Physiology, Proteins

Excerpt

Proteins are biopolymeric structures composed of amino acids, of which 20 are commonly found in biological chemistry. Proteins serve as structural support, biochemical catalysts, hormones, enzymes, building blocks, and initiators of cellular death. Proteins can be further defined by their four structural levels: primary, secondary, tertiary, and quaternary. The first level is the primary structure because it is the most basic protein structure. It is composed of the linear order of amino acid residues. All of the residues connect via peptide bonds. These linkages have designated carbon atom positions of alpha, beta, and gamma, corresponding to specific positions relative to the peptide linkage. This structure also has the name of the protein backbone.

The second level of protein structure is the secondary structure, and it consists of the various shapes formed via hydrogen bonding. These shapes include alpha helix, beta-pleated sheet, and beta-turn. As previously stated, hydrogen bonds stabilize all of these shapes. The third level of protein structure is the tertiary structure. It consists of the three-dimensional shape that forms when the polypeptide chain "backbone" interacts with an aqueous environment, which immediately begins to form when a newly synthesized polypeptide chain exits the terminal end of the ribosomal subunit complex. The polypeptide chain sequesters hydrophobic residues and exposes those that are hydrophilic to achieve thermodynamic stability. This thermodynamic stability is further driven by various chemical interactions, including hydrogen bonds, Vanderwall forces, and ionic bonding (the term ionic bonding includes electrostatic interactions and salt bridges). These interactions can produce energy ranging from 0.1 to 3 kilocalories per mole. The fourth and final level of protein structure is called the quarternary structure. This level is when complexes form from multiple polypeptide chains called subunits. An example is hemoglobin and how its tetrameric structure forms when chemical interactions hold 2 alpha and 2 beta subunits together. Therefore, it is appropriate to say that quaternary structure is the three-dimensional arrangement of two or more polypeptides in a protein, each folding independently. It is important to note that the term subunit is interchangeable with protomer. An example of clinical significance is in sickle cell anemia, whereby the hemoglobin protein possesses insoluble amino acids in an aqueous environment, driving the defective hemoglobin to aggregate to hide newly formed hydrophobic residues and achieve thermodynamic favorability. These altered hemoglobin molecules then form polymers that manifest as long, inflexible rods. These macromolecules continue to elongate until they eventually precipitate and distort the red blood cell's plasma membrane into the classic sickle shape seen in a sickle cell crisis.