Calcium-binding protein, helix

Strynadka, N. C. J., and James, M. N. G. (1989). Crystal structures of the helix-loop-helix calcium-binding proteins. Annu. Rev. Biochem. 58, 951-958. 

Fig. 12. Predicted domain structure of the small calcium-binding protein BM-40/os-teonectin/SPARC. Numbering is according to the sequence of SPARC (Mason et al., 1986a), but omits the signal peptide. A dashed line denotes predicted a helix, C identifies cysteine residues, and + indicates clusters of basic residues. A potential calcium-binding domain of the EF type is noted as well as a cluster of glutamic acids. Modified from Engel et al. (1987).

Redesign of Ca fMg Specificity. Proteins in the calmodulin superfamily (including troponin C, parvalbumin (PV) and oncomodulin (OM)) share similar overall structure and yet have different selectivity for Ca and Mg see Calcium-binding Proteins, Cation-activated Enzymes). For example, PV and OM have four helix-tum-helix domains, two of which contain mixed Ca /Mg sites and two Ca -specific sites. The mixed Ca /Mg sites have been converted to Ca -specific sites by replacing amino acids with the corresponding residues in the Ca -specific site. " ... 

In calcium-binding proteins such as calmodulin, oxygen atoms from five loop residues and one water molecule form ionic bonds with a Ca ion. (b)The zinc-finger motif is present in many DNA-binding proteins that help regulate transcription. A Zn ion is held between a pair of p strands (blue) and a single a helix (red) by a pair of cysteine residues and a pair of histidine residues. The two invariant cysteine residues are usually at positions 3 and 6 and the two invariant histidine residues are... 

The EF hand is by far the most common motif for intracellular calcium-binding proteins. Briefly, it is an approximately 30 amino acid-long peptide chain composed of a central calcium-binding loop flanked by two alpha helixes. The name EF-hand was coined by Kretsinger... 

Strynadka, N.C. James, M.N. 1989, "Crystal structures ofthe helix-loop-helix calcium-binding proteins",... 

Helix-loop-helix motif A binding motif that is found in calmodulin and some other calcium-binding proteins as well as in some DNA-binding proteins. It consists of two a-helix segments connected by a loop. 

C.d. may be used to obtain information that describes more than just the extent of secondary structure, for example in the study of cytochrome bm. In this pro-tein ° the intrinsic c.d. spectrum was typical of an a-helical protein with 52% a-helix and no j -structure in the case of the iron(in) form and 49% helix (no / -structure) in the iron(ii) form. The two iron forms show differences in their aromatic c.d. spectra and it was concluded inter alia that the oxidation-reduction process results in the alteration of the environment of the aromatic chromo-phores. The use of c.d. and absorption spectra has indicated that on binding of Ca + ions to the calcium-binding protein from pig intestine, the single L-tyrosyl residue and one or more of the five L-phenylalanyl residues were perturbed. The c.d. spectrum in the peptide absorption region was typical of an a-helical protein and showed no changes in interaction with any cations. 

Protein families are proteins related by structure or function. A protein family may be structurally diverse but have a particular cluster of amino acids at the active site that defines the class according to some catalytic function (e.g., dehydrogenases and kinases). Alternatively, proteins may have a structural motif that defines the class (e.g., helix-loop-helix motif of the EF-hand calcium-binding proteins). Proteins with identical function in different organisms often have slightly different primary structures (see below). The presence of certain amino acids relative to others in primary sequences allows putative protein sequences from the Human Genome Project, for example, to be classified into general protein families. Whether this initial classification is valid remains to be seen. 

EF-hand helix-loop-helix structural domain in calcium-binding proteins... 

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