264 PORPHYRINS 



veniently purified. Probably in its functional form it is bound loosely to protein, and 

 chlorophyll -protein complexes have been isolated from leaves by special techniques (1, 2). 

 Non-polar solvents such as benzene are unable to split chlorophyll from its complex, but 

 only slightly polar ones such as acetone readily extract the pigment. Since hematin com- 

 pounds can be isolated only in special cases, they must be studied and identified by in- 

 direct techniques in crude homogenates or in situ. The study of chlorophyll derivatives, 

 on the contrary, usually begins with their purification. The quantitative difference in 

 these two pigment types becomes apparent when one considers that on a dry weight basis 

 a green leaf may have about 1% chlorophyll pigments whereas total hematin in the richest 

 tissues is practically never more than 0. 01% and usually about 0. 001%. 



Vitamin B12 has a porphyr in-like structure, but its synthesis by higher plants is 

 questionable. It has been reported to occur in turnip greens but may actually be synthe- 

 sized by associated microorganisms (3), 



HEME AND RELATED COMPOUNDS 



The hematin (or iron porphyrin) pigments have been well-known in animal tissues 

 for many years (e.g. hemoglobin), but they are equally wide-spread and important in 

 plants. Hemoglobin itself occurs in the root nodules of legumes but is produced only in 

 the presence of the bacterial partner. The enzymes catalase and peroxidase occur 

 throughout the plant kingdom and have iron porphyrin prosthetic groups. However, the 

 predominant hematin compounds are the cytochrome respiratory pigments which in all 

 aerobic tissues transport electrons along the chain (or slight variants of it): 



cytochrome b -^ cytochrome Cj -^ cytochrome c -> cytochrome a -^ 

 cytochrome a.^ (cytochrome oxidase) -^ O2 



Other cytochromes are known in plants (e. g. h^, b^, f) but their functions are not as clear. 

 Small letters are used to refer to individual cytochrome molecules, whereas capital 

 letters refer to a group of cytochromes with a particular type of structure (4). The re- 

 views of Hartree (5), Smith and Chance (6), James and Leech (7) and King (8) discuss the 

 distribution and participation of these pigments in metabolism. 



The most common porphyrin structure among the hematin pigments is protoporphyrin 

 IX. (Here the Roman numeral refers to the particular side -chain arrangement among all 

 the isomers having the same groups as substituents. ) Its structure is shown in Figure 

 13-3, p. 271. As the ferrous chelate complex this is the pigment known specifically as 

 "heme." Peroxidase, catalase, hemoglobin, oxyhemoglobin, methemoglobin, and the 

 B -cytochromes all contain the same porphyrin as a ferrous or ferric complex. Four of 

 iron's six coordinate valences are utilized in this complex; the other two are directed as 

 summarized in Table 1. Cytochromes c and f apparently differ in structure from the B 

 type in having their vinyl side-chains condensed with suLfhydryl groups of the protein as 

 well as bound through iron to the protein: 



