Prosthetic groups primary structure

Tyr 143, 36 293 Tyr 254, 36 291-293 NMR spectroscopy, 36 271-272 pH dependence, 36 274-275 primary stmcture, 36 261-263 prosthetic groups structure, 36 258 quaternary structure, 36 261-262 reduction potentials, 36 268-269 short electron transport chain, 36 258-259 site-directed mutagenesis, 36 289-290 substrate specificity, 36 272-274 Flavocytochrome C552 electrochemistry, 36 365-367, 369... 

Hemoglobin has quaternary structure as it is made up of four polypeptide chains two a-chains and two (3-chains (a2 32), each with a heme prosthetic group. Despite little similarity in their primary sequences, the individual polypeptides of hemoglobin have a three-dimensional structure almost identical to the polypeptide chain of myoglobin. 

National Biomedical Research Foundation specializes in providing a database for protein primary structure. This database contains all the information from the Atlas of Protein Sequence and Structure edited by M.O. Dayhoff. In this database proteins are categorized according to their super family grouping. In addition to the primary structure information, detailed descriptions of proteins, including active site, prosthetic group, etc., are included. 

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. 

These are difficult enzymes to work with and only recently have crystal structures become available for two catalase-peroxidases Haloarcula marismortui (HMCP) and Burkholderia pseudomallei (BpKatG). A typical subunit is approximately 80 kDa in molecular mass, with a single heme b prosthetic group. The primary structure of each subunit can be divided into two distinct domains, N terminal and C terminal. The N-terminal domain contains the heme and active site, while the C-terminal domain does not contain a heme binding motif and its function remains unclear. The clear sequence similarity between the two domains suggests gene duplication and fusion. Curiously, despite many years of study, the actual in vivo peroxidatic substrate of the catalase-peroxidases has not been identified. 

Although aggregation is the predominant means by which proteins become inactivated during refolding, several other inactivation pathways have also been observed. Proteins can be inactivated by thiol-disulfide exchange or alteration of the primary structure by chemical modification of amino acid side chains. In addition, refolded proteins may be inactivate due to the absence of prosthetic groups and metals or because of improper association of the subunits in multimeric proteins (79). 

The copper-containing amine oxidases (copper amine oxidases, diamine oxidases) possess either a topaquinone or a 6-hydroxydopamine cofactor (Fig. 16.7-15), generally integrated in the oxidase primary structure. Tyrosine residues of the enzyme backbone in the active site are discussed as precursors for the prosthetic group[37]. 

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 spite of the close theoretical relationship between EPR and NMR spectroscopy, EPR has only very narrow applications. The primary reason for this is that the EPR phenomenon is spectroscopically silent unless there are unpaired electrons. Most biological macromolecules are closed shell molecules and contain no unpaired electrons. Therefore, EPR is of little real value for biological macromolecular structure characterisation. The only exception to this rule is that certain prosthetic groups in proteins may contain redox active metal centres/clusters that have transient or even permanent unpaired electrons (see Chapter 4). These metal centres/ clusters can be studied by EPR spectroscopy in order to demonstrate the presence of unpaired electrons. Thereafter, EPR data may then be used to derive the relative structural arrangements of metals within centres or clusters, and to assign putative distributions of redox states should there be any obvious redox heterogeneity. EPR is also useful to detect transient or even metastable radical formation during bio catalysis (see Chapter 8). 

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