Multichannel circular dichroism investigations of the structural stability of bacterial cytochrome P-450

Effect of alcohols on binding of camphor to cytochrome P450cam: Spectroscopic and stopped flow transient kinetic studies

Addition of alcohols to cytochrome P450cam (CYP101) was shown to release the substrate camphor from the heme pocket of the enzyme. The release of the substrate was found to be caused both due to increased solubility of the substrate in solution in presence of alcohol and due to change in the tertiary structure of the active site of the enzyme. The far-UV CD and near-UV CD spectra reveal that addition of alcohols to cytochrome P450cam cause a small change in the secondary structural elements but a significant change in the tertiary structural organization of this enzyme. The CD spectra at the heme region at various concentrations of alcohols indicate a substantial change in the tertiary structural organization around the heme moiety too. The equilibrium constant associated with the binding of camphor to Cyt P450cam is strongly dependent on the concentration of alcohols and the corresponding free energy associated with the binding is found to scale linearly with the concentration of alcohols. Kinetic experiments on binding of camphor to Cyt P450cam show that both k(on) and k(off) rate constants are strongly affected by addition of alcohols suggesting that alcohol expel camphor out of the heme cavity of Cyt P450cam by affecting tertiary structure of Cyt P450cam as well as by modifying the solubility properties of camphor in aqueous medium.

Role of Threonine 101 on the Stability of the Heme Active Site of Cytochrome P450cam: Multiwavelength Circular Dichroism Studies†

The role of the threonine 101 residue that resides close to the heme propionic acid side chain of cytochrome P450cam on the conformational properties of the active site of the enzyme has been investigated by circular dichroism (CD) spectroscopy. Site-specific mutation of the threonine by valine has been carried out that does not affect the size of the residue but significantly alters the hydropathy index. The T101V mutant of cytochrome P450cam showed distinct differences in the CD spectra near the heme region, indicating a subtle effect of the mutation on the properties of the heme active site. Thermal stabilities of the mutant and wild-type enzyme have been studied by temperature dependence of the ellipticity (intensity of the CD band) in the far-UV region for the secondary structure and at different wavelengths in the visible region that arise from the heme moiety for the tertiary structure around the prosthetic group. The thermal unfolding data from variations of the CD intensity at different wavelengths were analyzed using a generalized multistep unfolding model, and two distinct equilibrium intermediate conformational states of the enzyme were identified. The mutation of the T101 residue by valine was found to decrease the thermal stability of both the intermediates in the presence of the substrate. On the other hand, this mutation had no apparent effect on the thermal stability of the enzyme in the absence of the substrate. These results suggested that the threonine residue stabilizes the protein cavity around the heme center in the case of the substrate-bound species, possibly by hydrogen bonding with one of the propionate side chains of the heme moiety. Such hydrogen bonding of the heme propionate with threonine is absent in the substrate-free form of the enzyme.

P450scc mutant nanostructuring for optimal assembly

Molecular modeling and protein engineering were synergically employed to improve the fabrication of cytochrome P450scc mutant nanostructures for biodevice assembly. The optimization of protein three-dimensional structure by molecular modeling was performed using two models: in vacuum and simulating the presence of a polar solvent. Calculations were performed on a model to predict a P450scc mutant which could improve the process of molecules' immobilization onto solid supports. Engineerized cytochrome P450scc thin films were prepared and characterized by various biophysical techniques such as pi-A isotherms, surface potential measurements, Brewster angle microscopy, UV-vis spectroscopy, circular dichroism, nanogravimetry, and electrochemical analysis. This paper takes into consideration biomolecules modified by protein engineering that represent a new and powerful approach for obtaining synthetic simpler artificial structures with new or improved properties (i.e., specificity, stability, sensitivity, etc.) useful for biosensors development.

Structural stability and dynamics of hydrogenated and perdeuterated cytochrome P450cam (CYP101)

Perdeuterated and hydrogenated cytochrome P450cam (P450cam), from Pseudomonas putida, has been characterized concerning thermal stability and structural dynamics. For the first time, Fourier transform infrared (FTIR) spectroscopy was used to characterize a perdeuterated protein. The secondary structure compositions were determined from the fitted amide I' spectral region, giving band populations at 10 degrees C for the perdeuterated protein of 22% between 1605 and 1624 cm(-1) (beta-sheets), 47% between 1633 and 1650 cm(-1) (alpha-helix (29%) plus unordered/3(10)-helix (18%)), and 28% between 1657 and 1677 cm(-1) (turns) and for the hydrogenated protein of 22% between 1610 and 1635 cm(-1) (beta-sheets), 52% between 1640 and 1658 cm(-1) (alpha-helix (41%) plus unordered/3(10)-helix (11%)), and 24% between 1665 and 1680 cm(-1) (turns). Thermal unfolding experiments revealed that perdeuterated P450cam was less stable than the hydrogenated protein. The midpoint transition temperatures were 60.8 and 64.4 degrees C for the perdeuterated and hydrogenated P450cam, respectively. Step-scan time-resolved FTIR was applied to the P450cam-CO complex to study the ligand-rebinding process after flash photolysis. Rebinding of the ligand occurred with the same kinetics and rate constants k(on), 8.9 x 10(4) and 8.3 x 10(4) M(-1) s(-1) for the perdeuterated and hydrogenated P450cam, respectively.Perdeuterated P450cam was expressed for a neutron crystallographic study to determine the specific hydration states and hydrogen-bonding networks at the active site. The analyses presented here show that perdeuterated P450cam is structurally similar to its hydrogenated counterpart, despite its reduced thermal stability, suggesting that information obtained from the neutron structure will be representative of the normal hydrogenated P450cam.

