Haem ring

X-ray crystallography has shown that the haem prosthetic group is located in the protein with one edge of the haem ring exposed at the surface... 

Both magnesium and porphyrin are essentially colourless, but mixing solutions of magnesium and haem together causes a colour to form. The magnesium ion resides at the centre of the haem ring, which acts as a polydentate ligand. 

Haemoglobin is an essential component of the blood. The central, active part of the molecule comprises a haem ring (as above) at the centre of which lies an iron ion. We call it a charge transfer couple, since it requires two constituents, both the iron and the haem. 

After a while, sodium ions from the salt swap for the iron ion at the centre of the haem ring. There is no longer a couple (one component is lost), and consequently no scope for an MLCT transition, so the red colour of the blood fades. 

Similar reactions occur in red algae and cyanobacteria but because the full spectrum of sunlight does not always penetrate very far below the surface of the sea they make use of different pigments. These may be structurally similar to those used by plants, and are therefore classed as bacteriochlorophylls, or belong to the structurally distinct phycobilin class of compounds. Unlike chlorophyll, which has a cyclic tetrapyrrole structure containing magnesium, the phycobilins are acyclic tetrapyrroles with similarities to the breakdown product of the haem ring, bilirubin. 

Haptoglobin/haemopexin These proteins bind free haemoglobin/haem, thus protecting delocalised haemoglobin from influences which might otherwise activate the protein or destabilize the haem ring and promote iron release. 

The chemical rational for the adoption of the d haem ring has not been rigorously determined. Nitrite reduction is one of many examples where biology can use copper or iron. It is striking that no organic cofactor is needed for the copper enzyme. 

Table 9.11 Proton magnetic resonance chemical shifts for dipyridine haem dimethyl esters in pyridine-d Haem Ring substituent resonances ...

The structure of the diamagnetic, cherry-red vitamin B12 is shown in Fig. 26.6 and it can be seen that the coordination sphere of the cobalt has many similarities with that of iron in haem (see Fig. 25.7). In both cases the metal is coordinated to 4 nitrogen atoms of an unsaturated macrocycle (in this case part of a corrin ring which is less symmetrical and not so unsaturated as the porphyrin in haem) with an imidazole nitrogen in the fifth position. A major... 

The whole process of chlorophyll disappearance in vascular plants is a complex multistep pathway, much as chlorophyll biosynthesis is, but for didactic reasons it can be abbreviated into two main stages. The first group of reactions produces greenish derivatives while the more advanced steps produce colorless compounds by an oxidative ring opening, analog to the oxygenolytic rupmre of the porphynoid macrocycle of haem. It is a very rapid process and despite considerable efforts, the detection of intermediates is difficult. ... 

Figure 7.1 The overall pathway of haem biosynthesis. 5-AminolaevuIinate (ALA) is synthesized in the mitochondrion, and is transferred to the cytosol where it is converted to porphobilinogen, four molecules of which condense to form a porphyrin ring. The next three steps involve oxidation of the pyrrole ring substituents to give protoporphyrinogen fX, whose formation is accompanied by its transport back into the mitochondrion. After oxidation to protoporphyrin IX, ferrochelatase inserts Fe2+ to yield haem. A, P, M and V represent, respectively acetyl, propionyl, methyl and vinyl (—CH2=CH2) groups. From Voet and Voet, 1995. Reproduced by permission of John Wiley Sons, Inc.

The chlorophyll molecule comprises a central magnesium ion surrounded by an organic nitrogen-containing cyclic structure called haem (or heme ), (V) which is based on a porphyrin ring. 

The corrins and porphyrins are another important class of natural chelator molecules (Figure 2.4). They are thermodynamically very stable and have four nearly coplanar pyrrole rings, the nitrogen atoms of which can accommodate a number of different metal ions in different oxidation states like Fe2+ in haem, Mg2+ in chlorophyll and Co3+ in vitamin B12. 

The haem is tightly bound to the protein in a hydrophobic pocket formed principally by helices E and F and by a single coordinate bond between the imidazole of His F8, termed the proximal histidine (Figure 13.6), and the ferrous iron, which is some 0.6 A out of the plane of the domed porphyrin ring. A second His residue, His E7 (the distal histidine), is too far away from the iron atom to coordinate with it in the deoxy state. 

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