Antonio Díaz-Quintana

Structure of the complex between plastocyanin and cytochrome f from the cyanobacterium Nostoc sp. PCC 7119 as determined by paramagnetic NMR. The balance between electrostatic and hydrophobic interactions within the transient complex determines the relative orientation of the two proteins.

The complex between cytochrome f and plastocyanin from the cyanobacterium Nostoc has been characterized by NMR spectroscopy. The binding constant is 16 mm–1, and the lifetime of the complex is much less than 10 ms. Intermolecular pseudo-contact shifts observed for the plastocyanin amide nuclei, caused by the heme iron, as well as the chemical-shift perturbation data were used as the sole experimental restraints to determine the orientation of plastocyanin relative to cytochrome f with a precision of 1.3 Å. The data show that the hydrophobic patch surrounding tyrosine 1 in cytochrome f docks the hydrophobic patch of plastocyanin. Charge complementarities are found between the rims of the respective recognition sites of cytochrome f and plastocyanin. Significant differences in the relative orientation of both proteins are found between this complex and those previously reported for plants and Phormidium, indicating that electrostatic and hydrophobic interactions are balanced differently in these complexes.

Nitration of tyrosine 74 prevents human cytochrome c to play a key role in apoptosis signaling by blocking caspase-9 activation

Tyrosine nitration is one of the most common post-transcriptional modifications of proteins, so affecting their structure and function. Human cytochrome c, with five tyrosine residues, is an excellent case study as it is a well-known protein playing a double physiological role in different cell compartments. On one hand, it acts as electron carrier within the mitochondrial respiratory electron transport chain, and on the other hand, it serves as a cytoplasmic apoptosis-triggering agent. In a previous paper, we reported the effect of nitration on physicochemical and kinetic features of monotyrosine cytochrome c mutants. Here, we analyse the nitration-induced changes in secondary structure, thermal stability, haem environment, alkaline transition and molecular dynamics of three of such monotyrosine mutants--the so-called h-Y67, h-Y74 and h-Y97--which have four tyrosines replaced by phenylalanines and just keep the tyrosine residue giving its number to the mutant. The resulting data, along with the functional analyses of the three mutants, indicate that it is the specific nitration of solvent-exposed Tyr74 which enhances the peroxidase activity and blocks the ability of Cc to activate caspase-9, thereby preventing the apoptosis signaling pathway.

A comparative structural and functional analysis of cyanobacterial plastocyanin and cytochrome c 6 as alternative electron donors to Photosystem I

Plastocyanin and cytochrome c6 are two soluble metalloproteins that act as alternative electron carriers between the membrane-embedded complexes cytochromes b6f and Photosystem I. Despite plastocyanin and cytochrome c6 differing in the nature of their redox center (one is a copper protein, the other is a heme protein) and folding pattern (one is a β-barrel, the other consists of α-helices), they are exchangeable in green algae and cyanobacteria. In fact, the two proteins share a number of structural similarities that allow them to interact with the same membrane complexes in a similar way. The kinetic and thermodynamic analysis of Photosystem I reduction by plastocyanin and cytochrome c6 reveals that the same factors govern the reaction mechanism within the same organism, but differ from one another. In cyanobacteria, in particular, the electrostatic and hydrophobic interactions between Photosystem I and its electron donors have been analyzed using the wild-type protein species and site-directed mutants. A number of residues similarly conserved in the two proteins have been shown to be critical for the electron transfer reaction. Cytochrome c6 does contain two functional areas that are equivalent to those previously described in plastocyanin: one is a hydrophobic patch for electron transfer (site 1), and the other is an electrically charged area for complex formation (site 2). Each cyanobacterial protein contains just one arginyl residue, similarly located between sites 1 and 2, that is essential for the redox interaction with Photosystem I.

