Prosthetic groups haemoproteins

Biosynthesis of the polypeptide chain is realised by a complicated process called translation. The basic polypeptide chain is subsequently chemically modified by the so-called posttranslational modifications. During this sequence of events the peptide chain can be cleaved by directed proteolysis, some of the amino acids can be covalently modified (hydroxylated, dehydrogenated, amidated, etc.) or different so-called prosthetic groups such as haem (haemoproteins), phosphate residues (phosphoproteins), metal ions (metal-loproteins) or (oligo)saccharide chains (glycoproteins) can be attached to the molecule by covalent bonds. Naturally, one protein molecule can be modified by more means. 

The electronic properties of haemoproteins have been measured and discussed in recent years by workers whose primary interests cover a wide range of scientific disciplines, from theoretical physics to medicine and biology. In fact there can be few other fields in which so many disciplines have pooled their resources, both experimental and theoretical. In spite of the prodigious development of other physical methods electronic absorption spectroscopy remains the most widely-used tool in the study of these proteins. A proper understanding of their spectra is clearly of the greatest importance in the investigation of the molecular electronic structure of the haem chromophore, and of the effects of the structure and conformation of the polypeptide chain on the properties of the prosthetic groups derived from it. 

The iron atom in haems and haemoproteins is usually five- or six-coordi-ftate, since it can bind ligands at the axial positions. Haems such as tron(II) protoporphyrin IX will readily coordinate neutral bases such as NHj and pyridine, small unsaturated molecules like CO, and some anions. In haemoproteins, at least one of the axial ligands is provided by the polypeptide, and with the exception of some cytochromes, this is the only linkage between the polypeptide and the prosthetic group. Where only We axial position is occupied by the polypeptide, the other is thought to be taken up by a water molecule in ferric haemoproteins. This is readily replaced by other ligands. Ferrous haemoproteins, in the absence of potential ligands such as CO, can remain five-coordinate. 

The probable axial ligands of common haemoproteins are given in Table 2. Even in cytochromes, where the protein may occupy both axial positions, one of the axial ligands can be replaced by anions. The effects of changing the axial field in this way are useful in probing the electronic structure of the prosthetic group. 

Protein fluorescence is also a useful signal in such studies. In some cases, in addition to its aromatic amino acid residues, an enzyme may possess other useful chromophores. Thus rapid reaction studies of haemoproteins and flavoproteins, for example, have relied heavily on the spectral properties of the prosthetic groups of these enzymes. 

The nature of the molecular recognition between the haem and proteins in Z -type haemoproteins is not sufficiently specific to force a single orientation of the prosthetic group within its protein matrix. Although their crystal structures have invariably shown that the haem possesses a unique orientation within the protein, a solution NMR study has clearly demonstrated that the haem in these proteins is seated in two different orientations that differ by 180° rotation of the haem plane, about the 5,15-H we.so-proton axis, relative to the protein matrix (Fig. 3), and that the haem orientational disorder is present in almost all native Z -type haemopro-The dominant component in Mb possesses the same haem orientation... 

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