HEMATIN COMPOUNDS IN PHOTOMETABOLISM 155 



iron porphyrin protein but is a hybrid compound hke the cyto- 

 chrome of the green bacteria and the yellow pigment of the faculta- 

 tive bacteria. 



No enzymic function has been found for this compound, just as in 

 the case of cytochrome /. Cytochrome c reductase fails to activate the 

 Chroniatium pigment, although extracts of Chromatium contain re- 

 ductase acti\'ity when cytochrome c is present as a substrate (27). 



It is apparent that the photochemical apparatus in photosjaithetic 

 systems, regardless of overall metabolic pattern, contains hematin 

 compounds. Not only do these hematin compounds appear to be 

 unique but they also do not occur outside the photochemical ap- 

 paratus under conditions where there is no close coupling between 

 normal respiration and photosynthesis. 



THE FUNCTION OF HEMATIN COMPOUNDS IN PHOTOSYNTHESIS 



In green plant photosynthesis, the net result of the photochemical 

 act is the transport of four electrons against a potential drop of 1.2 

 volts, which is the difference betAveen the potentials of the hydrogen 

 and oxygen electrodes at physiological pH. If the unitary theory of 

 photosynthesis is adhered to (34), a similar process may be supposed 

 to occur in bacterial photosynthesis, although no oxygen evolution 

 concomitant with reduction at the level of the hydrogen electrode can 

 be demonstrated. 



In 1939, Hill proposed (see 15) that the initial phase of the photo- 

 chemical process did not push electrons all the way from the oxygen 

 potential to the hydrogen potential, but only part way. A back oxi- 

 dation was invoked to provide the additional energy storage. He 

 suggested that the substrate for the back oxidation was a part 

 of the reduced material formed in the initial photochemical act, and 

 the H acceptor was a part of the oxygen liberated by the partial 

 movement of electrons away from the potential of oxygen. He also 

 raised the possibility that the transport system for this movement of 

 electrons consisted of respiratory pigments similar to, but not identi- 

 cal with, those normal to respiratory systems. It was inferred that 

 this photorespiratory system was separated in the cell, spatially or 

 otherwise, so that it could function in close coupling with the for- 

 ward reaction of light absorption and act independently of normal 

 respiration. At the time of these proposals, no basis existed for the 

 notion of chloroplast respiratory pigments. As we have seen, however, 



