Myoglobin secondary structure analysis

The structural information gained from the analysis of myoglobin served as the basis for hypotheses on tertiary structure. However, myoglobin is an atypical globular protein since it lacks cysteinyl residues (no disulphide bonds) and P-con-formation, but contains a very large proportion of a-helical structure. Studies on over 200 globular proteins have demonstrated that polypeptide chains frequently contain both a-helical and P-sheet secondary structures (Figure 4.16). 

Perhaps one major uncertainty in the CD analysis of the secondary structures of proteins is choice of reference proteins which will adequately represent the protein to be analyzed [86Y1, 91V1]. In the early 1970s the selection was dictated by the then available X-ray structures such as myoglobin, lysozyme, ribonuclease, lactate dehydrogenase and papain. Today we have more than 300 proteins of known 3-dimensional structure at our disposal. What constitutes a representative set of reference proteins is still a subject to be investigated. Therefore, this problem may not be completely solved until different sets of reference proteins are extensively tested for the four classes of proteins. 

Many of these features are evident in the three-dimensional structures of ribonuclease and myoglobin shown in Figure II-3. To understand the common and distinguishing features of the secondary and tertiary structure of various proteins as determined by x-ray crystallographic analysis, the student should study several examples in standard textbooks of biochemistry. 

The first x-ray analysis of a protein, identifying the active site and giving details of secondary and tertiary structure, was that of lysozyme, carried out by Phillips [30] in 1966. The subsequent structure determinations of myoglobin [31] and haanoglobin [32] by Kendrew and Perutz represent great triumphs for both x-ray crystallography and protein science. 

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