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.