Site-directed Mutagenesis of Cytochromec 6 from Synechocystis sp. PCC 6803 THE HEME PROTEIN POSSESSES A NEGATIVELY CHARGED AREA THAT MAY BE ISOFUNCTIONAL WITH THE ACIDIC PATCH OF PLASTOCYANIN

This paper reports the first site-directed mutagenesis analysis of any cytochrome c 6, a heme protein that performs the same function as the copper-protein plastocyanin in the electron transport chain of photosynthetic organisms. Photosystem I reduction by the mutants of cytochromec 6 from the cyanobacteriumSynechocystis sp. PCC 6803 has been studied by laser flash absorption spectroscopy. Their kinetic efficiency and thermodynamic properties have been compared with those of plastocyanin mutants from the same organism. Such a comparative study reveals that aspartates at positions 70 and 72 in cytochrome c 6 are located in an acidic patch that may be isofunctional with the well known “south-east” patch of plastocyanin. Calculations of surface electrostatic potential distribution in the mutants of cytochromec 6 and plastocyanin indicate that the changes in protein reactivity depend on the surface electrostatic potential pattern rather than on the net charge modification induced by mutagenesis. Phe-64, which is close to the heme group and may be the counterpart of Tyr-83 in plastocyanin, does not appear to be involved in the electron transfer to photosystem I. In contrast, Arg-67, which is at the edge of the cytochrome c 6 acidic area, seems to be crucial for the interaction with the reaction center.

Site-directed Mutagenesis of Cytochromec 6 from Anabaena Species PCC 7119 IDENTIFICATION OF SURFACE RESIDUES OF THE HEMEPROTEIN INVOLVED IN PHOTOSYSTEM I REDUCTION

A number of surface residues of cytochromec 6 from the cyanobacterium Anabaenasp. PCC 7119 have been modified by site-directed mutagenesis. Changes were made in six amino acids, two near the heme group (Val-25 and Lys-29) and four in the positively charged patch (Lys-62, Arg-64, Lys-66, and Asp-72). The reactivity of mutants toward the membrane-anchored complex photosystem I was analyzed by laser flash absorption spectroscopy. The experimental results indicate that cytochrome c 6 possesses two areas involved in the redox interaction with photosystem I: 1) a positively charged patch that may drive its electrostatic attractive movement toward photosystem I to form a transient complex and 2) a hydrophobic region at the edge of the heme pocket that may provide the contact surface for the transfer of electrons to P700. The isofunctionality of these two areas with those found in plastocyanin (which acts as an alternative electron carrier playing the same role as cytochromec 6) are evident.

Tyrosine phosphorylation turns alkaline transition into a biologically relevant process and makes human cytochrome c behave as an anti-apoptotic switch

Cytochrome c (Cc) is a key protein in cell life (respiration) and cell death (apoptosis). On the one hand, it serves as a mitochondrial redox carrier, transferring electrons between the membrane-embedded complexes III and IV. On the other hand, it acts as a cytoplasmic apoptosis-triggering agent, forming the apoptosome with apoptosis protease-activating factor-1 (Apaf-1) and activating the caspase cascade. The two functions of cytochrome c are finely tuned by the phosphorylation of tyrosines and, in particular, those located at positions 48 and 97. However, the specific cytochrome c-phosphorylating kinase is still unknown. To study the structural and functional changes induced by tyrosine phosphorylation in cytochrome c, we studied the two phosphomimetic mutants Y48E and Y97E, in which each tyrosine residue is replaced by glutamate. Such substitutions alter both the physicochemical features and the function of each mutant compared with the native protein. Y97E is significantly less stable than the WT species, whereas Y48E not only exhibits lower values for the alkaline transition pKa and the midpoint redox potential, but it also impairs Apaf-1-mediated caspase activation. Altogether, these findings suggest that the specific phosphorylation of Tyr48 makes cytochrome c act as an anti-apoptotic switch.

The interactions of cyanobacterial cytochrome c6 and cytochrome f, characterized by NMR.

During oxygenic photosynthesis, cytochromec 6 shuttles electrons between the membrane-bound complexes cytochrome bf and photosystem I. Complex formation between Phormidium laminosum cytochromef and cytochrome c 6 from bothAnabaena sp. PCC 7119 and Synechococcus elongatus has been investigated by nuclear magnetic resonance spectroscopy. Chemical-shift perturbation analysis reveals a binding site on Anabaena cytochrome c 6, which consists of a predominantly hydrophobic patch surrounding the heme substituent, methyl 5. This region of the protein was implicated previously in the formation of the reactive complex with photosytem I. In contrast to the results obtained for Anabaena cytochromec 6, there is no evidence for specific complex formation with the acidic cytochrome c 6 fromSynechococcus. This remarkable variability between analogous cytochromes c 6 supports the idea that different organisms utilize distinct mechanisms of photosynthetic intermolecular electron transfer.

