Thomas Vedvick
and Michael Coates Deportment of Blochemistry Univers~tyof Oregon Medical School Portland, 97201
Hemoglobin: A Simple "Backbone" Type of Molecular Model
Recent advances in stereochemintry have made available the structures of hemoglobin and many other proteins (1). For most investigators the spatial features of the molecules are best visualized in the form of a three dimensional model. Several different types of hemoglobin models have been built (2-4). The final design of our model evolved from experimenting with different construction methods. The general conformations of the alpha and beta chains of hemoglobin are represented by a backbone made of 12 gauge galvanized tie wire (Fig. 1). These wire backbones give t,he chains considerable strength without the need for extensive bracing. Each chain has a single internal brace. The alpha and beta chains of hemoglobin consist of two types of segments, helical and nonhelical. To represent the helical segments we bent smaller diameter, 16-gauge, galvanized tie mire in the shape of alpha helices and soldered these to the backbone (Figs. 1, 2, and 3). Figure 4, which is modified from Perutz, et al. ( 5 ) , indicates the location of the helical and nonhelical segments and their dimensions in our model, as well as the total number of turns in each helix, the proper orientation of the helices in both the alpha and beta chains and the approximate positions of the centers of the amino acid residues along the chains. I n this figure the structural notation of Kendrew, et al. (6) is used. The helical segments are designated by letters A t o H, and the nonhelical segments are either interhelical, AB, BC, etc., or terminal, NA and HC. The amino acid residues are numbered consecutively, from the amino end
Figure 1.
Top view of the two beta chain, in the deoxygended position.
within these segments. There is approximately one amino acid residue every 100" of turn around a helix. The scale used in this model is the same as that used by Schroeder and Jones (S), 0.625 em to the Angstrom. The heme group is also attached to the backbone. We made the hemes out of 17 gauge brass sheeting cut t o the likeness of a protoporphyrin ring structure. The hemes are supported by 16 gauge mire and soldered in place between the E and F helical regions. We had the goal of building a model capable of showing the relative change in the location of the subunits when going from the oxygenated to the deoxy-
Figure 2.
Side view with one olpho chain removed,
Figure 3.
Side view with all four chains in place.
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Figure 4. Schematic representation of the a and 0 chains of hemoglobin. Heiicol segments are indicated by sine waves, nonhelicd segments b y straight liner. Demensions of these segments in the model ore given in centimeters. Dots represent the approximate centers of the amino acid residues; the flrst and lost numbers of these residues within each segment are given. Residues at the lops of the sine wover are internal with respect to the rubunih while those at the bottom ore external. This flgure is modifled from Figure 1 of M. F. Perutz, et ol., J. Mol. Biol., 13,669-678 (1 9651 and is published here with permission of the authors and Acodemie Press.
genated form of hemoglobin. From X-ray diffraction studies a t 5.5 A resolution Muirhead, et al. (2) concluded that the change in hemoglobin during deoxygenation can be almost completely interpreted as a simple rotation of the chains about their "rotational axes." The alpha chains rotate 9.38' and the beta
' IMRC Laboratory
of Molecular Biology, University Postgraduate Medical School, Hills Road, Cambridge, England.
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chains 7.43" around these axes. A model capable of showing this change has not been built previously to this one, probably for the reason t,hat earlier models have all needed many contact, points with a base for support. I n our model the chains are fast,ened direct,ly to the rot,ational axes as shown in Figures 1, 2, and 3; no other support rods are necessary. Joyce M. Cox1 supplied us with the coordinat,es for the rotational axes of both u and 6 chains in the two horizontal planes y = 0 and y = 15. We JVere able to accurately place the rotational axes in our model by extrapolation through these two points to the base. The rotational axes are made of 0.625-cm copper tubing and slide into a brass holder at t,he extrapolated points in the base. A 12.5-cm lever arm is attached to the rotational axis just above the brass holder. The lever arm moves in a Teflon guide which strictly defines the degrees of rotation of the axes (Figs. 1,2, and 3). Unlike the space-filling model, our model clearly shows the helical and nonhelical segments of bemoglobin. The spatial relationship of all four subunits and the general outline of these polypeptide chains are emphasized. Our model is useful for demonstrations because the subunits, with axes attached, can be removed easily from the base for inspection (Figs. 1 and 2). Finally, this model shows how the quat,ernary structure of hemoglobin is believed to change upon taking up or releasing oxygen. A model like ours could be useful to many readers. With some modifications this same type of construction could be used in building models of other proteins. The cost of materials for constructing our model was less than $20.00. Detailed directions for building this model are available from the authors. We thank Drs. R. T. Jones and W. A. Schroeder for their valuable suggestions during the building of this model. This work was supported in part by PIlS grants AMCA 13173, F02 HE 20330 and 2 TO1 GM01200. Literature Cited (1) Dms~nson.R. E., A N D G~18. I.. "ThsStructure end Aotion of Proteina." H a r ~ e rand Row, New York. 1969. (2) Mmnasro, H.. Cox, J. M., M * z z ~ n m . ~ nL... A N D PBRUTI. M. F., J . Mol. Biol., 28, 117 (1967). (3) S c a n o m ~ n W. . A., AND J o m a , R. T. i n "Progress i n the Chemistry of Organio Natural Products.'' Springer-Veriag. New York. 1965. Voi. XXIII. (4) Mun*r*nr*, M.. ~emonalcommunication. (5) P s n u ~ s M. , F.. KENDREIV. J. C., AND WATBON.H. C.. J. M o i . Bid.. 13,
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(6) Kbwnnew, J. C.. W ~ ~ e o H. r . C., S ~ n ~ w n e s n oH. . C., DICBEB~ON. B. E.. PHILLIPB, R. E., PAILLIPB. D. C.. A N D SXORE, U. C., Nnturc. 190, 663 (1861).
