In a globular protein, the amino acid chain twists and folds in a manner that enhances the protein's solubility in water by placing polar groups of atoms at the protein's surface (where they can participate in attractive interactions with water molecules). This twisting and folding that determine the overall shape of a protein molecule (its tertiary structure) are due largely to the very complex interplay of intramolecular forces that exists among different groups of atoms within the molecule, and to intermolecular forces acting between groups of atoms on the protein and molecules in the protein's immediate surroundings. Some of the common amino acids have polar side chains; this polarity is the result of chemical bonds between atoms in these side chains that have very different electronegativities (for instance, oxygen and hydrogen). A protein's polar side chains tend to exert strong attractive forces toward other polar groups of atoms within the protein molecule, or toward polar molecules in the protein's surroundings. Similarly, nonpolar side chains exert attractive forces (of a different nature) toward other non-polar side chains within the protein. The shape assumed by a globular protein molecule tends to maximize both types of attractive forces, whereby
nonpolar side chains "point" inward and interact with one another and polar side chains are oriented outward such that they are exposed to adjacent polar water molecules. Over the last few decades, molecular shape and structure have been experimentally determined for several thousand proteins.
The aqueous solubility of globular proteins allows them to exist in biological fluids as individual molecules or in small clusters and to accomplish a wide range of critical biological functions, for example, the enzymatic catalysis of chemical reactions. Globular proteins also function as anti-bodies in the body's immune system and as transport vehicles for other molecules in circulating blood, and they are heavily involved in the replication and repair of DNA . Figure 1 shows the molecular structure of polymerase β , a much-studied globular protein that catalyzes reactions having to do with the repair of damaged DNA. The structure of polymerase β was determined by Huguette Pelletier and her coworkers using the technique of x-ray diffraction at the University of California, San Diego.
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Pelletier, Huguette; Sawaya, Michael R.; Kumar, Amalendra; Wilson, Samuel H.; and Kraut, Joseph (1994). "Structures of Ternary Complexes of Rat DNA Polymerase Beta, a DNA Template Primer and ddCTP." Science 264:1891.