2.3.4—Globular proteins

Globular proteins are compact, roughly spherical proteins that are generally soluble in water. Their folded shape places many non-polar hydrophobic R groups inside and exposes polar hydrophilic R groups to the surrounding water.

• Folding: R-group interactions fold the polypeptide into a compact tertiary structure rather than a long open chain.
• Solubility: hydrophobic R groups are mostly shielded inside, while hydrophilic R groups interact with water at the surface, helping the protein remain dispersed.
• Specific shape: the fold creates a particular three-dimensional surface, pocket or binding site.
• Physiological roles: solubility allows transport in body fluids and access to aqueous reactions; the specific shape allows roles such as enzyme catalysis, antibody recognition or transport.

Haemoglobin is a globular protein whose folded subunits remain soluble in blood and position haem groups for oxygen transport. Enzymes are another example: their globular folds create active sites that bind particular substrates. These examples show different functions arising from the same broad globular class, not one universal globular-protein job.

“Globular” describes typical shape and solubility, not a single function or a guarantee that every protein is perfectly spherical. A change in conditions or sequence can alter the interactions that maintain the fold, reducing the shape-dependent function or solubility.