Haemoglobin is a globular protein found inside red blood cells. Since it is globular it is therefore soluble in water and folded into a complex 3D structure.
Each haemoglobin molecule is made up of four polypeptides which are two alpha globins and two beta globins. In the center of each polypeptide there is a haem group that has a atom of ferrous ion in the center of the haem which forms a temporary bond with an oxygen molecule
Diagram of haemoglobin
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Therefore each haemoglobin molecule can carry four oxygen molecules since there are four polypeptides in each haemoglobin.
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Collagen is a fibrous protein that has structural properties and is found in structures such as ligaments, and bone. Since it is a fibrous protein it is insoluble in water and have simple shapes such as a helix.
Collagen is made up of three identical polypeptides that form left handed helices which are wound together to form a triple helix.
diagram of collagen
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The triple helix is then joined together by covalent bonds to form a network of collagen fibrils
Diagrams of triple helices joined by covalent bonds
The end of the triple helices do not codincide so there are no areas of weakness which makes collagen suitable for structural properties
