Most knowledge concerning secondary and tertiary structure of globular proteins has been obtained by the examination of their crystals using\u00a0X-ray diffraction. In this technique, X-rays are allowed to strike the crystal; the X-rays are diffracted by the crystal and impinge on a photographic plate, forming a pattern of spots. The measured intensity of the diffraction pattern, as recorded on a photographic film, depends particularly on the\u00a0electron\u00a0density\u00a0of the atoms in the protein crystal. This density is lowest in hydrogen atoms, and they do not give a visible diffraction pattern. Although\u00a0carbon,\u00a0oxygen, and\u00a0nitrogen\u00a0atoms yield visible diffraction patterns, they are present in such great number\u2014about 700 or 800 per 100 amino acids\u2014that the resolution of the structure of a protein containing more than 100 amino acids is almost impossible. Resolution is considerably improved by substituting into the side chains of certain amino acids very heavy atoms, particularly those of heavy metals.\u00a0Mercury\u00a0ions, for example, bind to the sulfhydryl (\u2015SH) groups of cysteine.\u00a0Platinum\u00a0chloride has been used in other proteins. In the iron-containing proteins, the iron\u00a0atom\u00a0already in the molecule is adequate.<\/p>\n\n\n\n
Although the X-ray diffraction technique cannot resolve the complete three-dimensional\u00a0conformation\u00a0(that is, the secondary and\u00a0tertiary\u00a0structure of the\u00a0peptide\u00a0chain), complete resolution has been obtained by combination of the results of X-ray diffraction with those of\u00a0amino acid\u00a0sequence analysis. In this way the complete conformation of such proteins as\u00a0myoglobin, chymotrypsinogen, lysozyme, and ribonuclease has been resolved.<\/p>\n\n\n\n
The X-ray diffraction method has revealed regular structural arrangements in proteins; one is an extended form of antiparallel peptide chains that are linked to each other by hydrogen bonds between the carbonyl and imino groups. This conformation, called the pleated sheet, or \u03b2-structure, is found in some fibrous proteins. Short strands of the \u03b2-structure have also been detected in some globular proteins.<\/p>\n\n\n\n
A second important structural arrangement is the \u03b1-helix; it is formed by a sequence of amino acids\u00a0wound\u00a0around a straight axis in either a right-handed or a left-handed spiral. Each turn of the helix corresponds to a distance of 5.4 angstroms (= 0.54 nanometre) in the direction of the screw axis and contains 3.7 amino acids. Hence, the length of the \u03b1-helix per amino acid residue is 5.4 divided by 3.7, or 1.5\u00a0angstroms\u00a0(1 angstrom = 0.1 nanometre). The stability of the \u03b1-helix is maintained by hydrogen bonds between the carbonyl and imino groups of neighbouring turns of the helix. It was once thought, based on data from analyses of the myoglobin molecule, more than half of which consists of \u03b1-helices, that the \u03b1-helix is the\u00a0predominant\u00a0structural element of the globular proteins; it is now known that myoglobin is exceptional in this respect. The other globular proteins for which the structures have been resolved by X-ray diffraction contain only small regions of \u03b1-helix. In most of them the peptide chains are folded in an apparently random fashion, for which the term\u00a0random coil<\/em>\u00a0has been used. The term is misleading, however, because the folding is not random; rather, it is dictated by the primary structure and modified by the secondary and tertiary structures.<\/p>\n\n\n\n The first proteins for which the internal structures were completely resolved are the iron-containing proteins myoglobin and hemoglobin. The investigation of the hydrated crystals of these proteins by Austrian-born British biochemist\u00a0Max Perutz\u00a0and British biochemist\u00a0John C. Kendrew, who won the 1962\u00a0Nobel Prize\u00a0for Chemistry for their work, revealed that the folding of the peptide chains is so tight that most of the\u00a0water\u00a0is displaced from the centre of the globular molecules. The amino acids that carry the ammonium (\u2015NH3<\/sub>+<\/sup>) and carboxyl (\u2015COO\u2212<\/sup>) groups were found to be shifted to the surface of the globular molecules, and the nonpolar amino acids were found to be concentrated in the interior.<\/p>\n","protected":false},"excerpt":{"rendered":" Results of\u00a0X-ray diffraction\u00a0studies Most knowledge concerning secondary and tertiary structure of globular proteins has been obtained by the examination of their crystals using\u00a0X-ray diffraction. In this technique, X-rays are allowed to strike the crystal; the X-rays are diffracted by the crystal and impinge on a photographic plate, forming a pattern of spots. The measured intensity […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[5],"tags":[],"class_list":["post-1985","post","type-post","status-publish","format-standard","hentry","category-proteins"],"Cooking_time":"","jetpack_featured_media_url":"","_links":{"self":[{"href":"https:\/\/cake.appscodestudio.com\/index.php\/wp-json\/wp\/v2\/posts\/1985","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/cake.appscodestudio.com\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/cake.appscodestudio.com\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/cake.appscodestudio.com\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/cake.appscodestudio.com\/index.php\/wp-json\/wp\/v2\/comments?post=1985"}],"version-history":[{"count":1,"href":"https:\/\/cake.appscodestudio.com\/index.php\/wp-json\/wp\/v2\/posts\/1985\/revisions"}],"predecessor-version":[{"id":1986,"href":"https:\/\/cake.appscodestudio.com\/index.php\/wp-json\/wp\/v2\/posts\/1985\/revisions\/1986"}],"wp:attachment":[{"href":"https:\/\/cake.appscodestudio.com\/index.php\/wp-json\/wp\/v2\/media?parent=1985"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/cake.appscodestudio.com\/index.php\/wp-json\/wp\/v2\/categories?post=1985"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/cake.appscodestudio.com\/index.php\/wp-json\/wp\/v2\/tags?post=1985"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}