555 



R. A. Olson 



Another insight into the nature of the configuration of tetraporphyrin 

 plane in protein is offered by the hemoglobin macromolecule . In this molecule 

 both sequence and configuration of amino acid residues have been established 

 as well as the location of the tetraporphyrin, or heme plate.' ' While normal 

 hemoglobin does not appear to be molecularly organized, a genetically induced 

 alteration of this molecule causes a molecular phenomenon in erythrocytes 

 known as "sickling". Under conditions of anoxia "sickled cells" beconne elon- 

 gated and stretched into a sickle shape by the extension of long chains of stacked 

 hemoglobin molecules. Sickled cells exhibit dichroism in vivo consistent with 

 the absorption of heme chromophores stacked in the direction of cell exten- 

 sion.^ ' The axis of maximum polarized absorption is perpendicular to 

 the long axis of the sickled cells. This is shown in Plate II E and F taken from 

 a recent paper of Murayama, Olson and Jennings. ''^°' The plane of the analyzer 

 is indicated by the arrows. An explanation of this selective molecular orienta- 

 tion of tetraporphyrin in such a hemoglobin structure has been offered by 

 Murayama.''^') It involves stacks of the globin molecules which are stabilized 

 by cyclization of the N terminus of the beta polypeptide chains. In the case of 

 Hb-S, cyclization occurs by the formation of hydrophobic bonds and in the case 

 of Hb-C Georgetown by the formation of electrostatic bonds. 



The sickled cell thus offers a convincing demonstration of a mesophase 

 or paracrystalline protein structure in vivo in which a tetraporphyrin is molec- 

 ularly oriented. The relationship of the heme configuration to that of the spher- 

 ical macromolecule could provide a useful structural analogue to the chloroplast 

 pigment complex. Much more information regarding the structure of chloro- 

 phyll protein in vivo (amino acid sequence, electron density distribution, etc.) 

 will be required before similar sub-molecular aspects of chlorophyll molecular 

 orientation are resolved. 



Finally, we must consider the significance and virtue, if any, of oscillator 

 orientation at the site of chemical transformation of radiant energy. The rela- 

 tion between organization and specificity has not yet been established in biolog- 

 ical systems. A structurally dependent biochemical system in mitochondria 

 has been proposed by Green. \^^i A structural array of enzyme molecules is 

 implied, featuring fixed positions in space located to facilitate successive re- 

 actions. Such "solid state enzymology" has been disputed by Dixon and Webb.'^^' 

 (The subject is discussed by Lehninger and by Kauzmann' ' ■^'). 



Perhaps the organization of molecules for specific reaction is limited to 

 photochemical sites of energy transformation. Pronounced molecular orienta- 

 tion occurs in the visual apparatus. Liebman has measured in vivo dichroic 

 ratios as high as 6.0 in the lamellar structure of the retinal rod outer seg- 

 ment.'"^ ' Similarly, Jaffe has demonstrated a pronounced orientation of the 

 oscillators responsible for photonnorphogenesis in Fucus egg cells. '■^^' An 

 intensive search for other more subtle properties common to these systems 

 and oriented chlorophyll may well resolve the role of oriented oscillators in 

 photobiological systems. 



