3'-Arylazido-8-azido ATP--a cross-linking photoaffinity label for ATP binding proteins

The ADP and ATP transport in mitochondria and its carrier

Different from some more specialised short reviews, here a general although not encyclopaedic survey of the function, metabolic role, structure and mechanism of the ADP/ATP transport in mitochondria is presented. The obvious need for an "old fashioned" review comes from the gateway role in metabolism of the ATP transfer to the cytosol from mitochondria. Amidst the labours, 40 or more years ago, of unravelling the role of mitochondrial compartments and of the two membranes, the sequence of steps of how ATP arrives in the cytosol became a major issue. When the dust settled, a picture emerged where ATP is exported across the inner membrane in a 1:1 exchange against ADP and where the selection of ATP versus ADP is controlled by the high membrane potential at the inner membrane, thus uplifting the free energy of ATP in the cytosol over the mitochondrial matrix. Thus the disparate energy and redox states of the two major compartments are bridged by two membrane potential responsive carriers to enable their symbiosis in the eukaryotic cell. The advance to the molecular level by studying the binding of nucleotides and inhibitors was facilitated by the high level of carrier (AAC) binding sites in the mitochondrial membrane. A striking flexibility of nucleotide binding uncovered the reorientation of carrier sites between outer and inner face, assisted by the side specific high affinity inhibitors. The evidence of a single carrier site versus separate sites for substrate and inhibitors was expounded. In an ideal setting principles of transport catalysis were elucidated. The isolation of intact AAC as a first for any transporter enabled the reconstitution of transport for unravelling, independently of mitochondrial complications, the factors controlling the ADP/ATP exchange. Electrical currents measured with the reconstituted AAC demonstrated electrogenic translocation and charge shift of reorienting carrier sites. Aberrant or vital para-functions of AAC in basal uncoupling and in the mitochondrial pore transition were demonstrated in mitochondria and by patch clamp with reconstituted AAC. The first amino acid sequence of AAC and of any eukaryotic carrier furnished a 6-transmembrane helix folding model, and was the basis for mapping the structure by access studies with various probes, and for demonstrating the strong conformation changes demanded by the reorientation mechanism. Mutations served to elucidate the function of residues, including the particular sensitivity of ATP versus ADP transport to deletion of critical positive charge in AAC. After resisting for decades, at last the atomic crystal structure of the stabilised CAT-AAC complex emerged supporting the predicted principle fold of the AAC but showing unexpected features relevant to mechanism. Being a snapshot of an extreme abortive "c-state" the actual mechanism still remains a conjecture.

Photoaffinity labeling and photoaffinity cross-linking of phosphofructokinase-1 from Saccharomyces cerevisiae by 8-azidoadeninenucleotides

Phosphofructokinase-1 from Saccharomyces cerevisiae is composed of four alpha- and four beta-subunits, each of them carrying catalytic and regulatory bindings sites for MgATP. In this paper, various photoaffinity labels, such as 8-azidoadenosine 5'-triphosphate, 8-azido-1,N6-ethenoadenosine 5'-triphosphate, and 8-N3-3'(2')-O-biotinyl-8-azidoadenosine 5'-triphosphate have been used to study their interaction with the enzyme in the dark and during irradiation. All nucleotidetriphosphates function as phosphate donor forming fructose 1,6-bisphosphate from fructose 6-phosphate. However, the kinetic analysis revealed distinctly differences between them. Photolabeling causes a decrease in enzyme activity to a similar extent, and ATP acts as competitive effector to inactivation. Three bifunctional diazidodiadeninedinucleotides (8-diN3AP4A, monoepsilon-8-diN3AP4A, and diepsilon-8-diN3AP4A) were applied for studying the spatial arrangement of the nucleotide binding sites. No cross-linking of the subunits was obtained by irradiation of the enzyme with 8-diN3AP4A. Photolabeling with diepsilon-8-diN3AP4A resulted in the formation of two alpha-beta cross-links with different mobilities in the SDS-polyacrylamide gel electrophoresis, while monoepsilon-8-diN3AP4A yielded only one alpha-beta cross-link. Because an interfacial location of the catalytic sites between two subunits is less likely, we suggest that the formation of cross-linked subunits may be the result of specific interactions of the bifunctional photolabels with regulatory sites at the interface of both subunits.

