626 



I 



Rufus Lumry 



(7) 

 process itself have been carried out by Itoh, Izawa and Shibata using the 



electron microscope and the Coulter counter. They find that chloroplasts 

 shrink in light with a large defornnation of the chloroplast such that their 

 axial ration increases from 1,7 to Z. 6 on illumination. Again a close rela- 

 tionship between photophosphorylation and the new effect is shown. The 

 evidence also suggests that chloroplast fragments and presumably grana are 

 altered in shape in the process and may indeed be the primary level of the 

 effect. The process of shrinking is rapid when it is produced by ATP, but 

 much too slow to be related to individual photosynthetic reactions when shrink- 

 ing is caused by illumination. The action spectrum is that of a chlorophyll 

 with two major peaks, one at 435 m|j. and the other at 680 mjj. . There is, how- 

 ever, a pronounced and unexplained peak at 720-740 m|j.. The effect is similar 

 to that observed in mitochondria. The phenomenon attracts our attention to 

 the possibility of a rather large structure -function relationship in these parti- 

 cles, but the results to date do little to convince us that photosynthesis or 

 oxidative phosphorylation actually depend on these morphological changes. 

 The latter may prove to be related to some other behavior of the organisnns as 

 for example the well-known ability of chloroplasts to adjust their positions 

 within plant cells so as to innprove their ability to collect light. We can do 

 little to shed light on the fundamental nature of the morphological processes 

 or to explain them at the present time. We can, however, review some of the 

 present evidence for structure -function relationships in proteins and extrap- 

 olate from this to the possibilities for such relationships in the granum. 



THE MECHANISM OF CERTAIN HEME -PROTEINS 



At present there are three most promising explanations for the unusual 

 adaptability, specificity and efficiency of the specific physiological reactions 

 of proteins. It is probable that all three appear together or in pairwise com- 

 binations depending on the required protein function. The first of these is the 

 passive organization of reacting molecules near ionizable groups of the pro- 

 tein to provide both specificity of binding and the needed reduction in activation 

 free energy for enzymic processes. This is the oldest of the major proposals 

 for enzymic mechanism but has been out of favor for sonae years. Recent ex- 

 periments on new and nriore complex model systems, particularly by Bruice 

 and his collaborators, ' suggest that it may be more important than we have 

 believed. 



The second explanation is that the protein is so able to order dipolar and 

 nonpolar groups of its own structure about bound reactants as to produce an 

 electrostatic field highly favorable to an acceleration of the limiting step of an 

 enzymic reaction chain. In a case simpler than an enzyme this effect can be j 



illustrated by the oxidation -reduction potential of cytochrome c. In Fig. 1 is 

 shown the reaction between the innidazole hemochrome and oxidized cytochrcme 

 c. The major effect of electron transfer in this reaction is to transport a 

 positive charge from the octahedral hemichrome complex ion buried in the 

 protein to the free hemichrome closely surrounded by water molecules. Ex- 

 perimentally the free energy change is large ° and if one makes the reason- 

 able guess that the dielectric constant within the protein is about 20, the entire 

 value of the free energy change can be accounted for by the difference in di- 

 electric constants at the positive charge. There is no doubt that the effect is 

 important i^n biological reactions and it may be very important. However, 

 Sullivan' ^^' has carried out a number of experiments which show that the 

 effect is quantitatively less important than my sinnple calculation would indi- 

 cate. The iron complex ion of cytochrome c has six ligand atoms. Four of 

 these are porphyrin nitrogen atoms and the other two are thought to be 



