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As a result, the read length for Edman sequencing varies from 5 to 30 amino acid residues depending upon the quantity and purity of the peptide. indd 29 03/11/14 3:58 PM E Structures & Functions of roteins & nzymes P nI sectio 30 MOLECULAR BIOLOGY REVOLUTIONIZED THE DETERMINATION OF PRIMARY STRUCTURE S C N + O H N N H NH2 R O R′ While the reactions that sequentially derivatize and cleave PTH amino acids from the amino-terminal end of a peptide typically are conducted in an automated sequenator, DNA sequencing is far more rapid and economical.

Cells contain thousands of different proteins, each in widely varying amounts. The isolation of a specific protein in quantities sufficient for analysis of its properties thus presents a formidable challenge that may require successive application of multiple purification techniques. Selective precipitation exploits differences in relative solubility of individual proteins as a function of pH (isoelectric precipitation), polarity (precipitation with ethanol or acetone), or salt concentration (salting out with ammonium sulfate).

The resulting disruption of the urea cycle leads to ammonia toxicity. Finally, -β-methylaminoalanine, a neurotoxic amino acid present in Cycad seeds, has been P l-α-Amino acids fulfill vital metabolic roles in addition to serving as the “building blocks” of proteins. As discussed in later chapters, thyroid hormones are formed from tyrosine; glutamate serves as a neurotransmitter as well as the precursor of g-aminobutyric acid (GABA); ornithine and citrulline are intermediates in urea biosynthesis; and homocysteine, homoserine, and glutamate-g-semialdehyde participate in the intermediary metabolism of the protein amino acids (Table 3–3).