Structural and Functional Analysis of Novel Human Cytochrome c Targets in Apoptosis

Since the first description of apoptosis four decades ago, great efforts have been made to elucidate, both in vivo and in vitro, the molecular mechanisms involved in its regulation. Although the role of cytochrome c during apoptosis is well established, relatively little is known about its participation in signaling pathways in vivo due to its essential role during respiration. To obtain a better understanding of the role of cytochrome c in the onset of apoptosis, we used a proteomic approach based on affinity chromatography with cytochrome c as bait in this study. In this approach, novel cytochrome c interaction partners were identified whose in vivo interaction and cellular localization were facilitated through bimolecular fluorescence complementation. Modeling of the complex interface between cytochrome c and its counterparts indicated the involvement of the surface surrounding the heme crevice of cytochrome c, in agreement with the vast majority of known redox adducts of cytochrome c. However, in contrast to the high turnover rate of the mitochondrial cytochrome c redox adducts, those occurring under apoptosis led to the formation of stable nucleo-cytoplasmic ensembles, as inferred mainly from surface plasmon resonance and nuclear magnetic resonance measurements, which permitted us to corroborate the formation of such complexes in vitro. The results obtained suggest that human cytochrome c interacts with pro-survival, anti-apoptotic proteins following its release into the cytoplasm. Thus, cytochrome c may interfere with cell survival pathways and unlock apoptosis in order to prevent the spatial and temporal coexistence of antagonist signals.

New Arabidopsis thaliana Cytochrome c Partners: A Look Into the Elusive Role of Cytochrome c in Programmed Cell Death in Plants

Programmed cell death is an event displayed by many different organisms along the evolutionary scale. In plants, programmed cell death is necessary for development and the hypersensitive response to stress or pathogenic infection. A common feature in programmed cell death across organisms is the translocation of cytochrome c from mitochondria to the cytosol. To better understand the role of cytochrome c in the onset of programmed cell death in plants, a proteomic approach was developed based on affinity chromatography and using Arabidopsis thaliana cytochrome c as bait. Using this approach, ten putative new cytochrome c partners were identified. Of these putative partners and as indicated by bimolecular fluorescence complementation, nine of them bind the heme protein in plant protoplasts and human cells as a heterologous system. The in vitro interaction between cytochrome c and such soluble cytochrome c-targets was further corroborated using surface plasmon resonance. Taken together, the results obtained in the study indicate that Arabidopsis thaliana cytochrome c interacts with several distinct proteins involved in protein folding, translational regulation, cell death, oxidative stress, DNA damage, energetic metabolism, and mRNA metabolism. Interestingly, some of these novel Arabidopsis thaliana cytochrome c-targets are closely related to those for Homo sapiens cytochrome c (Martínez-Fábregas et al., unpublished). These results indicate that the evolutionarily well-conserved cytosolic cytochrome c, appearing in organisms from plants to mammals, interacts with a wide range of targets on programmed cell death. The data have been deposited to the ProteomeXchange with identifier PXD000280.

Specific nitration of tyrosines 46 and 48 makes cytochrome c assemble a non‐functional apoptosome

Under nitroxidative stress, a minor fraction of cytochrome c can be modified by tyrosine nitration. Here we analyze the specific effect of nitration of tyrosines 46 and 48 on the dual role of cytochrome c in cell survival and cell death. Our findings reveal that nitration of these two solvent‐exposed residues has a negligible effect on the rate of electron transfer from cytochrome c to cytochrome c oxidase, but impairs the ability of the heme protein to activate caspase‐9 by assembling a non‐functional apoptosome. It seems that cytochrome c nitration under cellular stress counteracts apoptosis in light of the small amount of modified protein. We conclude that other changes such as increased peroxidase activity prevail and allow the execution of apoptosis.