Pyruvate kinase in pig liver mitochondria

Mitochondrial Transport in Glycolysis and Gluconeogenesis: Achievements and Perspectives

Some metabolic pathways involve two different cell components, for instance, cytosol and mitochondria, with metabolites traffic occurring from cytosol to mitochondria and vice versa, as seen in both glycolysis and gluconeogenesis. However, the knowledge on the role of mitochondrial transport within these two glucose metabolic pathways remains poorly understood, due to controversial information available in published literature. In what follows, we discuss achievements, knowledge gaps, and perspectives on the role of mitochondrial transport in glycolysis and gluconeogenesis. We firstly describe the experimental approaches for quick and easy investigation of mitochondrial transport, with respect to cell metabolic diversity. In addition, we depict the mitochondrial shuttles by which NADH formed in glycolysis is oxidized, the mitochondrial transport of phosphoenolpyruvate in the light of the occurrence of the mitochondrial pyruvate kinase, and the mitochondrial transport and metabolism of L-lactate due to the L-lactate translocators and to the mitochondrial L-lactate dehydrogenase located in the inner mitochondrial compartment.

Nonalcoholic Fatty Liver Disease (NAFLD). Mitochondria as Players and Targets of Therapies?

Nonalcoholic fatty liver disease (NAFLD) is the most common chronic liver disease and represents the hepatic expression of several metabolic abnormalities of high epidemiologic relevance. Fat accumulation in the hepatocytes results in cellular fragility and risk of progression toward necroinflammation, i.e., nonalcoholic steatohepatitis (NASH), fibrosis, cirrhosis, and eventually hepatocellular carcinoma. Several pathways contribute to fat accumulation and damage in the liver and can also involve the mitochondria, whose functional integrity is essential to maintain liver bioenergetics. In NAFLD/NASH, both structural and functional mitochondrial abnormalities occur and can involve mitochondrial electron transport chain, decreased mitochondrial β-oxidation of free fatty acids, excessive generation of reactive oxygen species, and lipid peroxidation. NASH is a major target of therapy, but there is no established single or combined treatment so far. Notably, translational and clinical studies point to mitochondria as future therapeutic targets in NAFLD since the prevention of mitochondrial damage could improve liver bioenergetics.

The lid domain is important, but not essential, for catalysis of Escherichia coli pyruvate kinase

Pyruvate kinase catalyses the final step of the glycolytic pathway in central energy metabolism. The monomeric structure comprises three domains: a catalytic TIM-barrel, a regulatory domain involved in allosteric activation, and a lid domain that encloses the substrates. The lid domain is thought to close over the TIM-barrel domain forming contacts with the substrates to promote catalysis and may be involved in stabilising the activated state when the allosteric activator is bound. However, it remains unknown whether the lid domain is essential for pyruvate kinase catalytic or regulatory function. To address this, we removed the lid domain of Escherichia coli pyruvate kinase type 1 (PK^TIM+Reg) using protein engineering. Biochemical analyses demonstrate that, despite the absence of key catalytic residues in the lid domain, PK^TIM+Reg retains a low level of catalytic activity and has a reduced binding affinity for the substrate phosphoenolpyruvate. The enzyme retains allosteric activation, but the regulatory profile of the enzyme is changed relative to the wild-type enzyme. Analytical ultracentrifugation and small-angle X-ray scattering data show that, beyond the loss of the lid domain, the PK^TIM+Reg structure is not significantly altered and is consistent with the wild-type tetramer that is assembled through interactions at the TIM and regulatory domains. Our results highlight the contribution of the lid domain for facilitating pyruvate kinase catalysis and regulation, which could aid in the development of small molecule inhibitors for pyruvate kinase and related lid-regulated enzymes.

The draft nuclear genome sequence and predicted mitochondrial proteome of Andalucia godoyi, a protist with the most gene-rich and bacteria-like mitochondrial genome

Background

Comparative analyses have indicated that the mitochondrion of the last eukaryotic common ancestor likely possessed all the key core structures and functions that are widely conserved throughout the domain Eucarya. To date, such studies have largely focused on animals, fungi, and land plants (primarily multicellular eukaryotes); relatively few mitochondrial proteomes from protists (primarily unicellular eukaryotic microbes) have been examined. To gauge the full extent of mitochondrial structural and functional complexity and to identify potential evolutionary trends in mitochondrial proteomes, more comprehensive explorations of phylogenetically diverse mitochondrial proteomes are required. In this regard, a key group is the jakobids, a clade of protists belonging to the eukaryotic supergroup Discoba, distinguished by having the most gene-rich and most bacteria-like mitochondrial genomes discovered to date.

