Function Heme

the heme group of succinate dehydrogenase bound histidine, electron carrier in mitochondrial electron transfer chain. large semi-transparent sphere indicates location of iron ion. pdb: 1yq3​.

hemoproteins have diverse biological functions including transportation of diatomic gases, chemical catalysis, diatomic gas detection, , electron transfer. heme iron serves source or sink of electrons during electron transfer or redox chemistry. in peroxidase reactions, porphyrin molecule serves electron source. in transportation or detection of diatomic gases, gas binds heme iron. during detection of diatomic gases, binding of gas ligand heme iron induces conformational changes in surrounding protein. in general, diatomic gases bind reduced heme, ferrous fe(ii) while peroxidases cycle between fe(iii) , fe(iv) , hemeproteins involved in mitochondrial redox, oxidation-reduction, cycle between fe(ii) , fe(iii).

it has been speculated original evolutionary function of hemoproteins electron transfer in primitive sulfur-based photosynthesis pathways in ancestral cyanobacteria-like organisms before appearance of molecular oxygen.

hemoproteins achieve remarkable functional diversity modifying environment of heme macrocycle within protein matrix. example, ability of hemoglobin deliver oxygen tissues due specific amino acid residues located near heme molecule. hemoglobin reversibly binds oxygen in lungs when ph high, , carbon dioxide concentration low. when situation reversed (low ph , high carbon dioxide concentrations), hemoglobin release oxygen tissues. phenomenon, states hemoglobin s oxygen binding affinity inversely proportional both acidity , concentration of carbon dioxide, known bohr effect. molecular mechanism behind effect steric organization of globin chain; histidine residue, located adjacent heme group, becomes positively charged under acidic conditions (which caused dissolved co2 in working muscles, etc.) releasing oxygen heme group.