A simple three-dimensional-focusing, continuous-flow mixer for the study of fast protein dynamics.

We present a simple, yet flexible microfluidic mixer with a demonstrated mixing time as short as 80 µs that is widely accessible because it is made of commercially available parts. To simplify the study of fast protein dynamics, we have developed an inexpensive continuous-flow microfluidic mixer, requiring no specialized equipment or techniques. The mixer uses three-dimensional, hydrodynamic focusing of a protein sample stream by a surrounding sheath solution to achieve rapid diffusional mixing between the sample and sheath. Mixing initiates the reaction of interest. Reactions can be spatially observed by fluorescence or absorbance spectroscopy. We characterized the pixel-to-time calibration and diffusional mixing experimentally. We achieved a mixing time as short as 80 µs. We studied the kinetics of horse apomyoglobin (apoMb) unfolding from the intermediate (I) state to its completely unfolded (U) state, induced by a pH jump from the initial pH of 4.5 in the sample stream to a final pH of 2.0 in the sheath solution. The reaction time was probed using the fluorescence of 1-anilinonaphthalene-8-sulfonate (1,8-ANS) bound to the folded protein. We observed unfolding of apoMb within 760 µs, without populating additional intermediate states under these conditions. We also studied the reaction kinetics of the conversion of pyruvate to lactate catalyzed by lactate dehydrogenase using the intrinsic tryptophan emission of the enzyme. We observe sub-millisecond kinetics that we attribute to Michaelis complex formation and loop domain closure. These results demonstrate the utility of the three-dimensional focusing mixer for biophysical studies of protein dynamics.

Differential ordering of the protein backbone and side chains during protein folding revealed by site-specific recombinant infrared probes.

The time scale for ordering of the polypeptide backbone relative to the side chains is a critical issue in protein folding. The interplay between ordering of the backbone and ordering of the side chains is particularly important for the formation of ß-sheet structures, as the polypeptide chain searches for the native stabilizing cross-strand interactions. We have studied these issues in the N-terminal domain of protein L9 (NTL9), a model protein with mixed α/ß structure. We have developed a general approach for introducing site-specific IR probes for the side chains (azide) and backbone ((13)C═(18)O) using recombinant protein expression. Temperature-jump time-resolved IR spectroscopy combined with site-specific labeling enables independent measurement of the respective backbone and side-chain dynamics with single residue resolution. We have found that side-chain ordering in a key region of the ß-sheet structure occurs on a slower time scale than ordering of the backbone during the folding of NTL9, likely as a result of the transient formation of non-native side-chain interactions.

Implementation of time-resolved step-scan fourier transform infrared (FT-IR) spectroscopy using a kHz repetition rate pump laser.

Time-resolved step-scan Fourier transform infrared (FT-IR) spectroscopy has been shown to be invaluable for studying excited-state structures and dynamics in both biological and inorganic systems. Despite the established utility of this method, technical challenges continue to limit the data quality and more wide ranging applications. A critical problem has been the low laser repetition rate and interferometer stepping rate (both are typically 10 Hz) used for data acquisition. Here we demonstrate significant improvement in the quality of time-resolved spectra through the use of a kHz repetition rate laser to achieve kHz excitation and data collection rates while stepping the spectrometer at 200 Hz. We have studied the metal-to-ligand charge transfer excited state of Ru(bipyridine)(3)Cl(2) in deuterated acetonitrile to test and optimize high repetition rate data collection. Comparison of different interferometer stepping rates reveals an optimum rate of 200 Hz due to minimization of long-term baseline drift. With the improved collection efficiency and signal-to-noise ratio, better assignments of the MLCT excited-state bands can be made. Using optimized parameters, carbonmonoxy myoglobin in deuterated buffer is also studied by observing the infrared signatures of carbon monoxide photolysis upon excitation of the heme. We conclude from these studies that a substantial increase in performance of ss-FT-IR instrumentation is achieved by coupling commercial infrared benches with kHz repetition rate lasers.

Ligand trapping by cytochrome c oxidase: implications for gating at the catalytic center.

Cytochrome c oxidase is a member of the heme-copper family of oxygen reductases in which electron transfer is linked to the pumping of protons across the membrane. Neither the redox center(s) associated with proton pumping nor the pumping mechanism presumably common to all heme-copper oxidases has been established. A possible conformational coupling between the catalytic center (Fe(a3)(3+)-Cu(B)(2+)) and a protein site has been identified earlier from ligand binding studies, whereas a structural change initiated by azide binding to the protein has been proposed to facilitate the access of cyanide to the catalytic center of the oxidized bovine enzyme. Here we show that cytochrome oxidase pretreated with a low concentration of azide exhibits a significant increase in the apparent rate of cyanide binding relative to that of free enzyme. However, this increase in rate does not reflect a conformational change enhancing the rapid formation of a Fe(a3)(3+)-CN-Cu(B)(2+) complex. Instead the cyanide-induced transition of a preformed Fe(a3)(3+)-N(3)-Cu(B)(2+) to the ternary complex of Fe(a3)(3+)-N(3) Cu(B)(2+)-CN is the most likely reason for the observed acceleration. Significantly, the slow rate of azide release from the ternary complex indicates that cyanide ligated to Cu(B) blocks a channel between the catalytic site and the solvent. The results suggest that there is a pathway that originates at Cu(B) and that, during catalysis, ligands present at this copper center control access to the iron of heme a(3) from the bulk medium.

Proton interactions with hemes a and a3 in bovine heart cytochrome c oxidase.

Three forms of cytochrome c oxidase, fully oxidized CcO (CcO-O), oxidized CcO complexed with cyanide (CcO.CN), and mixed valence CcO, in which both heme a(3) and Cu(B) are reduced and stabilized by carbon monoxide (MV.CO), were investigated by optical spectroscopy, MCD, and stopped-flow for the pH sensitivity of spectral features. In the pH range between pH 5.7 and 9.0, both heme a and heme a(3) in CcO-O interact with a single protolytic group. From the variation of the position of the Soret peak with changes in pH, a pK(a) of 6.6 +/- 0.2 was determined for this group. The pH sensitivity of heme a(3) is lost in the CcO.CN complex, and only heme a responds to pH changes. In MV.CO the spectra of both hemes are almost independent of pH between 5.7 and 11.0. The stoichiometry of proton uptake in the conversion of CcO-O both to MV.CO and to fully reduced CcO was determined between pH 5.8 and pH 8.2. Formation of MV.CO from CcO-O was accompanied by the uptake of approximately two protons, and this value was almost independent of pH. Full reduction of oxidized CcO was associated with the uptake of approximately 2 H(+) at basic pH, and this value increases with decreasing pH. On the basis of these proton uptake measurements, it is concluded that the pK(a) of the group is independent of the redox state of CcO. It is suggested that Glu60 of subunit II, located at the entrance of the proton conducting K-channel, is the protolytic residue that interacts with both hemes through a hydrogen-bonding network.