H+ -PPases: a tightly membrane-bound family

The earliest known H+-PPase (proton-pumping inorganic pyrophosphatase), the integrally membrane-bound H+-PPi synthase (proton-pumping inorganic pyrophosphate synthase) from Rhodospirillum rubrum, is still the only alternative to H+-ATP synthase in biological electron transport phosphorylation. Cloning of several higher plant vacuolar H+-PPase genes has led to the recognition that the corresponding proteins form a family of extremely similar proton-pumping enzymes. The bacterial H+-PPi synthase and two algal vacuolar H+-PPases are homologous with this family, as deduced from their cloned genes. The prokaryotic and algal homologues differ more than the H+-PPases from higher plants, facilitating recognition of functionally significant entities. Primary structures of H+-PPases are reviewed and compared with H+-ATPases and soluble PPases.

H+-proton-pumping inorganic pyrophosphatase: a tightly membrane-bound family

The earliest known H+-proton-pumping inorganic pyrophosphatase, the integrally membrane-bound H+-proton-pumping inorganic pyrophosphate synthase from Rhodospirillum rubrum, is still the only alternative to H+-ATP synthase in biological electron transport phosphorylation. Cloning of several higher plant vacuolar H+-proton-pumping inorganic pyrophosphatase genes has led to the recognition that the corresponding proteins form a family of extremely similar proton-pumping enzymes. The bacterial H+-proton-pumping inorganic pyrophosphate synthase and two algal vacuolar H+-proton-pumping inorganic pyrophosphatases are homologous with this family, as deduced from their cloned genes. The prokaryotic and algal homologues differ more than the H+-proton-pumping inorganic pyrophosphatases from higher plants, facilitating recognition of functionally significant entities. Primary structures of H+-proton-pumping inorganic pyrophosphatases are reviewed and compared with H+-ATPases and soluble proton-pumping inorganic pyrophosphatases.

Major "anastrophes" in the origin and early evolution of biological energy conversion

The metabolism of living organisms has long been usefully divided into anabolism and catabolism. Anabolism is constructive, being concerned with the assembly of complex molecules, whereas catabolism is destructive in the sense that it involves the degradation of molecules. In addition to the well-known word catastrophe for sudden, drastic destruction and its consequences, it has been found practical to introduce in the evolutionary context the word "anastrophe" (an old greek word for turning back, in the opposite direction-from anastrephein, where ana = back and strephein = to turn) to cover sudden, drastic constructive events and their consequences. Mutations in genes and genomes giving selective advantage to an organism are typical anastrophic events. A single site mutation in a gene coding for a protein may be good, neutral or bad, or anastrophic, neutral or catastrophic, respectively, for the organism. The consequence of an anastrophic mutation may be a decisive first step on the long way to a new species, whereas a catastrophic mutation may lead to major cell damage or death. This example of the use of the anastrophe concept in biological evolution leads to the question about its applicability to other parts and paths of the cosmic evolutionary process, such as physical, chemical, social and cultural evolution. Here it will be mainly considered in connection with the energy conversion aspects of the chemical evolution leading to the origin of life and of the subsequent early biological evolution. More specifically, it will be attempted to describe possible major anastrophes in energy conversion both before and after the first occurrence of life on earth.

Alternative photophosphorylation, inorganic pyrophosphate synthase and inorganic pyrophosphate

This minireview in memory of Daniel I. Arnon, pioneer in photosynthesis research, concerns properties of the first and still only known alternative photophosphorylation system, with respect to the primary phosphorylated end product formed. The alternative to adenosine triphosphate (ATP), inorganic pyrophosphate (PPi), was produced in light, in chromatophores from the photosynthetic bacterium Rhodospirillum rubrum, when no adenosine diphosphate (ADP) had been added to the reaction mixture (Baltscheffsky H et al. (1966) Science 153: 1120-1122). This production of PPi and its capability to drive energy requiring reactions depend on the activity of a membrane bound inorganic pyrophosphatase (PPase) (Baltscheffsky M et al. (1966) Brookhaven Symposia in Biology, No. 19, pp 246-253); (Baltscheffsky M (1967) Nature 216: 241-243), which pumps protons (Moyle J et al. (1972) FEBS Lett 23: 233-236). Both enzyme and substrate in the PPase (PPi synthase) are much less complex than in the case of the corresponding adenosine triphosphatase (ATPase, ATP synthase). Whereas an artificially induced proton gradient alone can drive the synthesis of PPi, both a proton gradient and a membrane potential are required for obtaining ATP. The photobacterial, integrally membrane bound PPi synthase shows immunological cross reaction with membrane bound PPases from plant vacuoles (Nore BF et al. (1991) Biochem Biophys Res Commun 181: 962-967). With antibodies against the purified PPi synthase clones of its gene have been obtained and are currently being sequenced. Further structural information about the PPi synthase may serve to elucidate also fundamental mechanisms of electron transport coupled phosphorylation. The existence of the PPi synthase is in line with the assumption that PPi may have preceded ATP as energy carrier between energy yielding and energy requiring reactions.

