Cristina G. Timóteo

The production of cytochrome c peroxidase (CCP) from Pseudomonas ( Ps.) stutzeri (ATCC 11607) was optimized by adjusting the composition of the growth medium and aeration of the culture. The protein was isolated and characterized biochemically and spectroscopically in the oxidized and mixed valence forms. The activity of Ps. stutzeri CCP was studied using two different ferrocytochromes as electron donors: Ps. stutzeri cytochrome c(551) (the physiological electron donor) and horse heart cytochrome c. These electron donors interact differently with Ps. stutzeri CCP, exhibiting different ionic strength dependence. The CCP from Paracoccus ( Pa.) denitrificans was proposed to have two different Ca(2+) binding sites: one usually occupied (site I) and the other either empty or partially occupied in the oxidized enzyme (site II). The Ps. stutzeri enzyme was purified in a form with tightly bound Ca(2+). The affinity for Ca(2+) in the mixed valence enzyme is so high that Ca(2+) returns to it from the EGTA which was added to empty the site in the oxidized enzyme. Molecular mass determination by ultracentrifugation and behavior on gel filtration chromatography have revealed that this CCP is isolated as an active dimer, in contrast to the Pa. denitrificans CCP which requires added Ca(2+) for formation of the dimer and also for activation of the enzyme. This is consistent with the proposal that Ca(2+) in the bacterial peroxidases influences the monomer/dimer equilibrium and the transition to the active form of the enzyme. Additional Ca(2+)does affect both the kinetics of oxidation of horse heart cytochrome c (but not cytochrome c(551)) and higher aggregation states of the enzyme. This suggests the presence of a superficial Ca(2+)binding site of low affinity.

Nitric Oxide Reductase (NOR) is an integral membrane protein performing the reduction of NO to N(2)O. NOR is composed of two subunits: the large one (NorB) is a bundle of 12 transmembrane helices (TMH). It contains a b type heme and a binuclear iron site, which is believed to be the catalytic site, comprising a heme b and a non-hemic iron. The small subunit (NorC) harbors a cytochrome c and is attached to the membrane through a unique TMH. With the aim to perform structural and functional studies of NOR, we have immunized dromedaries with NOR and produced several antibody fragments of the heavy chain (VHHs, also known as nanobodies). These fragments have been used to develop a faster NOR purification procedure, to proceed to crystallization assays and to analyze the electron transfer of electron donors. BIAcore experiments have revealed that up to three VHHs can bind concomitantly to NOR with affinities in the nanomolar range. This is the first example of the use of VHHs with an integral membrane protein. Our results indicate that VHHs are able to recognize with high affinity distinct epitopes on this class of proteins, and can be used as versatile and valuable tool for purification, functional study and crystallization of integral membrane proteins.

Crystals of cytochrome c peroxidase from Pseudomonas stutzeri were obtained using sodium citrate and PEG 8000 as precipitants. A complete data set was collected to a resolution of 1.6 A under cryogenic conditions using synchrotron radiation at the ESRF. The crystals belong to space group P2(1), with unit-cell parameters a = 69.29, b = 143.31, c = 76.83 A, beta = 100.78 degrees. Four CCP molecules were found in the asymmetric unit, corresponding to a pair of dimers related by local dyads. The crystal packing in the structure shows that the functional dimers can dimerize, as suggested by previous biochemical studies.