Fibrous versus globular proteins
Fibrous proteins are long strands or sheets that give tissues strength, whereas globular proteins are compact, usually water-soluble shapes that carry out catalysis (speeding up chemical reactions), transport and regulation.
Remember
Fibrous proteins form tough strands or sheets that give tissues strength and typically do not dissolve freely in body fluids, while compact globular proteins fold to carry out chemical jobs.
Memory aid
fibrous proteins are rope, globular proteins are balls
Fibrous means rope, a long protein built to hold things up, and globular means ball, a compact protein built to do chemistry.
Fibrous proteins are elongated (long and thread-like). Their polypeptide chains (long chains of amino acids) run alongside each other as strands or sheets, and the same type of secondary structure (a regular local fold, such as a helix) repeats along the length.
Because the chains lie close together, many hydrogen bonds form between them. The result is a tough assembly that is poorly soluble, meaning it typically does not dissolve freely in body fluids. It resists stretching and suits support, protection and shape.
Collagen is the classic example. In collagen, three chains are wound together into a rope-like triple helix, and the molecules are further tied together by covalent cross-links (bonds tying the molecules together). This gives connective tissue its tensile strength, meaning resistance to being pulled apart.
Globular proteins are the contrast. They fold into compact, rounded shapes, are usually water-soluble, and can adjust their shape slightly, which lets them act as enzymes, receptors, transporters and regulators. Haemoglobin is a globular protein, whereas collagen is fibrous.
Alpha helices and beta sheets (the two common regular folds of a polypeptide chain) are found in both groups, so the presence of these folds does not separate the two groups. Overall shape does: fibrous proteins are elongated and strong, and globular proteins are compact and active.
Key facts
- Shape
- fibrous proteins are elongated strands or sheets, whereas globular proteins are compact and rounded.
- Main job of fibrous proteins
- structural support and mechanical strength, as in collagen.
- Jobs of globular proteins
- catalysis, transport, signalling and recognition.
- Solubility
- fibrous proteins are typically poorly soluble and stay in place in tissues, whereas globular proteins are usually water-soluble.
- Secondary structure
- fibrous proteins repeat one type of secondary structure along long strands or sheets, whereas a mixture of several kinds is typical of globular proteins.
- Collagen
- three chains wind into a rope-like triple helix held by many hydrogen bonds, and the collagen molecules are further tied together by covalent cross-links.
Exam traps
- Peptide bonds are not chemically stronger in fibrous proteins than in globular proteins: they have the same basic chemistry, and fibrous proteins gain mechanical strength from their ordered assembly and interactions between chains.
- High solubility and rapid diffusion are features of many globular proteins, whereas fibrous proteins are typically hydrophobic (water-repelling) and poorly soluble, so they do not diffuse rapidly within cells.
- Alpha helices and beta sheets are found in both globular and fibrous proteins, so they do not mark a protein as globular; it is the compact folding that points to a globular protein.
Read the full notes
Tested in 6 SimpleMed+ questions
Practice it in Cell Physiology and Biology, with the answer and a full explanation after every question.
Practice these questions