Inorganic pyrophosphate gives a membrane potential in yeast mitochondria, as measured with the permeant cation tetraphenylphosphonium

Evidence for a membrane potential (delta psi) generating capacity of inorganic pyrophosphate (PPi) with inorganic pyrophosphatase in mitochondria from the yeast Saccharomyces cerevisiae is presented. Addition of PPi increases the accumulation of the permeant cation tetraphenylphosphonium (TTP+) in mitochondria, as measured with a TPP(+)-selective electrode or using [3H]TPP+. This accumulation is strongly inhibited by the uncoupler carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) but not by oligomycin, whereas accumulation of TPP+ induced by ATP is strongly inhibited by both FCCP and oligomycin. The values of delta psi obtained upon addition of PPi were similar to those obtained when yeast mitochondria were energized by NADH or ATP. Energization of the wild type yeast mitochondria with ATP resulted in higher delta psi values than those achieved when PPi was hydrolyzed, whereas the effect of PPi was more pronounced in mitochondria from a pyrophosphatase (PPase) overproducing yeast strain. These results clearly indicate that PPi is significant as an energy donor and that the yeast mitochondrial PPase, the gene for which we have recently cloned and sequenced, may participate in energy transfer reactions.

Studies on the utilization of inorganic pyrophosphate by different metabolic systems in yeast mitochondria

A mitochondrial pyrophosphatase (PPase) from yeast cells (Saccharomyces cerevisiae) was studied and characterized. The hydrolytic activity towards inorganic pyrophosphate (PPi) was inhibited by different SH-reagents and increased in the presence of uncouplers, indicating a possible involvement of this enzyme in energy-linked processes. This view was also supported by the observation that these mitochondria were able to hydrolyze PPi, generating an electrical membrane potential (delta psi) of the same magnitude as that obtained with ATP. Both ATP and PPi inhibited the pyruvate dehydrogenase complex and it was demonstrated that PPi can be used as substrate by mitochondrial kinases leading to the same pattern of protein phosphorylation as when ATP is used.

Computer modeling of two inorganic pyrophosphatases

The yeast Saccharomyces cerevisiae has two inorganic pyrophosphatases that are structurally related. One, PPA1, is a cytoplasmic enzyme. The other, PPA2, is located in the mitochondria and appears to be energy-linked. The sequence similarity of PPA1 and PPA2 is about 66% and the identity is about 50%. All amino acids known to be important for catalysis are conserved, except one glutamate which is substituted by an aspartate in PPA2. The structures of PPA2 and the cytoplasmic PPase from Schizosaccharomyces pombe were modeled based on the three dimensional structure of PPA1. Two cysteines in PPA2 and one in the S. pombe enzyme are located at the catalytic cleft. Four residues form an unique insertion near the entrance of the catalytic cleft in the mitochondrial enzyme.

