Protein Structure And Properties

Amino acid residues in protein are linked by peptide bonds ( CO NH ). There are four orders of protein structure:[1] primary structure (the sequence of amino acids along the polypeptide chain); secondary structure (the conformation of the polypeptide backbone); tertiary structure (the three-dimensional arrangement of protein); and quaternary structure (the spatial arrangement of polypeptide subunits). The forces stabilizing polypeptide aggregates are hydrogen and electrostatic bonds between amino acid residues.

Proteins can be classified according to their overall shape (globular or fibrous), solubility in water (hydrophobic or hydrophilic), three-dimensional structure, or biologic function (Table 1). For example, albumin and hemoglobin are globular proteins. Fibrous proteins include collagens, elastin, a-keratins (wool and hair), and p-keratins (the feathers, skin, beaks, and scales of most birds and reptiles). Collagens are rich in proline and glycine (approximately 1/3 each), and constitute approximately 30% of total proteins in animals. Keratins are rich in cysteine; wool protein contains approximately 4% sulfur.

All proteins can be denatured by heat, acids, bases, alcohols, urea, and salts of heavy metals. The suscepti bility of proteins to heat damage is increased in the presence of carbohydrates, owing to the Maillard reaction, which involves a condensation between the carbonyl group of a reducing sugar with the free amino group of an amino acid residue (e.g., lysine).

Crude protein content in animal tissues and feeds is often obtained by multiplying the nitrogen content by a factor of 6.25, on the basis of the average nitrogen content (16%) in protein. Such calculation, however, is not very precise, because some proteins contain less or more nitrogen and because some nitrogenous compounds (e.g., ammonia, urea, amides, choline, betaine, purines, py-rimidines, nitrite, and nitrate) are neither proteins nor amino acids. The composition of amino acids in protein is often determined using liquid or gas chromatography.

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