The functions of respiration and photosynthesis in protoplasm serve to provide energy for maintenance and renewal. In this paper I wish to present some hypotheses on how these functions may have been accomplished in a primitive form in inorganic mineral catalysts, how organic molecules came to take over these functions, and how these organic molecules were modified as the biosynthetic chains developed. Finally, I should like to discuss the recent appearance in evolution of a heme oxidative enzyme.
THE Two FUNCTIONS OF THE PORPHYRIN BIOSYNTHETIC CHAIN
One of the problems that fascinated me as a student was whether the two major pigments of protoplasm, heme and chlorophyll, were related. This was an old problem. It had its roots in the realization of the basic unity of protoplasm which became evident when the cell theory was proposed about 125 years ago. The philosophical question of the unity of protoplasm on a molecular level was also being considered at that time, since some of the biologically occurring molecules had already been synthesized by organic chemists. There is a paper in the Comptes Rendus of about 1850 which stated that the two major pigments were indeed related, since both contained iron. The author of the paper should have been given an A in philosophy but a zero in qualitative analysis. By the 1940's, the massive and difficult work on the structural chemistry of heme and chlorophyll wasi almost complete . The methods of organic chemistry had shown that heme and chlorophyll were constructed on the same porphyrin plan. , but there were a number of differences. It was only through a biochemical approach that the reason for their similarity was found about fifteen years ago. It was then shown that both pigments arise from the same precursor, protoporphyrin. To make heme, iron is inserted into protoporphyrin. To make chlorophyll, magnesium is inserted into protoporphyrin; then in a number of steps the side chains, i.e., the fringes around the porphyrin ring, are modified. One propionic acid group is esterified with methanol and oxidized to form the cyclopentanone ring; one vinyl is reduced to an ethyl group; one pyrrole is reduced by addition of two Ç atoms; and finally, the other propionic acid group is esterified with phytol. Thus, it is seen that the basic plan of the two pigments is essentially the same, but in chlorophyll there are fringe benefits.
Heme and chlorophyll are related not only because they are two end products of the same biosynthetic chain. They are also related in function. Heme serves as the catalyst for respiration to release the energy stored in organic bonds to be used for useful work. Chlorophyll serves as a catalyst to convert the energy of sunlight into the stored chemical energy of organic bonds. The basic energetics of protoplasm are thus catalyzed by these two pigments derived from the same biosynthetic chain. When we examine the structures of heme and chlorophyll and attempt to relate them to their functions, a simple relation is evident. The redox activities of heme which are responsible for its catalytic function reside in the properties of the iron atom; the properties of the porphyrin surrounding the iron atom merely modify the properties of the iron atom. On the other hand the porphyrin or chlorophyll molecules are dyestuffs that have properties suitable for photochemical reactions; these dyes absorb light and fluoresce intensely in the visible region of the spectrum.
A number of biological pigments are known which can act photochemically, e.g., derivatives of flavins or carotenoids. However, the ability of the porphyrin to do two jobs, i.e., both redox and photochemical reactions,
gave it such an advantage that it easily preempted the biological stage.