Porphyrin
Porphyrins (after Greek πορφυρά, Porphyra, the purple dye) are organic chemical dyes, which consists of four pyrrole rings ( tetrapyrrole ) exist, which are linked by four methine groups cyclically with each other. The simplest example is porphine. Porphyrins or porphyrinverwandte compounds occur as chlorophyll and heme as in the heme -based proteins hemoglobin and the various cytochromes, for example. Heme is a complex compound with protoporphyrin IX and ferrous ion as the central atom instead of the two hydrogen atoms. An iron-free heme is eg hematoporphyrin.
Importance of porphyrins
Porphyrins play a central role in human metabolism. In addition to the transport of oxygen through the blood bound in red blood pigment heme group they are present in many enzymes, such as catalase ( hydrogen peroxide detoxification ) and the enzymes of the respiratory chain in the mitochondrion. Also, the bacterially produced vitamin B12 contains a porphyrin-like moiety, namely Corrin. Open-chain degradation products of hemoglobin in the body are responsible for the coloration of urine, feces and bile. Diseases related to porphyrins are, for example hepatitis, in which the impaired degradation of heme to the yellowing of the skin leads ( jaundice ), or the inherited structure of heme disorder ( porphyria ), which accumulate precursors and lead to severe intolerance to light, stomach pain and nerve damage.
Some porphyrins, the determination of drugs in the blood, eg by means of FPIA (Fluorescence Polarization Immunoassay ) make impracticable or distort.
Biosynthesis
In the human body the biosynthesis of all porphyrins of succinyl -CoA and the amino acid glycine goes out. It takes place partly in the cytosol and partly in the mitochondrial intermembrane space. The reaction substrates and products must therefore repeatedly pass through the mitochondrial membrane; the competent transport proteins are still unknown.
The δ - aminolevulinate synthase catalyzes the combination of glycine and succinyl -CoA to α -amino- β - ketoadipat which decarboxylates spontaneously δ - aminolevulinate:
Plants, algae, bacteria ( with the exception of Alphaproteobacterien ) and Archeaebakterien can also be prepared from glutamic acid producing what is known as C5 - or δ - aminolevulinate Beale pathway. The bound tRNA amino acid is reduced by glutamyl -tRNA reductase in glutamate -1 -semialdehyde with NADPH consumption. This is finally converted into δ - aminolevulinate, which is catalyzed by glutamate -1 -semialdehyde aminotransferase. It was, however, not yet been demonstrated that bacteria possess both the C5 - way as well as the synthesis of the δ - aminolevulinate synthase described above.
Four of these molecules are converted by the porphobilinogen deaminase with elimination of four ammonia molecules to Hydroxymethylbilan. Other transformations take place gradually over uroporphyrinogen III ( uroporphyrinogen III synthase) and coproporphyrinogen III ( uroporphyrinogen decarboxylase ) to protoporphyrinogen IX ( coproporphyrinogen oxidase ) and protoporphyrin IX ( protoporphyrinogen oxidase).
Degradation and excretion
The heme oxygenase converts heme to the red, with opening of the porphyrin ring and under iron and elimination of carbon monoxide to the green biliverdin. The biliverdin reductase reduces it to orange - red bilirubin. Bilirubin is excreted via the bile into the intestine, and 75 to 80 percent of the secreted bilirubin is excreted in the feces, the rest is subject to enterohepatic circulation.
Further degradation products of bilirubin as the colorless Sterkobilinogen ( which is converted by intestinal bacteria to brown Dipyrrolen Mesobilifuchsin and Bilifuchsin ) and the orange - yellow stercobilin contribute to the normal stool color. Partial Bilirubinabbauprodukte be reabsorbed and contribute as urobilinogen and urobilin to the yellow color of urine at.