The Relation between Structure and Function in Some Glycoproteins

THE RELATION BETWEEN STRUCTURE AND FUNCTION IN SOME GLYCOPROTEINS ALFRED GOTTSCHALK, M.D., D.Sc* Proteins play a key role in the life ofevery cell. They contribute a share to the cellular framework. They provide the tools for the catalysis ofall chemical reactions proceeding in the cell. They controlby various mechanisms the rate ofcatalyzed reactions. Their constituent amino acids, when released by hydrolysis ofthe peptide bonds, may undergo oxidation or enter the pathways ofsynthesis ofcarbohydrate, fatty acids, porphyrin, and other cell components. They are the material basis ofbiological toxins and, in the form ofantibodies, they defend vertebrate animals against invaders. No other natural material could conceivably fulfil such a great number of widely different functions. The reason for the astounding versatility ofproteins in biological events is to be found in their composition, their covalent structure, and in the arrangement in space of the covalent structure—conformation. Broadly, proteins may be envisaged as consisting ofone or more polypeptide chains, cross-linked and coiled through chemical bonds varying in strength. The polypeptide chains are composed ofa variety of amino acids in differing numbers and sequences, linked together by amide bonds. The general properties of proteins are essentially a reflection of the properties of the constituent amino acids, whereas the biological specificity ofproteins resides in the sequence ofamino acids in the polypeptide or parts thereofand in the conformation ofthe chain. Thus, it is the properties ofthe aliphatic, aromatic, heterocyclic, acidic and basic, hydroxyl- and sulfur-containing amino acids, their sequence and their arrangement in space which endow proteins with the ability to form as many specific enzymes as are required for the synthesis, breakdown, and conversion ofall natural * Department of Microbiology, John Curtin School of Medical Research, Australian National University, Canberra, Australia. 327 products. Each individual enzyme is thought to possess at its surface some area complementary in structure to some feature ofthe substrate molecule, the forces available at the enzyme surface for attracting and binding the substrate being hydrogen bonds, hydrophobic bonds, ion-pair bonds, and other electrostatic forces and dispersion forces. Assume that for the specific activity ofan enzyme (active center and attachment sites) io amino acids must occupy strictly defined positions within the polypeptide chain. If each of these io amino acids is selected from the set of 25 natural amino acids, the number ofpotential enzymes with different specificity would be 2510 = 1014. Ifthe choice ofamino acids is in any way restricted by either the composition or the conformation ofthe enzyme molecule, the above maximum number ofpossibilities would be reduced somewhat.Just as the near-infinite number ofpotential proteins with different surface structure will easily provide specific enzymes for any number ofnatural substrates, the balanced complement of readily accessible hydrophilic and hydrophobic , ofpositively and negatively charged, and ofpolar and non-polar groups will enable proteins to attract and bind compounds of such different character as, for instance, linolenic acid and sucrose. Even if ribonucleic acid (RNA) wereableto match protein intheability to provide, by appropriate arrangement ofits components, an infinite number ofspecific structures, it could not concentrate the requisite specific features into such minute areas as an ordinary protein can and, indeed, must do for reasons of space economy. Enzymes are proteins not by chance or choice but of necessity. Another feature characteristic ofproteins is their ability to incorporate into their covalent structure elements distinct from protein or amino acids, the non-proteinacious component of these "conjugated proteins" being referred to as "prosthetic group." The best known conjugated proteins are the heme proteins—for instance, hemoglobin, myoglobin, cytochromes b, catalases, and peroxidases. The aforementioned heme proteins have the same iron-porphyrin nucleus, though they differ in the valence state ofthe iron. Since their biological activity is intimately connected with the iron atom, their distinct functions are ascribed to their distinct protein structures and to the type oflinkage between prosthetic group and protein. Another group of conjugated proteins is represented by the carbohydrate -proteins. In the polysaccharide-protein complexes the carbohydrate is a homo- or heteropolysaccharide such as chitin, hyaluronic acid, chon328 Alfred Gottschalk · Structure and Function in Some Glycoproteins Perspectives in Biology and Medicine · Spring 1962 droitin 4-sulfate, chondroitin 6-sulfate, dermatan sulfate, heparin, etc., characterized by a small repeating...