Parvalbumin structure

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. 

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.

The wide structural application of dipolar couplings is demonstrated by its use to validate models built by sequence homology methods. Additionally, dipolar couplings have been shown to reduce the RMSD between these models and the target structure. One example is the work reported by Chou et al., in which the RMSD of sequence homology models of the protein calmodulin, built from the structure of recoverin and parvalbumin, is reduced using heteronuclear dipolar couplings [110]. 

Parvalbumin is another protein in search of a function. It contains three HLH motifs (Mr 1 IK), but only the second and third are functional Ca " -binding sites. These are high-affinity Ca /Mg sites, and both are filled with Ca in the known crystal structures (references in Table I). In fast twitch muscle, where most parvalbumins are found, the protein is postulated to act as a Ca " buffer (Haich a/., 1979 Gi s etal., 1982). As Ca is released from troponin C after muscle contraction, the Ca may be bound by parvalbumin to prevent reinitiation of contraction. In resting cells parvalbumin likely binds Mg ", rather than Ca (Haiech etal., 1979). 

We now consider two calcium binding proteins, parvalbumin and troponin. The first, carp parvalbumin, is a small protein of known crystal structure.41 It binds two Ca2+ ions per molecule. One of the Ca2+ ions is essential for a folded structure to be maintained the second may be lost with a relatively small conformational change.42... 

Parvalbumins, which are also found in other vertebrates, are high-affinity Ca2+-buffers." Additional calcium buffers with EF-hand structures are the vitamin D-induced calbindins. One 9-kDa calbindin is found in mammalian intestinal tissue and in skin. It has two helix-loop-helix Ca2+-binding sites of differing affinity101102 that presumably function in the absorption of calcium. A 28-kDa vitamin D-dependent protein from chicken intestine contains six similar Ca2 +-binding loops.97 103... 

In order to understand these effects in a detailed way for the calcium-binding proteins, it is necessary to compare the structure of the calcium-free and calcium-bound forms of each protein. The structures of parvalbumin and the intestinal calcium-binding protein (Wasserman protein) are known (see Table 6), but at present there are no crystal structures available for calcium-free proteins. Some structural information has been deduced from changes in the NMR spectrum as calcium is removed from the calcium-bound proteins.209,210... 

The calcium sites in troponin C have been studied by X-ray absorption near edge structure (XANES).244 In all four cases, Ca2+ appears to be coordinated to carboxylate and carbonyl groups, and no structural differences could be found between the two classes of sites. Binding of Mg2+ causes a distortion of the geometry of the calcium site. Thus, the reduced affinity for Ca2+ of the Ca2+-Mg2+ sites in the presence of Mg2+ may not simply be due to competition with Mg2+, but due to some conformational change induced at these sites by Mg2+. The similarity of all four Ca2+ sites means that local bonding effects do not explain the inability of Mg2+ to bind to the calcium-specific sites I and II. The XANES of parvalbumin differs from that of troponin C. 

These are soluble, sarcoplasmic calcium-binding proteins found in vertebrates. Invertebrates contain different sarcoplasmic calcium-binding proteins.247 Parvalbumins are low molecular weight (10 000-12 000), acidic proteins which contain two binding sites for Ca2+. The structure of parvalbumin has been determined, and details of the calcium sites are given in Table 6. XANES studies indicate that the calcium sites are similar to those in calmodulin, but different from troponin C.244,248 The two parvalbumin sites have different coordination numbers, six and eight.249... 

Protein sequence and structure of troponin (TN-C) and carp muscle calcium binding parvalbumin (MCBP) are known199-201). MCBP has two calcium binding regions each consisting of an a-helix, a loop about the calcium ion and another a-helix, as so-called EF hand (Fig. 21)202,203). 

Fig. 17. Model of the three-dimensional structure of carp parvalbumin showing the a-carbon backbone and a group of phenylalanine side chains, which form an enthalpically favorable set of aromatic-aromatic interactions and, to a large extent, define the core structure of this protein.

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