Some aspects of the structure of starch and glycogen.

Abstract

In many scientific careers, certain years have a special significance. For the writer, 1949 and 1989 are such years. In 1949, I began my research career in Cambridge, by studying the structure of glycogen, under the direction of the late Dr. D. J. Bell, who since 1934 had published a series of papers on this subject. At this time, periodate oxidation methods of anal-ysis and paper chromatography were being developed, and these techniques were amongst those widely used in these and later studies. The research also involved an examination of a series of dextrins prepared for me, by an amylolytic degradation of rabbit liver glycogen. One of these, a “β-dextrin, ” had an apparent chain length of nine glucose residues, compar-ed to 12 for the original glycogen, suggesting that on the average, three glucose residues were removed by β-amylase from each chain. How-ever, the “β-dextrin” was, in fact, further degraded by crystalline β-amylase, and the true β-limit dextrin had a chain length of seven. Although this observation was a disappointment at that time, it led to a lasting understanding of the importance of enzyme and substrate con-centrations, which was invaluable over the next four decades. The researches on glycogen also involved classical methylations by the Haworth and Purdie procedures, followed by chromatographic separations of the methylated sugars. From this work came an awareness of the great debt that is owed to the pioneers of polysaccharide chemistry for their painstaking efforts in the 1930 s and 1940s. This fact tends to be overlooked by many who use modern facile and rapid methods of analysis. In 1989, our researches on starch and glycogen have been greatly honoured by this Society. In this review, a selection of this work will be described in four sections. The first covers a period in the 1950s and 1960s when it seemed probable that both polysaccharides contained a small proportion of (1→3)-α-D-glucosidic lin-kages. This would have important implications with respect to the biosynthesis and degradation of these two α-D-glucans. Fortunately, but with one possible exception, the proposition appears to be untrue. However, there is no guarantee that newly discovered samples of starch and glycogen from unusual biological sources such as uncommon algae and protozoa will not contain some anomalous linkages. The second section describes briefly the detection of anomalous linkages in amylose, and their identification by enzymic rather than chemical methods. The key to this work was yeast isoamylase, a debranching enzyme first described by Japanese biochemists. Debranching enzymes also play a major part in the third section which describes studies on the degree of multiple branching in amylopectin and glycogen. Multiple branching originally described the presence of chains containing more than one branch point as an integral part of the molecular structure. This was later described in terms of the ratio of A-chains to B-chains. This ratio is an important parameter which relates to the actual molecular models for amylopectin and glycogen. This topic forms the final part of this review.