Nature's Robots: A History of Proteins

By Charles Tanford; Jaqueline Reynolds | Go to book overview

CHAPTER 8
Amino acid sequence

The enzymatic breakdown of proteins always leads to mixtures of
many substances of unequal molecular size…. The isolation of
definite chemically-characterized albumoses or peptones from these
mixtures is a thankless task.

F. Hofmeister(1908)1

Knowing the amino acid composition of a protein hydrolysate, even with high accuracy, is just like having a sackful of letters of the alphabet, all mixed up. Knowing the arrangement of these amino acids along a polypeptide chain—or, sometimes, several connected polypeptide chains —is another matter altogether: words and sentences are now formed. This experimental transition was first achieved by Cambridge biochemist Frederick Sanger (1918–..) during a period lasting more than ten years, from about 1945 to 1955.


The amino acid sequence of beef insulin

The beginning of the project may be marked by a curious feature of Erwin Brand’s amino acid analyses. He was the great authority on overall amino acid composition, to whose work we referred in the preceding chapter. He especially favoured the use of microbial assays, each specific for a single amino acid, but his data invariably included a figure for free α-amino groups—that is, amino groups at the ends of polypeptide chains, not involved in peptide links. These figures could not, of course, be measured by microbiological methods, but were based instead on combining a direct chemical procedure for the total content of free amino groups, as measured before proteolysis, with the specific value in the hydrolysate for lysine, the one amino acid known to have a free amino group on its side chain. Subtracting one from the other should yield the desired number of chain-terminal α-amino groups. This pro-

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