Principles of Biochemistry, Volume 1 - A. Lehninger 1985

Biomolecules
Proteins: Covalent Structure and Biological Functions
During hydrolysis, proteins break down into amino acids

Many Proteins have been obtained in pure crystalline form. Fig. 6-2 shows crystals of pure Trypsin, a digestive enzyme secreted into the intestine, and cytochrome c, a mitochondrial protein that Functions as an electron carrier. Upon acid or alkaline Hydrolysis, proteins—much like simple Peptides (sec. 5.22)—are cleaved to yield a mixture of free Amino Acids that served as the Building Blocks of these proteins. Each individual protein is characterized by a specific ratio of various amino acids produced by its complete hydrolysis. Table 6-2 presents data on the composition of amino acid mixtures obtained from the complete hydrolysis of cytochrome c and bovine chymotrypsinogen, the inactive precursor of the digestive enzyme Chymotrypsin. It is evident that these two proteins, which perform entirely different functions, also differ markedly in the relative Abundance of their constituent amino acids. The 20 Amino acids are never present in a protein in equimolar amounts. Some amino acids occur only once per protein molecule, whereas others appear much more frequently. Moreover, far from all proteins contain all 20 amino acids. Each individual protein possesses a characteristic ratio of amino acid building blocks.

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Fig. 6-2. A. Crystals of pure cytochrome c isolated from horse Heart. Cytochrome c is a protein that serves as an electron carrier in Cell/35.html">Mitochondria. B. Crystals of bovine trypsin.

Table 6-2. Amino Acid Composition of two proteins

Values indicate the number of residues of each amino acid per protein molecule