Calcium binding groups

The three-dimensional structure of HRP C is largely a-helical, although there is also a small region of (3-sheet. There are two domains, the distal and proximal, between which the heme group is located. These domains probably originated as a result of gene duplication, a proposal supported by their common calcium binding sites and other structural elements (Veitch 2004). 

Many calcium-binding proteins can be described in terms of loops containing potential metal-binding sites linked by helices of various shapes and lengths. There are several families of closely related proteins, in particular the groups of EF-hand species and... 

Several other properties of selected dietary fibers may influence the bioavailability of calcium directly or indirectly. Those fibers which have cation exchange capabilities such as acid polysaccharides due to free carboxyl groups on the sugar residues may bind minerals such as calcium (3,17,33,36). Loss of calcium binding protein as a result of mucosa injury caused by the feeding of some kinds of dietary fiber was credited as being the partial cause of decreased calcium absorption in everted, rat gut sac studies by Oku et al. (16). 

The tyrosinate fluorescence observed with bovine testes calmodulin is argued to be due to tyrosinate in the ground state.(123) Of the two tyrosine residues in this calmodulin, Tyr-99 apparently has a low pKa near 7 for the formation of tyrosinate, which is most likely due to nearby side chains that are involved in calcium binding. These groups could then also account for the complex pH dependence of the 345-nm emission intensity. Besides the tyrosine and tyrosinate emissions at 305 and 345 nm, respectively, Pundak and Roche(123) also reported the existence of a third emission band between 312 and 320 nm. This band was similar in its pH and calcium dependence to the other residue, Tyr-138, and was speculated to be a result of a combination of contributions from the tyrosine and tyrosinate emissions. Since this band has its excitation profile shifted to the red, however, it could be that a hydrogen-bonded tyrosine exists in this calmodulin. Alternatively, it has also been found that the presence of the 345-nm emission depends upon the method of preparation (G. Sanyal, personal communication). 

The proximal calcium binding site is coupled to the heme group by virtue of the fact that one of its ligands, Thrl71, is adjacent to the proximal histidine residue, Hisl70 (Fig. 4). The results of site-directed mutagenesis studies at this position are awaited with interest. An illustration of the importance of both calcium sites to the structure and function of HRP C is afforded by the need to incorporate calcium as a component of in vitro folding mixtures to obtain active recombinant enzyme from solubilized inclusion bodies (64). 

---