SCHEMATIC MODELS OF BIOCHEMICAL POLYMERS R. QUENTIN BLACKWELL Northwestern University D ~ n t a School, l Chicago, Illinois
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various commercially available atom models are invaluable for instructional purposes as well as in research. However, in the area of biological polymers one frequently needs a simple model to represent monomer units. For example, a polypeptide for purpose of discussion, may be advantageously considered as a string of beads in which the individual bead represents the individual amino acid. Likewise, the amylose type molecule can be compared and cont,rasted to the amylopectin and to the glycogen t,ype by suitable models which depict the glucose unit as a single sphere. For mch purposes the ordinary plastic heads made for novelty necklaces and bracelets serve satisfactorily. These beads can be obtained in many colors at nominal cost.' They are self-fastening and they have the advantage of extreme ease of assembly. Examples in which such schematic models are useful include: Models t o show the gross structural nature of such glucose polymers as amylose, nmylopectin, glycogen, and cellulose. Multicolored m d e l s to demonstrate peptides and proteins. Demon~trationof the vast possibilities of sequential isomerism in peptides and proteins. Comparison of the known amino acid sequences in biologically important peptide8 and prateins such a8 hormones, portions of enzymes, and antihiotics. Structures of nurleosides, nurleotides, and nuoleic acids. Piature of the action of a- and &amylase enzymes in the hydrolyfiis of polysareharides. Demonstration oi the directed hydrolysis oi specific peptidelinkages hy p~oteolyticenzymes. Hydroly~isof nucleie acids. Polymerization oi actin molwules, i.e., G-actin F-actin conversion in muscle.
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POLYSACCHARIDE MODELS
White beads are effective in depirting glucose polymers. The a-1,4-polymer amylose is shown simply as a long string of beads whereas amylopectin and glycogen models require u-1,G-branching. Reads are available which provide for such branching; hovever an extra hole is introduced easily in the side of the regular head mith a heated metal rod. No differentiati011 between a- and 8- type linkages is attempted in such models; therefore amylose and cellulose models would appear identical except for the difference in the number of glucose units. Figure 1 shows the model of a portion of an amylopectin molecule. The model was spread on a flat surface with no attempt to provide a t,hree-dimensional model. The model contains approximately 580 glucose units and would represent approximately 'Plastic "Pop-It" hcada with diameter slightly over one eentimeter cost about one cent enrh.
Figure 1.
A m y l o p e c t i n M ~ l e s v l eModel
one-third of an average amylopectin molem~le. It ma.y be seen that only one reducing group appears in the molecule (the prong extending from the bead in the left central portion of t,he illustration). All of the remaining glucose units are involved in u-1-linkages. To conserve space no model of amylose is depicted; it would appear as a string of glucose units. Glycogen would resemble the amylopectin model except that branching would occur at more frequent intervals along each chain. The action of a- and pamylases on such molecules may be demonstrated by pulling the model apart at the appropriate points; that is, at the glucose units where branching occurs. The projecting knobs, in this case, can be taken to represent the reducing groups. Typical fragments, such as illustrated by Bernfeld2 may be shown effectively. MODELS OF PEPTIDES AND PROTEINS
The individual amino acids in peptides are represented by colored heads. There are more than sufficient head colors commercially available to represent all of the amino acids commonly found in peptides; however, it was felt that careful color-coding mould allow easier recognition of the individual amino acids. (This was particularly import,ant for the systematic study of amino acid sequences in peptides with known structure.) Accordingly, the basic amino acids were represented by shades of blue, the acidic ones by shades of red, the sulfur-containing ones by yellows, the unsnbstituted aliphatic monoamino monocarboxylic acids hy n~hite heads, banded mith st,ripes of color, phenylalanine by hlack, tyrosine by hlack witaha white striue, and so on. 2
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~P., Advances ~ ~ in Enrp~oloqlj, ~ ~ ~12, 379-428 ~ , (1951). JOURNAL OF CHEMICAL EDUCATION
720 possible arrangements. The demonstration should be tempered with the added discussion that certain sequences and end groups may be favored; the obvious effect of departure from the random mould be to reduce the number of probable arrangement~.~ NUCLEOTIDE MODELS
( I ) Oxutorin. (2) vasopressin, (3)insulin A. (4) insulin B, ( 5 ) eortieotropin
strepwenin aotivity. (10)bacitraoin I. (11) qramicidin S. (12) one small segment of the A, (6) MSA, (7)Ikypertensin, (8) glueagon, ( 9 ) a peptide with
rihomdease moleoale.
Unfortunately, without being shown in color the examples in Figure 2 do not appear to full advantage. (All cysteines are represented as entities, i.e., no disulfide linkages are shown.) At the end of the peptide chain model the bead wit,h the extending prong represents the amino acid with the free a-carboxyl group whereas the amino acid with the free or-amino group a t the opposite end of the chain is distinguished by a bead with a n open hole. Demonstration of the directed hydrolysis of specific peptide linkages by the proteolytic enzymes can be accomplished by assembling a long multicolored string of beads to represent a hypothetical protein and shorn.ing the actual cleavages favored by the individual enzymes. The use of such a model clearly demonstrates the production of peptide units from the original protein molecule. To demonstrate the vast possibilities for sequential isomerism as discussed so entertainingly by Asimov3 one may hand out to each member of the class a small sack containing six different colored beads and instruct the class to shake the sacks and, without looking a t the beads, remove and assemble. Generally, no two resulting peptide models mill he identical because of the
VOLUME 34, NO. 10, OCTOBER, 1957
The relationship among the nitrogenous base, pentose, and phosphorir acid components of a nucleotide is made clear by snnple models using, for example, shades of blue and green to represent adenine, guanine, cytosine, uracil, thymine, and 5-methylcytosine, white beads to represent ribose and deoxyribose, and red beads to represent phosphoric acid. The additional linkages necessary between the pentose and phosphoric acid units may be effected by pieces of heavy nichrome wire heated and thrust into the sides of the beads where they become bonded. Such models allow demonstration of the numerous suggested linkages between the individual nucleotide units. Figure 3 illustrates a model of a section of a hypothetical nucleic acid containing two spiraled polynncleotide chains. No attempt was made to depict actual dimensional relationships or number of nucleotide units per spiral turn. Again, the possibilities of isomerism are made apparent by the order of appearance of the purine and pyrimidine unit,% a
Asnrov, ISAAC,J. CHEM.EDUC.,31, 1 2 5 2 7 (1954). Fox,S. W.,interiean Scientisl, 44, 347-50 (1056).
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