IB Biology Topic B1.2.12: Relationship of Form and Function in Globular and Fibrous Proteins Notes

Fibrous and Globular Proteins: Shape Determines Function

1. Proteins fall into two broad structural categories: fibrous proteins and globular proteins.
2. The difference lies in their shape, and their shape directly determines what they do.

Fibrous Proteins: Built for Structural Support

1. Fibrous proteins are long, elongated polypeptide chains arranged into rope-like or sheet-like structures rather than compact globular shapes.
   1. Multiple chains link together into extended fibres or filaments, held together mainly by hydrogen bonds between the chains.
2. This elongated shape gives fibrous proteins high tensile strength, making them suited to structural roles.
3. They are generally insoluble in water because their surfaces are dominated by hydrophobic R-groups.

Globular Proteins: Compact and Chemically Active

How Globular Shape Supports Function

1. Globular proteins fold into compact, roughly spherical shapes.
   1. This folding is stabilised by R-group interactions: hydrogen bonds, ionic bonds, disulfide bonds, and hydrophobic interactions.
   2. The result is a specific three-dimensional conformation that is essential for function.
2. Their hydrophilic R-groups face outward, so most globular proteins are soluble in water.
   1. This lets them work in the cytoplasm, blood plasma, and other aqueous environments, where they carry out catalysis, signalling, transport, and immune defence.

Comparing Fibrous and Globular Proteins