2.2 Porphyrins promote oxygen transport in animals and humans (excellent biocompatibility)
Heme (Fe-protoporphyrin IX) is an endogenous porphyrin derivative that is an important molecule in aerobic organisms(Kabe et al. 2006). Heme (Fig. 3) is a metal compound consisting of an iron atom coordinated through its nitrogen atom to a tetrapyrrole ring system known as the protoporphyrin part. The iron in heme binds to the histidine residues of the pearlin chain to form hemoglobin. Notably, the iron in hemoglobin (the primary carrier of oxygen) is in the ferrous state (Fe++), which allows for reversible binding to molecular oxygen. Hemoglobin is oxidized and converted into methemoglobin, where iron is oxidized to Fe+++ and its oxygen-carrying capacity changes significantly(Tsiftsoglou et al. 2006), so that hemoglobin can participate in respiration as an oxygen molecular carrier. Besides, heme is an important cofactor in mitochondrial electron transport (complexes II-IV), drug and steroid metabolism (cytochromes), signal transduction (nitric oxide synthase, soluble guanylate cyclase), transcription (N-PAS2, Bach I) and regulation of antioxidant defense enzymes(Krishnamurthy et al. 2007; Ponka 1999). The many functions of heme in animals and humans have attracted the interest of scientists. It is worth mentioning that the widespread presence of heme in animals as well as humans indicates that porphyrins are biocompatible in humans, offering the possibility of developing porphyrin-based photosensitizing drugs.
Fig. 3 Chemical structure of heme.