Differences between collagen and hemoglobin regarding STRUCTURE are:

Haemoglobin consists of four polypeptides while collagen has three. Haemoglobin is globular and has a tertiary structure while collagen is fibrous and lacks a tertiary structure. Haemoglobin is conjugated (contains heme groups - example of a prosthetic group) whereas collagen is not conjugated. Haemoglobin has a functional role (carries oxygen) while collagen has a structural role.

Haemoglobin's quaternary structure is four polypeptide subunits (2 alpha and 2 beta chains) whereas Collagen's structure is 3 polypeptide chains wound around each other like rope. Haemoglobin has a prosthetic group – each chain contains a haem group (Fe2+) but collagen has no prosthetic group.

Organism, organ system, organs, tissue, cells