Chemical modification of active sites in relation to the catalytic mechanism of F1

Beef heart mitochondrial F1-ATPase: inhibition by azidoadenyl-5'-yl imidodiphosphates and cooperative binding of substrate

Two ATP analogs, 2- and 8-azidoadenyl-5'-yl imidodiphosphate, were synthesized, purified and utilized as inhibitors of soluble beef heart mitochondrial F1-ATPase under non-photolytical conditions. In the range of 5 microM to 3 mM ATP, the initial rates of ATP hydrolysis in the presence and absence of the inhibiting ATP analogs can be adequately described by two pairs of Km and Vmax values (3 microM, 8.5 mumol ATP/min per mg; 255 microM, 42.0 mumol ATP/min per mg). With increasing inhibitor concentrations, the apparent Km,2 increases as in competitive inhibition, while Vmax,1 decreases as in non-competitive inhibition. The Ki values derived for both types of inhibition are similar, but strongly different for 2- and 8-azido-AMP-PNP (4 microM and 460 microM, respectively). The decrease of the high-affinity Vmax is compensated by an increase in low-affinity catalysis, resulting in a constant sum of maximal velocities. These data can be described by a model where two sites interact with negative cooperativity in binding of substrate.

Inhibitory effect of NaN3 on the F0F1 ATPase of submitochondrial particles as related to nucleotide binding

The inhibitory effects of NaN3 on the F0F1 ATPase of beef heart submitochondrial particles were investigated. It was shown that NaN3 inhibited the ATPase activity only in the presence of ATP or ADP and the inhibition proceeded slowly. Analysis of the time-course of the inhibition process lead to a conclusion that an ATP binding site which has an apparent Kd of 14.0 +/- 8.7 microM is responsible for the increase of NaN3 sensitivity. This value agreed well with the low Km of ATP hydrolysis characterized before (Muneyuki, E., and Hirata, H. (1988) FEBS Lett. 234, 455-458) and in the range of so-called bi-site catalysis. The same conclusion was derived as for isolated F1 ATPase. From similar analysis, the Kd of this site for ADP was deduced to be 1.34 +/- 0.45 microM, which also agreed with that reported by Pedersen (Pedersen, P.L. (1975) Biochem. Biophys. Res. Commun. 64, 610-616) and also in the same range as reported for the low Km of ATP synthesis by activated submitochondrial particles. These results suggest that hydrolysis through the low Km mode of ATPase reaction leads the enzyme NaN3 sensitive form and this reaction cycle corresponds to the low Km mode of ATP synthesis.

Immunoelectron microscopy of enzymes, multienzyme complexes, and selected other oligomeric proteins

The collective term "immunoelectron microscopy" subsumes a number of techniques in which the biological material is decorated with specific antibodies, prior to being visualized in the electron microscope. In this article, we have reviewed literature on immunoelectron microscopy that focusses on the analysis of the molecular architecture of proteins, in particular of enzymes and of multienzyme complexes. Molecular immunoelectron microscopy has been remarkably successful with multi-subunit enzymes of complex quaternary structures, and in many cases the data have been the basis for the eventual development of detailed three-dimensional molecular models. The elucidation of subunit composition and juxtaposition of a given enzyme, an important accomplishment in itself, has in turn stimulated and guided discussions on the catalytic mechanism; illustrative examples include F1 ATPase and citrate lyase, among others. Here we have chosen a variety of enzymes, multienzyme complexes, and non-enzymatic proteins to demonstrate the versatility of immunoelectron microscopy, to illustrate methodological prerequisites and limitations, and to discuss significance and implications of individual immunoelectron microscopy studies.

Beta subunit of mitochondrial F1-ATPase from the fission yeast. Deduced sequence of the wild type protein and identification of a mutation that increases nucleotide binding

The Schizosaccharomyces pombe nuclear gene, atp2, encoding the beta subunit of the mitochondrial ATP synthase, was sequenced and found to contain a 1575-bp open reading frame. Two adjacent transcription-initiation sites were found at positions 34 and 44 nucleotides upstream of the translation-initiation codon. The deduced polypeptide sequence was composed of 525 amino acid residues (molecular mass = 56875 Da). The mature polypeptide starts at residue 45 (molecular mass = 51,685 Da), indicating the presence of a presequence of 44 residues, presumably involved in mitochondrial targeting. The atp2 mutant B59-1 [Boutry, M. & Goffeau, A. (1982) Eur. J. Biochem. 125, 471-477] and its related revertant allele R4-3 [Jault, J. M., Di Pietro, A., Falson, P., Gautheron, D. C., Boutry, M. & Goffeau, A. (1989) Biochem. Biophys. Res. Commun. 158, 392-399] were also cloned and sequenced. A single nonsense mutation, CAG (Gln170)----TAG (stop) in mutant B59-1, became a missense mutation, TAG (stop)----TAC (Tyr) in revertant R4-3. Gln170 is located between the first and second elements belonging to the nucleotide-binding site. Its substitution by a tyrosine residue increases the enzyme affinity towards ADP, the amount of endogenous nucleotides and the apparent negative cooperativity for ATPase activity.

Intrinsic tryptophan fluorescence of Schizosaccharomyces pombe mitochondrial F1-ATPase. A powerful probe for phosphate and nucleotide interactions

Mitochondrial F1 from the yeast Schizosaccharomyces pombe, in contrast to the mammalian enzyme, exhibits a characteristic intrinsic tryptophan fluorescence with a maximal excitation at 291 nm and a maximal emission at 332 nm. Low values of Stern-Volmer quenching constants, 4.0 M-1 or 1.8 M-1, respectively, in the presence of either acrylamide or iodide, indicate that tryptophans are mainly buried inside the native enzyme. Upon subunit dissociation and unfolding by 6 M guanidine hydrochloride (Gdn.HCl), the maximal emission is shifted to 354 nm, a value very similar to that obtained with N-acetyltryptophanamide, a solute-tryptophan model compound. The tryptophan content of each isolated subunit has been estimated by fluorescence titration in the presence of Gdn.HCl with free tryptophan as a standard. Two tryptophans and one tryptophan are found respectively in the alpha and epsilon subunits, whereas none is detected in the beta, gamma, and delta subunits. These subunit contents are consistent with the total of seven tryptophans estimated for native F1 with alpha 3 beta 3 gamma 1 delta 1 epsilon 1 stoichiometry. The maximal emission of the isolated epsilon subunit is markedly blue-shifted to 310-312 nm by interaction with the isolated delta subunit, which suggests that the epsilon subunit tryptophan might be a very minor contributor to the native F1 fluorescence measured at 332 nm. This fluorescence is very sensitive to phosphate, which produces a marked blue shift indicative of tryptophans in a more hydrophobic environment. On the other hand, ADP and ATP quench the maximal emission at 332 nm, lower tryptophan accessibility to acrylamide, and reveal tryptophan heterogeneity.