Prosthetic groups chromophores

Table 4.2 Summary of the main prosthetic group chromophores found in redox-active or electron transfer proteins/enzymes with absorption characteristics given. See Fig. 4.2 and Table 4.1 for structures and abbreviations used in the table.

All proteins have CD spectra in the ultraviolet region due to the transitions of the peptide backbone and side-chain residues. The CD bands due to the protein itself are called intrinsic Cotton effects. However, many proteins have CD bands at wavelengths that do not overlap with the intrinsic Cotton effects. These bands are due to enzyme-bound chromophores such as coenzymes, prosthetic groups, metal ions, substrates, inhibitors, etc. and are called extrinsic Cotton effects. 

Fig. 15. Structure of the prosthetic group in (a) rhodopsin, where the retinal Schiff base is in an 1 l-co,6-j-cir-l2-s-ew conformation (b) delocalized excited singlet state (c) bathorhodopsin. where the chromophore is a hexaene amine-imidazole complex. From van der Meer et al. [127],...

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. 

Selective determination of the vibrational properties of individual chromophores possible in proteins with multiple chromophoric prosthetic groups. 

Illustrative examples for such a possibility are found with the cytochromes. The name of these proteins comes from the Greek words meaning colored substances in the cell. Cytochromes are intensely red-colored redox enzjunes containing a heme prosthetic group as their dominant chromophore. Hemes are iron complexes of protoporph5uin IX derivatives (10,26). One of the most frequently studied metalloproteins of this family is cytochrome c (27). The ribbon structure of a cytochrome c enzyme together with the protein-bound heme c cofactor 6 is shown in Fig. 4. 

If the two wavelengths. A,- and Aj, and the two chromophores are very close, however, the denominator of these two terms, R and Rjt, becomes quite small and these terms can dominate in the summations for Rj and Rj. This has been demonstrated in a number of dinucleotide prosthetic groups of enzymes, such as flavin-adenine dinucleotide and nicotinamide-adenine dinucleotide. An example is given below for the adenine and nicotinate chromophores. 

MAO catalyzes the oxidative deamination of catecholamines, 5-hydroxytryptamine (serotonin), and other monoamines, both primary such as NE, and secondary such as EP. It is one of several oxidase-type enzymes whose coenzyme is the flavin-adenine-dinucleotide (FAD) covalently bound as a prosthetic group (Fig. 9-3). The isoalloxazine ring system is viewed as the catalytically functional component of the enzyme. In a narrow view N-5 and C-4a is where the redox reaction takes place (i.e., +H+, +le or -H+, -le), although the whole chromophoric N-5-C-4a-C-4-N-3-C-2-N-l region undoubtedly participates. Figure 9-3 is a proposed structure of MAO isolated from pig brain (Salach et al., 1976).4... 

In this review the term fluorescent protein refers to proteins being able to autocatalytically form a chromophore, thus possessing the intrinsic property to emit light without the need for any substrate, prosthetic group or cofactor. 

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