Role of substrate on the conformational stability of the heme active site of cytochrome P450cam: effect of temperature and low concentrations of denaturants

The effect of 1R-camphor on the conformational stability of the heme active site of cytochrome P450cam has been investigated. The absorption spectra of the heme moiety showed the presence of two hitherto unknown intermediates formed at low urea concentrations or during small temperature perturbations. The corresponding thermodynamic parameters were obtained by global fitting of the experimental data to a generalized sequential unfolding model at different wavelengths, which showed that the active conformation of the enzyme is stabilized by binding of the substrate at the active site. Circular-dichroism spectra of the enzyme in the visible- and far-UV region were studied to identify the critical range of denaturant concentration and the temperature at which the tertiary structure around the heme center was affected with almost no change in the secondary structure of the enzyme. This critical range of urea concentration was 0-2.8 M in the presence of camphor and 0-1.5 M in the absence of camphor. The tertiary structure of the enzyme was found to undergo conformational change in the temperature range 20-60 degrees C in the presence of the substrate and 20-47 degrees C in its absence. The spectral assignments of the intermediate species of the heme active site with the intact secondary structure of the enzyme were made by deconvolution of the Soret absorption spectra, and the results were analyzed to determine stabilization of the heme active-site geometry by 1R-camphor. Results showed that subtle conformational changes due to melting of the tertiary contacts in the active site lead to formation of intermediates which are coordinatively similar to the native enzyme. Analogous intermediate species might be responsible for leakage in the redox catalytic cycle of the enzyme.

Comparative Fourier transform infrared studies of the secondary structure and the CO heme ligand environment in cytochrome P-450cam and cytochrome P-420cam

For the first time, Fourier transform infrared spectroscopy has been applied to cytochrome P-450 to analyze the protein secondary structure. From Fourier self-deconvolution and fitting the infrared spectra in the amide I' region (1600-1700 cm-1), we estimate 44% alpha-helix, 31% beta-sheet, and 18% turns for substrate-free cytochrome P-450cam. In the presence of camphor, 54% alpha-helix and 310-helix, 21% beta-sheet, and 21% turns are obtained which agree with the crystallographic data of 53% alpha-helix, 19% beta-sheet, and 16% turns [Poulos, T. L., Finzel, B. C., & Howard, A. J. (1987) J. Mol. Biol. 195, 687-700]. Cytochrome P-420cam is produced from substrate-free cytochrome P-450cam in two ways: (i) by temperature elevation up to 60 degrees C and (ii) by exposure to KSCN up to 1.5 M. The secondary structure composition is determined for each temperature and KSCN concentration and compared with the changes observed in the iron ligand CO stretch vibration bands appearing between 1900 and 2000 cm-1. Thermally induced cytochrome P-420 has an alpha-helix content of 19%, a beta-sheet content of 53%, 14% turns, and 5% antiparallel beta-sheets from intermolecular hydrogen bonds within protein aggregates. The formation of cytochrome P-420 as a function of the KSCN concentration indicates two types of cytochrome P-420. Up to 1 M KSCN, the induced cytochrome P-420 displays only little modification of the secondary structure, whereas at 1.5 M KSCN, larger changes are observed, resulting in 85% cytochrome P-420 without protein precipitation and containing 30% alpha-helix, 48% beta-sheet, and 17% turns. Infrared spectra in the iron ligand CO stretch region show several subconformers for cytochrome P-420. During the cytochrome P-420 formation, the CO stretch modes are shifted to higher frequencies by 3-11 cm-1, with a main feature at about 1964 cm-1, compared to those of substrate-free cytochrome P-450cam-CO.

Apocytochrome P450cam is a native protein with some intermediate-like properties

Holo- and apocytochrome P450cam were studied by differential scanning calorimetry (DSC), limited proteolysis, second-derivative spectroscopy, circular dichroism, and size-exclusion chromatography. The holoprotein shows three folding units (domains) in DSC. The prosthetic group is related to the most unstable domain, which has a thermal transition at 41.9 degrees C. Compared with the holoprotein, apocytochrome P450cam has a reduced helix content. The protein is compact as judged by the Stokes radius and is still able to undergo a two-state transition. However, the enthalpy change at thermal melting is reduced from 980 kJ/mol for the holoprotein to 135 kJ/mol for the apo form. Parts of the molecule have a destabilized tertiary structure. This is indicated by second-derivative spectroscopy, circular dichroism in the near-ultraviolet region, and a high susceptibility to proteolytic digestion. Apocytochrome P450cam is considered a native protein with the extremely low stability of delta G = 7.5 kJ/mol, thus showing at the same time intermediate-like properties. The importance of the properties for in vivo folding are discussed.