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Physiology, Proteins

https://www.ncbi.nlm.nih.gov/books/NBK555990/

Introduction

Proteins are biopolymeric structures composed of amino acids, of which there are 20 commons 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 level of 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, which correspond 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 form 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 will form 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; this is all to achieve thermodynamic stability. This thermodynamic stability is further driven by a variety of chemical interactions to include hydrogen bonds, Vanderwall forces, and ionic bonding (the term ionic bonding includes electrostatic interactions and salt bridges). The energy that these interactions can produce ranges 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 of this is hemoglobin and how its tetrameric structure forms when two alpha and two beta subunits are held together by chemical interactions. Therefore it is appropriate to say that quaternary structure is the three-dimensional arrangement of two or more polypeptides in a protein, each of which folds independently of the other. It is important to note that the term subunit is interchangeable with protomer.[1] An example of the clinical significance is in sickle cell anemia, whereby the hemoglobin protein made possesses amino acids that are insoluble 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.