Parvalbumins

One of these motifs, called the helix-turn-helix motif, is specific for DNA binding and is described in detail in Chapters 8 and 9. The second motif is specific for calcium binding and is present in parvalbumin, calmodulin, tro-ponin-C, and other proteins that bind calcium and thereby regulate cellular activities. This calcium-binding motif was first found in 1973 by Robert Kretsinger, University of Virginia, when he determined the structure of parvalbumin to 1.8 A resolution. 

Parvalbumin is a muscle protein with a single polypeptide chain of 109 amino acids. Its function is uncertain, but calcium binding to this protein probably plays a role in muscle relaxation. The helix-loop-helix motif appears three times in this structure, in two of the cases there is a calcium-binding site. Figure 2.13 shows this motif which is called an EF hand because the fifth and sixth helices from the amino terminus in the structure of parvalbumin, which were labeled E and F, are the parts of the structure that were originally used to illustrate calcium binding by this motif. Despite this trivial origin, the name has remained in the literature. 

Moews, P.C., Kretsinger, R.H. Refinement of the structure of carp muscle calcium-binding parvalbumin by model building and difference Fourier analysis. 

Nonrepetitive but well-defined structures of this type form many important features of enzyme active sites. In some cases, a particular arrangement of coil structure providing a specific type of functional site recurs in several functionally related proteins. The peptide loop that binds iron-sulfur clusters in both ferredoxin and high potential iron protein is one example. Another is the central loop portion of the E—F hand structure that binds a calcium ion in several calcium-binding proteins, including calmodulin, carp parvalbumin, troponin C, and the intestinal calcium-binding protein. This loop, shown in Figure 6.26, connects two short a-helices. The calcium ion nestles into the pocket formed by this structure. 

Parvalbumin (Fig 1) is a cytosolic protein expressed in fast-twitch skeletal muscles and in the nervous system. In muscles, parvalbumin controls the relaxation process. In the CNS, parvalbumin, expressed in a subpopulation of GABAergic neurons, is correlated with their firing rates, protecting the cells from Ca2+ overload. 

In the CNS, calbindin D28k is widely expressed in a subpopulation of neurons distinct from those immunoreactive for parvalbumin or calretinin. 

Parvalbumin was found to be a major fish allergen in approximately 70% of fish allergies, mediated by parvalbumin IgE antibodies, leading to respiratory and gastrointestinal symptoms. Parvalbumin also became of interest because of its potential use in gene therapy to correct cardiomyopathy. 

Parvalbumin is a cytosolic protein expressed mainly in skeletal muscles and brain. 

Fig. 13. The binding sites of calcium in (a) parvalbumin (41a), (b) annexin (41) and (c) calmodulin (42). The drawings show two bidentate carboxylates coordinated to Ca2 in the EF-hand site of parvalbumin, and one bidentate carboxylate coordinated to Ca2 in annexin and calmodulin. All the donor atoms coordinated to the calciums are oxygen donor atoms from carboxylates of asp = aspartate, or glu = glutamate, or else peptide carbonyl oxygens from gly = glycine or met = methionine. Redrawn after Refs. (41-42).

It should be pointed out that the addition of substances, which could improve the biocompatibility of sol-gel processing and the functional characteristics of the silica matrix, is practiced rather widely. Polyethylene glycol) is one of such additives [110— 113]. Enzyme stabilization was favored by formation of polyelectrolyte complexes with polymers. For example, an increase in the lactate oxidase and glycolate oxidase activity and lifetime took place when they were combined with poly(N-vinylimida-zole) and poly(ethyleneimine), respectively, prior to their immobilization [87,114]. To improve the functional efficiency of entrapped horseradish peroxidase, a graft copolymer of polyvinylimidazole and polyvinylpyridine was added [115,116]. As shown in Refs. [117,118], the denaturation of calcium-binding proteins, cod III parvalbumin and oncomodulin, in the course of sol-gel processing could be decreased by complexation with calcium cations. 

Flora, K. and Brennan, J.D. (1998) Fluorometric detection of Ca2 based on an induced change in the conformation of sol-gel entrapped parvalbumin. Analytical Chemistry, 70, 4505-4513. 

Table XI (346-390) lists a number of calcium-binding proteins and indicates very succinctly their role in biological systems. This table both illustrates the range of functions of calcium-binding proteins and serves to introduce those which appear subsequently in this chapter. The structures and functions of particularly important calcium-binding proteins such as calmodulin, parvalbumin, and troponin C are described in standard texts on biochemistry. The minimal Table XI entry for the particularly important calmodulins is amplified in the next paragraph. Table XI provides a sprinkling of references to enable readers to gain entry into the literature, for these and for most of the less-familiar species.

Oncomodulin Osteocalcin regulator (374) P-Parvalbumin (375) Extracellular protein abundant in bone and in dentin (376,377)... 

Parvalbumins Phospholipase A2 High-affinity calcium buffers Hydrolysis of phospholipids with the release of free... 

Parvalbumin (CD site) r 2 aspartates 2 glutamates (1 bidentate) 1 serine 1 phenylalanine (425,437)... 

Parvalbumin (EF site) rjd 3 aspartates 1 glutamate (bidentate) 1 lysine 1 water (425,437)... 

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