The structure, composition, and chemistry of proteins play an important role in how they function both in food systems and in the human body (Al-Holy and Rasco 2007). Proteins are comprised of chains of amino acids ranging from 50 to 5000 amino acids, often covalently bonded to other chains of amino acids to comprise its quaternary structure (Salway 2004). For example, all proteins contain an independent-R group, an amine group, and a hydroxyl group (Al-Holy and Rasco 2007). Amino acids are essentially zwitterions because they contain both an acidic and basic groups (positive and negative charges) which makes them amphoteric (Salway 2004). The R group is what makes the protein unique and provides chemical properties: glycine has a hydrogen atom for its R group, whereas alanine has a methyl group (Salway 2004). Consequently, amino acids can be classified based on their R group depending on charge, polarity, and hydrophobicity (Al-Holy and Rasco 2007). Amino acids that are polar are easily soluble in water (Al-Holy and Rasco 2007). Likewise, nonpolar amino acids are not readily soluble in water and contribute to the hydrophobic tendencies of proteins (Al-Holy and Rasco 2007). Amino acids are joined together by covalent, peptide bonds which present 40% double bond character owing to the fact that the peptide bond is planar and has no rotation, which makes them extremely stable (Friedli 1996). The amine group of an amino acid joins with the carboxyl group of another amino acid to form a peptide bond, leaving the R group free to partake in chemical reactions (Al-Holy and Rasco 2007). Such chemical reactions include protein hydrolysis and alkali reactions, discussed later (Al-Holy and Rasco 2007).
The structure of an amin...
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