy-hemocyanin, O₂ bridges two copper(II) ions side-on as peroxide, O₂^2-, and the weaker O-O bond stretches near 750 cm^-1; about 744 cm^-1 is observed in the resonance Raman spectrum.

1550 cm^-1 is the stretch of free O₂, not a coordinated one, and the reversed pairs swap the superoxo and peroxo assignments.

Answer: A

The correct matching is P-V, Q-III, R-I, S-II.

• Cu: tyrosinase, a type 3 dicopper enzyme that hydroxylates phenols and oxidises them to quinones.
• Mo: nitrate reductase, which uses a molybdenum cofactor (Mo-pterin) to reduce nitrate to nitrite.
• Co: the B₁₂ coenzyme, a cobalt corrin complex (adenosylcobalamin).
• Zn: carboxypeptidase, a zinc hydrolase that cleaves the C-terminal peptide bond.

Cytochrome P-450 (IV) has an iron porphyrin (heme) active site, so it matches none of the four metals listed and is the extra item. Every other option either pairs a metal with P-450 or swaps the cobalt and zinc enzymes.

Answer: B

The answer is 0, 2, 4.

Mineral acid releases H₂S only from acid-labile inorganic sulfide (S^2-) in an iron-sulfur cluster, not from the cysteine thiolates that bind the cluster to the protein.

• Rubredoxin has a single iron held by four cysteine sulfurs, Fe(S-Cys)₄, with no inorganic sulfide: 0.
• A 2-iron ferredoxin has an [Fe₂S₂] core with two bridging sulfides: 2.
• A 4-iron ferredoxin has a cubane [Fe₄S₄] core with four sulfides: 4.

'4, 6, 8' counts the total sulfur atoms including the cysteines, and '1, 2, 4' wrongly assigns a sulfide to rubredoxin. '0, 2, 3' undercounts the four sulfides of the [Fe₄S₄] cube.

Answer: D

In both forms the longest bond is to methionine-S.

The type 1 copper centre of plastocyanin is bound by two histidine nitrogens, the thiolate sulfur of a cysteine and the thioether sulfur of a methionine, in a distorted tetrahedral arrangement. The Cu-S(Cys) bond is short (about 2.1 Å) and strongly covalent; it gives the intense blue colour of the oxidised protein through S→Cu(II) charge transfer. The Cu-S(Met) bond is long, roughly 2.8 to 2.9 Å, and stays long whether copper is Cu(II) or Cu(I).

The protein holds the site in nearly the same geometry in both states (the 'entatic' or rack-induced state), which keeps the reorganisation energy small and electron transfer fast. So options naming cysteine for either form are wrong.

Answer: A

Zinc is essential for carboxypeptidase A.

Carboxypeptidase A is a pancreatic exopeptidase that removes C-terminal amino acids, and it is a classic zinc metalloenzyme: a Zn²⁺ ion, held by two histidines and a glutamate, polarises the carbonyl of the peptide bond and activates a water molecule for the attack.

The other molecules use different metals. Chlorophyll a has magnesium at the centre of its chlorin ring; myoglobin carries iron in its haem group; and vitamin B12 (cobalamin) is built around cobalt.

Answer: A

Carbonic anhydrase is classed among the options as a hydrolysis enzyme.

It is a zinc enzyme that catalyses the reversible hydration of carbon dioxide, CO₂ + H₂O ⇌ HCO₃⁻ + H⁺. The Zn²⁺ ion, held by three histidines, binds and activates a water molecule (as Zn-OH) that attacks CO₂; inorganic chemistry texts group it with the zinc hydrolytic enzymes such as carboxypeptidase. Formal enzyme nomenclature lists it as a hydro-lyase, but of the four options only this one fits.

It is not a redox enzyme, since Zn²⁺ has a full d shell and does not change oxidation state. It does not carry O₂ (haemoglobin, myoglobin and haemocyanin do), and it has no heme group.