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Classifications and Examples of Protein Interactions
Proteins are high molecular weight chain structures derived from some 20 amino acids joined together through the formation of peptide linkages, -(C=O)-NH2-. The sequence of amino acids synthesized intercellularly is predetermined by genetic code. Proteins interact with a variety of molecule types, subdivided into five main classifications:
- Protein - protein interactions
- Protein - polynucleotide interactions
- Protein - lipid interactions
- Protein - polyphenol interactions
- Protein - ligand interactions
- Protein - solvent interactions
It is the protein-protein interaction that is most commonly studied. One such important interaction occurs at the cell membrane, which has an outer and an inner layer of hydrophilic (water-loving) phospholipids. Many protons pierce through the entire thickness of the cell membrane and at the outside surface serve as signal receptors. Messengers from outside the cell—for example, adrenaline—trigger receptor proteins which then trigger the release of other messenger species at the other end of the protein, this time inside the cell. One internal messenger triggered is the so-called G-protein.
Credit: GM Cooper - The Cell: a Molecular Approach 2nd Ed, 2000.
Building new cells requires abundant raw materials including proteins. How are the correct proteins produced? This is by means of protein-polynucleotide interactions. Polynucleotides such as DNA and RNA serve as templates for the manufacture of molecules of specific proteins that are to be used. These templates may even dictate the way proteins are folded. In this way, polynucleotide contents of already-existent cells produce needed starting materials for the formation of more cells—cellular reproduction.
In addition to membrane-insertion proteins, some proteins do not penetrate the membrane, but attach to it directly. This is an example of protein-lipid interactions. Such interactions are immensely complex and not well understood; thus they are beyond the scope of this article.
Protein-polyphenol interactions are of interest, since many nutritional substances such as anti-oxidants, anti-carcinogens and anti-mutagens are polyphenols. These are known to interact with proteins, which leads to the formation of complexes between the two substances, some water-soluble and some insoluble. This is of interest because of the issue of bioavailability. Do these complexes reduce or ehance nutritional value?
Protein-ligand and protein-solvent interactions may be studied by use expensive instrumentation, including a variety of spectroscopies, fluorescence techniques and calorimetry. Computational methods are being used more and more frequently, due to the increased capabilities of personal computers. This latter method allows a comparison with empirical data that offers insights and suggests likely theories of molecular interaction.