Although we have just spent some time discussing fibrous proteins, many of the most interesting proteins are nonstructural but rather have specialized functional properties.
Specific functions include proteins as mediators of transport, as catalysts (enzymes), hormones, i.e. peptide hormones and immunoglobulins.
These proteins tend to be globular rather than fibrous and have more complex secondary structures. The tertiary structure of a protein is based on the relationship between secondary structures previously identified, i.e. alpha & beta helices.
As the protein's primary structural sequences defined implicitly by the nucleotide DNA sequences, the ultimate function of the globular protein is manifest at the quaternary and tertiary structural levels.
In other words, the sequence of amino acids by themselves wouldn't do anything until a highly regulated process of folding occurs dictated by the sequence and in accord with thermodynamically favored structures.
Much like our consideration of very small molecules, much larger protein molecules should not be regarded as static structures but rather a structures that exhibit internal motions exploring various configurations or confirmations.
Furthermore, some proteins such as drug receptors have specific regions called binding domains which interact with some specificity with small molecules.
The interaction itself between the small molecule in the proteins binding domain may induce conformational changes that in turn change protein function.
What we have examined earlier with respect to oxygen binding to hemoglobin affected by protons binding to special sites in influencing the oxygen-hemoglobin dissociation curve is analogous to the effect of benzodiazepines such as diazepam (Valium) on the chloride conductance property of GABA receptors.
To summarize our discussion of proteins in the components that yield higher-order structure we have:
amino acid linear sequences
alpha helices (beta sheets, turns etc.)
polypeptide chains (for example tropocollagen or mixtures of alpha & beta helices)
oligomeric proteins consisting of subunits, such as hemoglobin, nicotinic cholinergic receptors and many other receptor systems.
54Lehninger, AL, Nelson, DL, Cox, MM, "The Three-Dimensional Structure of Proteins" in Principles of Biochemistry with an Extended Discussion of Oxygen-Binding Proteins, Chapter 7, , 160-187, Worth Publishers, New York, 1993.