Characterization of a mitochondrial inorganic pyrophosphatase in Saccharomyces cerevisiae

We have studied a mitochondrial inorganic pyrophosphatase (PPase) in the yeast Saccharomyces cerevisiae. The uncoupler FCCP (carbonyl cyanide p-trifluoromethoxyphenylhydrazone) and the ionophores valinomycin and nigericin stimulate the PPase activity of repeatedly washed yeast mitochondria 2-3-fold. We have previously cloned a yeast gene, PPA2, encoding the catalytic subunit of a mitochondrial PPase. Uncouplers stimulate the PPase activity several-fold in mitochondria from both cells that overexpress PPA2 from a high copy number plasmid and cells with normal expression. These results indicate that the PPA2 polypeptide functions as an energy linked and membrane associated PPase. The stimulation of mitochondrial PPase activity by FCCP, but not by valinomycin and nigericin, was greatly enhanced by the presence of DTT. The antibiotics Dio-9, equisetin and the F0F1-ATPase inhibitor oligomycin also increase mitochondrial PPase activity several fold. This stimulation is much higher, whereas basal PPase activity is lower, in isotonic than in hypotonic solution, which indicates that intact membranes are a prerequisite for maximal effects.

Protein phosphorylation by inorganic pyrophosphate in yeast mitochondria

Inorganic pyrophosphate can function as phosphate donor in protein phosphorylation reactions in yeast mitochondria. It was shown that, when PPi substitutes for ATP as inhibitor of the pyruvate dehydrogenase reaction, maximal activity is reached after a lag-period of 30-60 minutes. 32P-labeling of peptides shows that [32P]PPi gives about 25% of the labeling obtained by [gamma-32P]ATP in the protein kinase reaction. The PPi dependent phosphorylation is increased several fold by the presence of cold ATP.

Yeast PPA2 gene encodes a mitochondrial inorganic pyrophosphatase that is essential for mitochondrial function

We have cloned a gene encoding a mitochondrial inorganic pyrophosphatase (PPase) in the yeast Saccharomyces cerevisiae by low stringency hybridization to PPA1, the yeast gene for cytoplasmic PPase. The new gene, PPA2, is located on chromosome 13 and encodes a protein whose sequence is 49% identical to the cytoplasmic enzyme. The protein differs from cytoplasmic PPase in that it has a leader sequence enriched in basic and hydroxylated residues, which is typically found in mitochondrial proteins. Yeast cells overproducing PPA2 had a 47-fold increase in mitochondrial PPase activity. This activity was further stimulated 3-fold by the uncoupler carbonyl cyanide p-trifluoromethoxyphenylhydrazone, which suggests that PPA2 is part of an energy-linked enzyme. Using gene disruptions, we found that PPA1 is required for cell growth. In contrast, cells disrupted for PPA2 are viable, but unable to grow on respiratory carbon sources. Fluorescence microscopy revealed that these cells have lost their mitochondrial DNA. We conclude that the mitochondrial PPase encoded by PPA2 is essential for mitochondrial function and maintenance of the mitochondrial genome.

Inorganic-pyrophosphate-dependent phosphorylation of spinach thylakoid proteins

It has been shown for the first time that several photosystem-II thylakoid proteins and the main chlorophyll-a/b light-harvesting complex can be phosphorylated with inorganic pyrophosphate as phosphate donor. With pyrophosphate, as with ATP, the protein-kinase reaction is dependent on light or a strong reducing agent. The reaction which can be demonstrated in well-washed spinach thylakoids is dependent on electron transport and is controlled by the redox state of the plastoquinone pool. It is suggested that the pyrophosphate-dependent thylakoid protein phosphorylation is mediated by the same kinase which is responsible for the ATP-dependent protein phosphorylation. This pyrophosphate-dependent kinase activity may be derived from an evolutionary precursor from which ATP-dependent protein phosphorylation also developed.

Energy-dependent formation of free ATP in yeast submitochondrial particles, and its stimulation by oligomycin

Yeast submitochondrial particles, in a Pi- and NADH-dependent reaction, produced low concentrations of free ATP in the absence of added ADP. This formation of free ATP, as measured by the luciferin-luciferase method, was strongly stimulated by oligomycin. For maximal stimulation, oligomycin was to be added not earlier than 5-10 min after the addition of NADH. Upon addition of antimycin or FCCP the system was completely inhibited. The amount of free ATP formed corresponded to one-third of the amount of bound ATP in submitochondrial particles. The stimulatory effect of oligomycin disappeared if the submitochondrial particles were spun down after oligomycin stimulation and then resuspended in the reaction medium, whereas submitochondrial particles with no oligomycin added initially were stimulated by oligomycin after the same procedure. A different picture emerged with addition of ADP. If the submitochondrial particles were preenergized with NADH in the presence of oligomycin before the addition of ADP the formation of free ATP upon subsequent addition of ADP was inhibited by oligomycin. In the presence of oligomycin, but lacking preenergization with NADH, a stimulation of free ATP formation was achieved with added ADP. A possible explanation for the stimulating effect of oligomycin on ATP formation in the absence of added ADP is that it enhances the release of bound ATP in an energy-requiring process. The release of only about one-third of the bound ATP could indicate that one of three nucleotide-binding subunits involved in the mechanism of ATP formation by ATP synthase is in a state suitable for such an energy-dependent release of ATP.

