Biomolecules: Haemoglobin found in red blood cells is best classified as which type of macromolecule?

Introduction / Context:
Haemoglobin is the oxygen-transport metalloprotein in vertebrate red blood cells. Knowing its biochemical class clarifies its structure–function relationship, cooperative binding behavior, and distinction from enzymes and simple amino acids.

Given Data / Assumptions:

• Haemoglobin consists of four polypeptide chains (globins) and four heme prosthetic groups.
• Each heme contains an iron ion that binds oxygen reversibly.
• Function is transport, not catalysis.

Concept / Approach:

Proteins are polymers of amino acids. Haemoglobin is a conjugated globular protein: a polypeptide (protein) plus a non-protein prosthetic group (heme). Its cooperative oxygen binding (sigmoidal curve) is a hallmark of multimeric proteins, not enzymes in general, though some proteins can be catalytic; haemoglobin is not.

Step-by-Step Solution:

Verification / Alternative check:

Structural biology shows α2β2 tetramer arrangement and heme–Fe2+ centers. Kinetics of O2 binding are transport-oriented, not catalytic turnover.

Why Other Options Are Wrong:

Amino acid: Monomer unit; haemoglobin is a polymeric protein. Biological catalyst/Enzyme: Haemoglobin is not an enzyme; it does not catalyze chemical reactions in vivo.

Common Pitfalls:

Equating all proteins with enzymes; overlooking the distinction between structural/transport proteins and catalytic enzymes.

Final Answer:

Protein (conjugated globular protein with heme)