Graphic Representation of Oligosaccharide and Polysaccharide Structures Containing Hexopyranose Units John F. Robyt Department of Biochemistry and Biophysics, Iowa State University. Ames. IA 5001 1
Carbohydrate monosaccharide molecules, with a hexopyranose structure, have an empirical formula of C6H1206. While this formula is relatively simple, the actual structures are complex in terms of stereo configuration and conformation (see Fig. l ) . This complexity is increased when these monosaccharides are joined together by an acetal linkage t o form oligosaccharides or polysaccharides. The monosaccharide residues can be joined together a t a number of positions, depending on the number of free hydroxyl groups. For Dglucopyranose there are five free hydroxyl groups, including the hydroxyl group attached to carhon-1, which is a hemiaceta1 hydroxyl, capable of two configurations (aand p), and involved in every acetal linkage. The other four hydroxyl groups are attached to carbons-2, -3, -4, and -6 and can react with the hemiacetal hydroxyl of carbon-l of a second Dglucose to form the acetal linkage. Thus, there can be four different position isomers and two configurational isomers (a and p), giving a total of eight possible isomers. Because of the relative complexity of drawing D-glucopyranose structures, especially when incorporated into oligoand polysaccharides containing different types of linkage positional and configurations, we have developed a simple graphic representation of D-glucopyranose as a circle with
or-0-Glucopyranose
8-D-Gluco~yranoee
five distinct linkage positions (hydroxyl groups) as shown in Figure 2. The five hydroxyl groups are drawn as lines extending from the circle with those hydroxyl groups that are above the plane of the Haworth and conformational rings extending into the circle and those hydroxyl groups below the plane of the ring extending outside the circle. The orientation of the circle is the same as the Haworth structure shown in Figure 1which is in the "standard position" with the hydroxyl group of carhon-1 to the right and the other hydroxyl groups following clockwise around the circle (com1and 2). Dare Fies. " The various combinations of D-glucopyranose residues ioined into the eieht a and B disaccharides with 1 6.1 4. i 3, and 1 linkages are shown in Figure 3 ~ ~: h e s d combinations can easily be extrapolated to more complex structures as tri- and tetrasaccharides with mixed linkages (see Fig. 3B) and to even more complex structures of polysaccharides with various combinations of mixed linkages and branch linkages (see Fig. 4). The simplified D-glucopyranose structure used in Figures 3 and 4 can be extended toall of the possible eight^-hexoses. This is shown in Figure 5A for the three epimers of Dglucopyranose-D-mannopyranose, D-galactopyranose, and D-allopyranose-by indicating the five stereochemical positions of the hydroxyl groups. When more than one type of monosaccharide residues are linked together to give hetero-
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->
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w-0-Gluco~yranos~ chair conformation
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Fioure 1. I.A.l and 181 and B-o" . . Hawonh snuctures for a-wlucaovranase .. glucapyranase, respectively. (C)Contormational chair structure for a-D-gluccpyranose. The numbers indicate the individual carbon atoms.
trisaccharide panose, l i n k e d 0-1-6 6 0-1-4
0-D-GIUCOpyranose
P-D-Glucopyranose
Figure 2. Graphic representation of (A) a-oglucopyranose and (6) 8-o-glum pyranose with tha numbers indicating the attachment of the hydroxyl groups to the respective carbon atoms.
560
Journal of Chemical Education
tetrasaccharide l i n k e d a-1+3. a-146 11 0-1-4
Figure 3. (A) The eight a- and ,%inked D-glucopyranose reducing disacchs rides. The hemiacstal hydroxyl of the a-linked disaccharides are drawn in the a-contiauration and the B-linked disaccharides are drawn In me B-canfimralmon In s a l ~ on. t tne hemaceta hydroxyl group of caroohydrates e x m n an equ lhor urn m x1.re 01 n- an0 3-anomers Inill may be oes gnalw al caroon-1 as -Oh (8) TI,- an0 telrasacchar de rlructdror wlth mlxeo nkages
8-1355 Alternan
i
,..
.. Amylopectin or Glycogen Chitin
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Flgure 4 US^ 01 me graph c rspre$entatoon tor me structures of some Dql~copyranosepalyssccnsr#des(A) Dextran. the u-1 6 llnked D q l ~ c a n wlth n.1 3 oranch I nkages e aoorated oy ~euconorlocmesentero!desB.512F,and me polysacchar de dsed as Me sbrtlng msterla lor Sephadex 18) L mesenteroldes B- 1355 alternant withDqlucopyranoseresidueslinkedalternateiyby a-I 6and a-1-3 linkages Inmemain chains witha-1 -3 branch linkages.(C)Chltin.the@-1-4 linked 2-ecetamido-2deoxy-~qIucopyranose polysaccharidefound in fungal call walls. mycellal yeasts, algae, and in the animal kingdom In the exoskeletons of arthro~ods.the shells of molluscs. and the mandibular tendon of lobsters. ID) Amylopectin w glycogen with a-I 4 linked Dqiucopyranose residues in the mein 6 branch linkages. chains and 0-1
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saccharide structures, they may be distinguished from each other by leaving the unsubstituted hydroxyl groups on the circles. Substituted hexopyranoses can also be drawn by placing the substirurnt o n t o the appropriate carbon. For example, 2-deoxyglucose or 2-amtnu-2-deoxyylucose would have . hydrogen i or an amino group attached to carbon-2 (see Fig. 4C and Fig. 5B and C).Figure 5B illustrates a-lactose, awhich is 4-O-8-D-galactopyranosyl-cu-o-glucopyranose, melibiose, which is 6-0-a-D-galactopyranosyl-a-D-gluconvranose. and 2-amino-2-deoxv-a-D-gluco~vranose. Figure %illustrates different comhin&ons i f mo&saccharid~residues and different tvpes of linkages . in a hetero-trisaccharide and -tetrasacchaiIde. The proposed simplified carbohydrate structures follow a logical pattern and allow the drawing of all eight types of glycosidic linkages, all eight of the D-hexopyranoses, and any number of substituted hexopyranoses. The system can be readilv extra~olatedto the L-hexo~vranosesas they are simply tge mirror images of their D-co&terparts. The use of the sim~lifiedstructures ~ e r m i tthe s rapid drawing of complex saccharide structur& that otherwise would-he slow, tedious, and difficult if the Haworth or conformational structures of Figure 1were used. These simplified structures give a rapid recognition of the different types of monosaccharide residues, the different types of glycosidic linkages between the monosaccharides, and the position of substituents on substituted monosaccharide residues. They are especially convenient when drawing complex heterosaccharide or large, complex polysaccharide structures.
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Acknowledgment I wish to thank Bernard J. White for valuable discussions and suggestions during the development of the system and the manuscript.
Figure 5. Graphic representation of (A) a-oqlucopyranose, a-D-mannopyranase, a-o-galactapyranase. and n-o-allopyranose, ( 0 )a-lactose, a-melibiose. and 2amina-2deoxy-a-~.glucopyr~nose, and (C)hetero-trisaccharide linked by a-1 4 linkages and hetern-tetrasaccharide llnked by 8-1 6. P - l 3. and 0-1 4 linkages.
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Volume 63
Number 7
July 1986
561
