Prosthetic groups of proteins

These are rarely found free but occur as a common prosthetic group of proteins. The first such protein to be extensively studied was the iron porphyrin (Greek porphyra, purple) haemoglobin. This and other iron-porphyrin proteins play a vital role in the physiological activity of nearly all forms of life.146 These forms have the same basic structure (39) but differ in the nature of the pyrrole substituents these are shown for the major porphyrins in Table 13. It has become common practice to refer to all the iron-porphyrin proteins as haem proteins. The function of haemoglobin is, of... 

Covalently bound prosthetic groups of proteins have been obtained in good yields from enzymatic hydrolyzates. Tables XV list several conjugated proteins that have been exhaustively degraded with enzymes. In each case prosthetic groups of the parent protein were isolated from the enzymatic hydrolyzates and were found to be linked to amino acid or peptides. 

There is an enormous amount of literature on the optical rotation and absorption of prosthetic groups of proteins. Only a few examples will be given here, which derive from the past interests of the reviewer. The points to be made, hopefully, will be of general utility and are chosen to relate to the previous discussions. 

Table 5.2. Hydrophobicity scale (preliminary T, and AGha values) for chemical modifications and prosthetic groups of proteins. ...

Structure of the MoFe Protein. Extensive spectroscopic studies of the MoEe proteia, the appHcation of cluster extmsion techniques (84,151), x-ray anomalous scattering, and x-ray diffraction (10,135—137,152) have shown that the MoEe proteia contains two types of prosthetic groups, ie, protein-bound metal clusters, each of which contains about 50% of the Ee and content. Sixteen of the 30 Ee atoms and 14—16 of the 32—34... 

Fig. 7. View of the FeMo-cofactor prosthetic group of the nitrogenase MoFe protein with some of the surrounding amino acid residues where ( ) represents the molybdenum coordinated to a-His-442 and homocitrate (at the top), ( ) represents the iron, interspersed with the sulfur (O) and carbon...

FIGURE 22.18 Model of the R. viridis reaction center, (a, b) Two views of the ribbon diagram of the reaction center. Mand L subunits appear in purple and blue, respectively. Cytochrome subunit is brown H subunit is green. These proteins provide a scaffold upon which the prosthetic groups of the reaction center are situated for effective photosynthedc electron transfer. Panel (c) shows the spatial relationship between the various prosthetic groups (4 hemes, P870, 2 BChl, 2 BPheo, 2 quinones, and the Fe atom) in the same view as in (b), but with protein chains deleted. 

Koda, P., and Lee, J. (1979). Separation and structure of the prosthetic group of the blue fluorescence protein from the bioluminescent bacterium Photobacterium phosphoreum. Proc. Natl. Acad. Sci. USA 76 3068-3072. 

Classes of metalloproteins. Transition ion prosthetic groups in proteins are... 

Note, however, that the -59In mV change per pH-unit is seldom found for prosthetic groups in proteins because association of protons is usually not directly on the coordination complex (which could result in loss of the metal) but rather on a nearby (or not-so-nearby) amino-acid side chain. So, the change can be anywhere between 0 and -59In mV. This information can be quite valuable for an understanding of the mechanism of action of the metalloprotein, but it does mean that we have to carry out EPR-monitored redox titrations at several different pH-values. 

The prosthetic groups of iron-sulfur proteins fall into several classes (Lov-enberg, 1977 Spiro 1982). Rubredoxins bind single iron atoms with four cys-teinyl sulfur ligands they function as electron carriers in some bacterial systems. Rubredoxins generally have two such centers per molecule in the ferric state each iron center is EPR detectable. 

Biotinidase (EC 3.5.1.12) is required for the recycling of biotin and for the utilization of protein bound biotin from the diet. Biotin (vitamin H) functions as a prosthetic group of four carboxylases in man the mitochondrial propionyl-CoA carboxylase,... 

In rats, the conversion of vinyl bromide to reactive metabolites occurs primarily in hepatocytes. Irreversible binding of such metabolites to proteins and RNA has been established both with rat-liver microsomes in vitro and in rats in vivo. They can also alkylate the cytochrome P450 prosthetic group of phenobarbital-treated rat-liver microsomes. Exposure of rats to vinyl bromide causes a decrease in hepatic cytochrome P450 (lARC, 1986). 

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