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Calcium-Binding Protein Protocols Calcium-Binding Protein Protocols

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Steady-State Fluorescence Spectroscopy 77<br />

Fig. 2. Steady-state tyrosine fluorescence spectra of CaM and a synthetic peptide<br />

encompassing the CaM-binding domain of constitutive nitric oxide synthase (cNOS).<br />

The Ca 2+ –CaM tyrosine fluorescence (filled ovals) is markedly intense compared with<br />

the apo- form of the protein (filled squares). The cNOS peptide alone (open squares)<br />

shows no tyrosine fluorescence, and the CaM–cNOS complex (filled triangles) fluorescence<br />

intensity is very similar to that of Ca 2+ –CaM alone (Weljie and Vogel, unpublished<br />

observations).<br />

concentrations. The tyrosine fluorescence is unchanged again when a synthetic<br />

peptide encompassing a CaM-binding domain is added to Ca 2+ –CaM. In spite<br />

of these transitions, applications for tyrosine fluorescence are somewhat limited,<br />

as the presence of tryptophan precludes exclusive tyrosine excitation.<br />

However, in spite of the fact that CaM and troponin C do not contain tryptophan<br />

residues, tryptophan fluorescence has been shown to be an important<br />

tool for probing intermolecular interactions. For example, our lab has studied<br />

the interaction of CaM with synthetic peptides encompassing the CaM-binding<br />

domains from various CaM target proteins. The tryptophan in many of these<br />

synthetic target peptides acts as excellent intrinsic fluorescent probes. <strong>Binding</strong><br />

of a target peptide to wild-type CaM is accompanied by a significant increase<br />

in tryptophan fluorescence intensity and a marked blueshift in the maximumfluorescence<br />

emission peak (see Fig. 3). Altering the chemical nature of the<br />

CaM side chains produces markedly different spectra, such as decreased<br />

fluoresence intensity when selenomethionine residues replace methionine,<br />

demonstrating the rapid use of fluorescence to provide useful information.

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