Hemoproteins heme groups

Catalase is a hemoprotein containing four heme groups. In addition to possessing peroxidase activity, it is able to use one molecule of H2O2 as a substrate electron donor and another molecule of H2O2 as an oxidant or electron acceptor. 

The hyperfine shifts in the proton NMR spectra of paramagnetic hemes and hemoproteins are closely related to the electronic structures of these molecules. At present the most extensive NMR studies of the electronic spin distribution in the heme groups have been done with low spin ferric compounds, which will be discussed in this section. Procedures similar to those described here would apply to the analysis of the NMR spectra of hemoproteins in the other paramagnetic states (Table 1). 

A more detailed treatment, including the effects of spin delocalization on the pseudocontact shifts, might be warranted once single crystal EPR data will become available for several of the low spin ferric heme compounds. In the hemoproteins it would then be of special interest to investigate pseudocontact shifts for amino acid residues near the heme groups which could yield structural information in a similar way as the ring current shifts. 

One of the obvious goals in the study of hemoproteins is an understanding of the mechanisms by which the physical and chemical properties of the heme group are modulated by the protein environment. The primary control of heme reactivity clearly is exercised in the selection of axial ligand(s) through molecular evolution. However, powerful secondary control mechanisms also exist. As the... 

In this context, the control of O2 affinity by the distal environment of the heme group in hemoproteins can be probed by comparison of wild-type compounds with mutants which do not contain distal histidine [105]. Replacement of distal histidine by glycine results in a 10-fold decrease in O2 affinity. Within the hemoproteins themselves, the lack of distal histidine in aplysia myoblobin compared to horse myoglobin increases the dissociation rate constant by a factor of six, whereas the association rate constants are identical for both systems. The data are again consistent with the stabilization of bound O2 via hydrogen-bonding in horse myoglobin. 

Catabolic pathway for the heme group from hemoproteins (predominantly hemoglobin). 

The heme group of hemoproteins is first converted to biliverdin and then to bilirubin. After undergoing conjugation reactions in the liver, bilirubin is excreted in bile. 

In studying the reaction of the heme group in hemoproteins it is always necessary to ensure that no alteration in protein structure is affecting the results. Complete denaturation is easily recognized by precipitation and flocculation and the liberation of heme of hematin, but minor changes can also occur. In the case of peroxidase two forms were isolated by Theorell (25) and named peroxidase I and II. The first of these, now called paraperoxidase (Theorell, 26), has been found to have undergone some alteration possibly the removal of the carbohydrate portion of the protein molecule or possibly reversible denaturation to a slight extent (Keilin and Hartree, 48). The paraperoxidase shows enzymatic activity comparable to that of the intact enzyme. 

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