Porphins for Apoproteins

It has been noted above that porphins lacking either the iron or the vinyl groups do not support the growth of smooth strains of Hemophilus influenzae. Indeed, they actually inhibit the growth which is supported by iron porphins. A competitive inhibition of growth is readily recognized between the iron porphin (for example, iron protoporphin) and the porphin into which iron cannot be inserted (for example, deuteroporphin). 

In a similar manner, a competitive inhibition can be observed between two iron porphins (for example, iron mesoporphin and iron protoporphin), both of which support growth of H. influenzae and only one of which (iron protoporphin) effects nitrate reduction. If these two iron porphins are added together in suboptimal amounts, growth is increased over that attained by one of them alone, but the nitrite produced is less than expected from the presence of iron protoporphin alone. 

For these competitions to be present one requirement is essential the propionic acid groups must be ionizable. For example, when deutero-phin is methylated at the propionic acid groups no inhibition of growth with this compound is observed. This is readily understandable if we assume that the propionic acid groups are required for attachment to the apo-enzyme. 

In connection with this problem of competition, an interesting situation becomes apparent when one considers the chloroplasts. These bodies are autonomous units of remarkable synthesizing ability. They contain 

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