6 I. INTRODUCTION 



demonstrated are the chromoproteins of red and blue algae (phy- 

 coerythrins and phycocyanins), which act as photosensitizers in 

 algal photosynthesis. 



3. VARIABILITY OF THE PYRROLE PIGMENTS 



In addition to the variations described — closed ring system or 

 open chain, porphin or hydroporphin — the pigments can be further 

 modified in a variety of ways: 



(1) By various side chains in the jS-positions of the pyrrole nuclei. 

 Numerous porphyrins exist which differ in the nature of these side 

 chains, e.g., protoporphyrin, coproporphyrin, and uroporphyrin. In 

 the hematin series, we find such differences between hemoglobin, 

 cytochrome c, and cytochromes a (to which latter the respiratory 

 ferment probably belongs) and in the chlorophyll series, between 

 chlorophylls a and b and bacteriochlorophyll. Several of the hematin 

 compounds (hemoglobin, myohemoglobin, erythrocruorins, catalase, 

 horse-radish peroxidase), contain however, the same porphyrin — 

 protoporphyrin — and do not differ in the side chains of the pros- 

 thetic group. 



(2) By a difference in arrangement of the same side chains on the 

 porphin nucleus. We know, for example, several isomeric copro- 

 porphyrins. 



(3) By the metal held in complex combination: magnesium in the 

 chlorophylls, iron in the hematin enzymes and hemoglobin. Com- 

 plexes of porphyrins with metals other than iron, e.g., copper and 

 zinc, occur in nature. A copper porphyrin (turacin) occurs as an 

 ornamental pigment in bird feathers, but compounds of this type do 

 not possess important biological functions. While the role of mag- 

 nesium in chlorophyll is still uncertain, that of iron in the respiratory 

 pigments and enzymes is of fundamental significance. 



(4) The metal held in complex combination, especially when this 

 is iron, may be of different valency. Thus hemoglobin is functionally 

 active only in the ferrous state, peroxidases and probably catalase 

 in the ferric, while in cytochromes and cytochrome oxidase the bio- 

 logical function is connected with the oscillation of the iron between 

 the divalent and trivalent forms. 



(5) Some metals, particularly iron, which can form coordination 

 compounds with more than four atoms, combine with two further 

 groups in addition to the four nitrogen atoms of the porphin ring. 



