Inaccessibility of tryptophan residues of recombinant human renin to quenching agents

Size of the aliphatic chain of sodium houttuyfonate analogs determines their affinity for renin and angiotensin I converting enzyme

Sodium houttuyfonate analogs (SHAs), CH(3)-(CH(2))(n)-CO-CH(2)-CH(OH)SO(3)Na, (n=6-14) were synthesized and their molecular interactions with renin and angiotensin I converting enzyme (ACE) studied using fluorescence quenching techniques. Unlike renin, inhibition of ACE activity was not directly proportional to the aliphatic chain length of SHAs. Ability of SHAs to inhibit enzyme activities and quench protein fluorescence was greater with renin than with ACE. The presence of an ACE substrate (angiotensin I) did not reduce quenching ability of SHAs, suggesting that enzyme-inhibitor interactions did not involve the active site or the substrate was displaced by inhibitor molecules. The results showed that renin is a more sensitive target than ACE for the potential antihypertensive ability of SHAs.

A fluorescence resonance energy transfer method for measuring the binding of inhibitors to stromelysin

A sensitive fluorescence resonance energy transfer method was developed for the direct measurement of the dissociation constants of stromelysin inhibitors. The method is applied to the thiadiazole class of stromelysin inhibitors and it takes advantage of the fact that, upon binding to the active site of enzyme, the thiadiazole ring, with its absorbance centered at 320 nm, is able to quench the fluorescence of the tryptophan residues surrounding the catalytic site. The changes in fluorescence are proportional to the occupancy of the active site: Analysis of the fluorescence versus inhibitor concentration data yields dissociation constants that are in agreement with the corresponding competitive inhibitory constants measured by a catalytic rate assay. The affinity of nonthiadiazole inhibitors of stromelysin-such as hydroxamic acids and others-can be determined from the concentration-dependent displacement of a thiadiazole of known affinity. Using this displacement method, we determined the affinities of a number of structurally diverse inhibitors toward stromelysin. Since the three tryptophan residues located in the vicinity of the active site of stromelysin are conserved in gelatinase and collagenase, the method should also be applicable to inhibitors of other matrix metalloproteinases.

Structural studies of the melibiose permease of Escherichia coli by fluorescence resonance energy transfer. II. Identification of the tryptophan residues acting as energy donors

In the accompanying paper, we demonstrated the presence of a fluorescence resonance energy transfer (FRET) between the tryptophans of the melibiose permease (MelB) of Escherichia coli and a fluorescent sugar, 2'-(N-5-dimethylaminonaphthalene-1-sulfonyl)aminoethyl-1-thio-beta-D- galactopyranoside (Dns2-S-Gal) bound at the sugar-binding site (Maehrel, C., Cordat, E., Mus-Veteau, I., and Leblanc, G. (1998) J. Biol. Chem. 273, 33192-33197). To identify the tryptophans that transfer their energy to the fluorescent sugar, we analyzed the FRET properties of MelB mutants carrying the replacement of each of the eight MelB tryptophans by a phenylalanine. The data indicate that Trp64, localized in loop 2-3 from the N-terminal domain, and Trp299, localized in helix IX in the C-terminal domain, are responsible for up to 80% of the FRET signal. Moreover, by assuming that only Trp299 transfers energy to Dns2-S-Gal in mutant W64F, whereas only Trp64 transfers energy to Dns2-S-Gal in mutant W299F, we calculated that Trp299 and Trp64 are about 14 and 20 A away from the probe, respectively. In addition, we observed that mutating Trp342, localized in helix X of the C-terminal domain, produces a significant increase of the polarity of the fluorescent sugar environment, suggesting its proximity to the sugar-binding site. Taken together, these data provide additional support for the suggestion that (i) the sugar-binding site is localized in the C-terminal part of the transporter, probably close to membrane segments IX and X, and (ii) the N-terminal domain, and particularly cytoplasmic loop 2-3, is also close to the sugar-binding site.

