Harper's Illustrated Biochemistry, Volume 1 - Murray R. 1993

Structure and Functions of Proteins and Enzymes
Proteins: Structure and Properties
Methods for Determining Primary Structure

First, Conjugated Proteins are stripped of their prosthetic groups (such as heme) and their Disulfide Bonds are oxidized, yielding linear Polypeptides (see Fig. 4.10). The sequencing Methods for these polypeptides were already discussed in Chapter 4. Most proteins contain only the Amino Acids listed in Table 3.3, though some proteins feature derivatives of these amino acids (Tables 5.4 and 5.5). Methods for identifying Amino Acid Derivatives fall outside The Scope of this chapter; we will simply note that their presence complicates Primary Structure determination.

Class="center">Table 5.4. Modification of a-COOH and a-NH₂ groups within proteins ¹⁾

¹⁾ Adapted, with kind permission From the Authors, from Uy R., Wold F.: Posttranslational Covalent Modification of proteins. Science: 1977: 198: 890.

Introduction/19.html">Primary structure of Insulin and Ribonuclease

Insulin consists of two polypeptide chains covalently linked by disulfide bonds (Fig. 5.9). The A-chain has a Gly residue at the N-terminus and an Asn residue at the C-terminus; in the B-chain, the N- and C-terminal residues are Phe and Ala, respectively. Oxidation of insulin with performic acid cleaves the disulfide bonds connecting the A- and B-chains. During Biosynthesis, both chains are initially part of a single proinsulin polypeptide chain, which undergoes post-synthesis proteolytic Processing to yield insulin (Chapter 51).

Table 5.5. Modified functional groups in amino acid side chains within proteins¹⁾

¹⁾ Adapted, with kind permission from the authors, from Uy R., Wold F.: Posttranslational covalent modification of proteins. Science: 1977: 198: 890.

Fig. 5.9. Human insulin A- and B-chains connected by disulfide bonds.

Ribonuclease consists of a single chain of 124 residues, with Lys at the N-terminus and Val at the C-terminus. Eight Cysteine residues are joined by disulfide bonds, forming four cross-links within the protein (Fig. 5.10).