Soluble succinate dehydrogenase from the halophilic archaebacterium, Halobacterium halobium

Succinate dehydrogenase activity was found in both the cytoplasmic and the membrane fractions from disrupted Halobacterium halobium cells. The cytoplasmic enzyme was found to be soluble in aqueous media and had an apparent molecular weight of 90,000. The enzyme activity of the cytoplasmic succinate dehydrogenase was salt dependent, with preference for KCl over KNO3. The Km values for succinate of the soluble and the membrane-bound succinate dehydrogenases from H. halobium were 2.3 +/- 0.3 and 0.7 +/- 0.1 mM, respectively. The soluble succinate dehydrogenase was obtained from two different strains of H. halobium and was obtained independently of the method used to disrupt the bacteria. Thus, the archaebacterium, H. halobium, contains a succinate dehydrogenase which differs from the succinate dehydrogenase in most eucaryotic and eubacterial cells, where the enzyme is tightly membrane-bound.

Stepwise molecular evolution of bacterial photosynthetic energy conversion

The concept of continuity in molecular evolution implies a stepwise formation of metabolic systems and processes. In this manner, chemical and biological evolution have given rise, step by step, to such complicated systems as the photosynthetic apparatus and thus, such elaborate processes as photosynthesis in the living cell. Among currently living organisms, the bacteria contain a much less complex photosynthetic system than the algae and higher plants, which uniquely are capable fo splitting H2O. But also the bacterial system is a very highly evolved and sophisticated, membrane-bound apparatus for the transformation of light energy to other biologically useful energy forms. The study of its molecular evolution is here undertaken by the method of attempting to break down the system into its main components and functions in order to elucidate how they had originated and evolved, and how, by divergent and convergent evolutionary steps, the stage was set for the arrival of bacterial photophosphorylation.

Properties of the nitrogenase system from a photosynthetic bacterium, Rhodospirillum rubrum

Soluble nitrogenase from Rhodospirillum rubrum has been isolated and separated into its two components, the MoFe protein and the Fe protein. The MoFe protein has been purified to near homogeneity and has a molecular weight or 215 000. It contains two Mo, 25--30 Fe and 19--22 acid-labile sulphide and consists of four subunits, Mw 56 000. The Fe protein has a molecular weight 65 000. It contains approximately four Fe and four acid-labile sulphide and consists of two subunits, Mw 31 500. The highest specific activities for the purified components are 920 and 1260 nmol ethylene produced per min per mg protein, respectively. The purified components require the membrane component for activity (Nordlund, S., Eriksson, U. and Baltscheffsky, H. (1977) Biochim. Biophys. Acta 462, 187--195). Titration of the MoFe protein with the Fe protein shows saturation and excess MoFe protein over Fe protein is inhibitory. Addition of Fe2+ or Mn2+ to the reaction mixture increases the activity apparently through interaction with the membrane component.

Necessity of a membrane component for nitrogenase activity in Rhodospirillum rubrum

Acetylene reduction catalyzed by nitrogenase from Rhodospirillum rubrum has low activity and exhibits a lag phase. The activity can be increased by the addition of a chromatophore membrane component and the lag eliminated by preincubation with this component, which can be solubilized from chromatophores by treatment with NaCl. It is both trypsin- and oxygen-sensitive. Titration of the membrane component with nitrogenase and vice versa shows a saturation point. The membrane component interacts specifically with the Fe protein of nitrogenase, the interaction being ATP- and Mg2+-dependent.

Inorganic pyrophosphate: formation in bacterial photophosphorylation

Inorganic pyrophosphate is identified as the major product of photophosphorylation by isolated chromatophores from Rhodospirillum rubrum in the absence of added nucleotides.