2,8-Diazido-ATP--a short-length bifunctional photoaffinity label for photoaffinity cross-linking of a stable F1 in ATP synthase (from thermophilic bacteria PS3)

To demonstrate the direct interfacial position of nucleotide binding sites between subunits of proteins we have synthesized the bifunctional photoaffinity label 2,8-diazidoadenosine 5'-triphosphate (2,8-DiN3ATP). UV irradiation of the F1-ATPase (TF1) from the thermophilic bacterium PS3 in the presence of 2,8-DiN3ATP results in a nucleotide-dependent inactivation of the enzyme and in a nucleotide-dependent formation of alpha-beta crosslinks. The results confirm an interfacial localization of all the nucleotide binding sites on TF1.

Photoaffinity cross-linking of F1ATPase from spinach chloroplasts by 3'-arylazido-beta-alanyl-8-azido ATP

UV irradiation of the ATPase (CF1) from spinach chloroplasts in the presence of 3'-arylazido-beta-alanyl-8-azido ATP (8,3'-DiN3ATP) results in a nucleotide-dependent inactivation of the enzyme and in a nucleotide-dependent formation of alpha-beta cross-links. The results demonstrate an interfacial localization of the nucleotide binding sites on CF1.

Inactivation of the F1-ATPase from the thermophilic bacterium PS3 by 5'-p-fluorosulfonylbenzoylinosine at 65 degrees C is accompanied by modification of beta-tyrosine-364

A major radioactive peptide, T1, was resolved by high-performance liquid chromatography from a tryptic digest prepared from the F1-ATPase from the thermophilic bacterium PS3 which had been inactivated with p-fluorosulfonylbenzoyl[3H]inosine. Two radioactive peptides, T1P1 and T1P2, were isolated from a peptic digest of T1 by high-performance liquid chromatography. The sequences of T1P1 and T1P2 were shown to be E-E-H-X-Q-V-A-R and E-E-H-X-Q, respectively, where X corresponds to derivatized Tyr-364 of the beta subunit.

Evidence that the Mg-dependent low-affinity binding site for ATP and Pi demonstrated on the isolated beta subunit of the F0.F1 ATP synthase is a catalytic site

Binding sites for one Pi and two ATP or ADP molecules have been identified on the isolated, reconstitutively active beta subunit from the Rhodospirillum rubrum F0.F1 ATP synthase. Chemical modification of this beta subunit by the histidine reagent diethyl pyrocarbonate or by the carboxyl group reagent Woodword's reagent K results in complete inhibition of Pi binding to beta. The same reagents inhibit the binding of ATP to a Mg-dependent low-affinity site but not to a Mg-independent high-affinity site on this beta subunit. The binding stoichiometry of ADP to either site is not affected by these modifications. The beta subunit modified by either one of these reagents retains its capacity to rebind to beta-less chromatophores but not its ability to restore their photo-phosphorylation. These results indicate that the low-affinity Pi binding site on beta is located at the binding site of the gamma-phosphate group of ATP in the Mg-dependent low-affinity nucleotide binding site. This site contains histidine and carboxyl group residues, both of which are required for the binding of Pi and of the gamma-phosphate group of ATP. The same residues must also be involved in the capacity of the isolated beta subunit to restore the catalytic activity of the beta-less ATP synthase. It is therefore concluded that the low-affinity Mg-dependent substrate binding site identified on the isolated beta subunit of the R. rubrum F0.F1 ATP synthase is the catalytic site of this enzyme complex.