The constituent tryptophans and bisANS as fluorescent probes of the active site and of a secondary binding site of stromelysin-1 (MMP-3)

The active site of the catalytic domain of stromelysin-1 (matrix metalloproteinase-3, MMP-3) was probed by fluorescence quenching, lifetime, and polarization of its three intrinsic tryptophans and by the environmentally sensitive fluorescent reporter molecule bisANS. Wavelength-dependent acrylamide quenching identified three distinct emitting tryptophan species, only one of which changes its emission and fluorescence lifetime upon binding of the competitive inhibitor Batimastat. Significant changes in the tryptophan fluorescence polarization occur upon binding by any of the three hydroxamate inhibitors Batimastat, CAS108383-58-0, and Celltech CT1418, all of which bind in the P2'-P3' region of the active site. In contrast, the inhibitor CGS27023A, which is thought to bind in the P1-P1' region, does not induce any change in tryptophan fluorescence polarization. The use of the fluorescent probe bisANS revealed the existence of an auxiliary binding site extrinsic to the catalytic cleft. BisANS acts as a competitive inhibitor of stromelysin with a dissociation constant of Ki=22 microM. In addition to this binding to the active site, it also binds to the auxiliary site with a dissociation constant of 3.40+/-0.17 microM. The auxiliary site is open, hydrophobic, and near the fluorescing tryptophans. The binding of bisANS to the auxiliary site is greatly enhanced by Batimastat, but not by the other competitive inhibitors tested.

Fluorescence energy transfer analysis of calmodulin-peptide complexes

The interactions between calmodulin and the tryptophan residues of synthetic peptides corresponding to the calmodulin binding domains of skeletal muscle myosin light-chain kinase and the plasma membrane calcium pump were examined. The single tryptophan residue contained in each peptide became relatively immobilized and inaccessible to iodide ion upon binding to calmodulin, indicating that the indole side chain was inserted into a hydrophobic cleft in the surface of calmodulin. Fluorescence energy transfer from peptidyl tryptophan residues to an AEDANS moiety attached to cysteine-26 of spinach calmodulin was measured. Included in these analyses was a tryptophan-containing peptide analog of the calmodulin binding domain of neuromodulin. These data indicated that the indole ring of each peptide inserted 32-35 A away from cysteine-26 and may therefore interact with the carboxyl-terminal lobe of CaM in its "bent" conformation [Persechini & Kretsinger (1988a) J. Cardiovasc. Pharmacol. 12 (Suppl 5), S1-S12; Ikura et al. (1992) Science 256, 632-638; Vorherr et al. (1992) Eur. J. Biochem. 204, 931-937]. The interchange of tryptophan-3 and phenylalanine-21 of the calcium pump peptide increased the efficiency of energy transfer to the AEDANS-moiety approximately 12-fold, reducing the calculated distance to 20 A. These data suggest that phenylalanine-21 of the calcium pump peptide interacts with the hydrophobic cleft in the amino-terminal lobe of CaM.

An experimental method for the determination of enzyme-competitive inhibitor dissociation constants from displacement curves: application to human renin using fluorescence energy transfer to a synthetic dansylated inhibitor peptide

We developed a facile procedure for the determination of enzyme-competitive inhibitor dissociation constants over a wide range of potencies at any ratios of enzyme, labeled ligand, and inhibitor. The assay uses displacement curves and a fluorescent-labeled ligand to allow experimental determination of dissociation constants (Kd's) of inhibitors of human renin, a highly specific enzyme, for which numerous high affinity (up to 100 pM) inhibitors have been synthesized. The procedure involves binding a dansylated competitive inhibitor, U80215, followed by its displacement by an unlabeled inhibitor of renin. Binding of U80215 is monitored by fluorescent energy transfer from the renin tryptophans to the dansyl moiety; displacement of U80215 by an unlabeled inhibitor is monitored by a reversal of this process. The procedures may be used to determine the potencies of unlabeled inhibitors up to 100 pM affinities and to determine kinetic binding constants. The concepts described should also be useful in other protein/ligand systems.