Magnesium tetrapyrrole macrocycles

Our knowledge of the stereochemistry of porphyrins and related tetrapyrrole macrocycles has expanded rapidly since the first reported x-ray structure determination in 1959 The structures of metallotetrapyrrole complexes are of interest because of the common occurrence of this type of macrocycle in biological systems. As is well known, foremost among these are the heme proteins (iron derivatives), the various photosynthetic pigments (magnesium complexes), the vitamin Bn coenzyme (cobalt corrinoids), and coenzyme F430 (nickel corphinoids) of the methanogenic bacteria. 

Chlorophylls are macrocyclic tetrapyrrole derivatives containing in their natural form a chelated magnesium ion. The basic structure of chlorophyll pigments is shown in Fig. 2.122. 

The basic structure of Chls is the tetrapyrrolic porphyrin macrocycle with an attached isocyclic ring (Fig. 1). This structure has been termed phytoporphyrin by the lUPAC commission (2). The four nitrogens bind a magnesium ion (Mg +) as central... 

Macrocyclic tetrapyrrole compounds such as heme (iron), chlorophyll (magnesium), siroheme (iron), and E12 (cobalt) contain specific metal ions at the center of their tetrapyrrole rings [17]. Metal ion chelatases can be divided into two classes based on their structural architecture. Class 1 chelatases are heteromultimeric enzymes that require three gene products for efficient catalysis [18] of the ATP-dependent chelation reaction [19]. Enzymes in this class include chlorophyll and bacteriochlorophyll magnesium chelatases [18] and aerobic cobalt chelatase (CobNST) [20]. 

The chlorophylls are macrocyclic tetrapyrrole pigments which in their native form contain a chelated magnesium ion. The general structures for the chlorophylls and the derivatives discussed in this section are given in Fig. 14. 

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