Results

In this study, we assembled the draft nuclear genome sequence for the jakobid Andalucia godoyi and used a comprehensive in silico approach to infer the nucleus-encoded portion of the mitochondrial proteome of this protist, identifying 864 candidate mitochondrial proteins. The A. godoyi mitochondrial proteome has a complexity that parallels that of other eukaryotes, while exhibiting an unusually large number of ancestral features that have been lost particularly in opisthokont (animal and fungal) mitochondria. Notably, we find no evidence that the A. godoyi nuclear genome has or had a gene encoding a single-subunit, T3/T7 bacteriophage-like RNA polymerase, which functions as the mitochondrial transcriptase in all eukaryotes except the jakobids.

Conclusions

As genome and mitochondrial proteome data have become more widely available, a strikingly punctuate phylogenetic distribution of different mitochondrial components has been revealed, emphasizing that the pathways of mitochondrial proteome evolution are likely complex and lineage-specific. Unraveling this complexity will require comprehensive comparative analyses of mitochondrial proteomes from a phylogenetically broad range of eukaryotes, especially protists. The systematic in silico approach described here offers a valuable adjunct to direct proteomic analysis (e.g., via mass spectrometry), particularly in cases where the latter approach is constrained by sample limitation or other practical considerations.

Understanding the interrelationship between the synthesis of urea and gluconeogenesis by formulating an overall balanced equation

It is well known that a strong metabolic interrelationship exists between ureagenesis and gluconeogenesis. In this paper, we present a detailed, overall equation, describing a possible metabolic link between ureagenesis and gluconeogenesis. We adopted a guided approach in which we strongly suggest that students, when faced with the problem of obtaining the overall equation of a metabolic pathway, carefully account for all atoms and charges of the single reactions, as well as the cellular localizations of the substrates, and the related transport systems. If this suggestion is always taken into account, a balanced, overall equation of a metabolic pathway will be obtained, which strongly facilitates the discussion of its physiological role. Unfortunately, textbooks often report unbalanced overall equations of metabolic pathways, including ureagenesis and gluconeogenesis. Most likely the reason is that metabolism and enzymology have been neglected for about three decades, owing to the remarkable advances of molecular biology and molecular genetics. In this paper, we strongly suggest that students, when faced with the problem of obtaining the overall reaction of a metabolic pathway, carefully control if the single reactions are properly balanced for atoms and charges. Following this suggestion, we were able to obtain an overall equation describing the metabolic interrelationship between ureagenesis and gluconeogenesis, in which urea and glucose are the final products. The aim is to better rationalize this topic and to convince students and teachers that metabolism is an important and rewarding chapter of human physiology.

The occurrence of l-lactate dehydrogenase in the inner mitochondrial compartment of pig liver

Although pig represents a model species in biomedical research including studies dealing with liver patho-physiology, some aspects of liver metabolism need to be addressed. In particular, whether and how pig mitochondria can metabolize l-lactate remains to be established. We show here that pig liver mitochondria (PLM) possess their own l-lactate dehydrogenase (mL-LDH). This was shown both via immunological analysis and by assaying photometrically the L-LDH reaction in solubilised PLM. The mL-LDH reaction shows hyperbolic dependence on the substrate concentration, it is inhibited by oxamate and proves to differ from the cytosolic activity (cL-LDH), as revealed by the difference found in both pH profiles and temperature dependence of m- and cL-LDH. Titration experiments with digitonin show that mL-LDH is restricted in mitochondrial inner compartment. In agreement with the above findings, three genes in Sus scrofa genome encoded for L-LDH subunits which are predicted to have mitochondrial localization, as investigated by Target P 1.1 and PredSL analysis.

Functional characterization of the oxidative capacity of mitochondria and glycolytic assessment in benthic aquatic organisms

The metabolism of benthic aquatic invertebrates, populating transitional water ecosystems, is influenced by both physiological and environmental factors, thus involving an adjustment of physiological processes which has a metabolic cost. In order to discover changes in metabolic pathways in response to specific factors, it's firstly necessary characterizing the principal cellular metabolic activities of the small benthic aquatic organisms. We approach here the bioenergetic state issue of two benthic organisms, i.e. Lekanesphaera monodi and Gammarus insensibilis, evidencing that no apparent and statistically significative differences between them in aerobic as well in glycolytic capacities are detected, except for COX activity.

Which way does the citric acid cycle turn during hypoxia? The critical role of α-ketoglutarate dehydrogenase complex

The citric acid cycle forms a major metabolic hub and as such it is involved in many disease states involving energetic imbalance. In spite of the fact that it is being branded as a "cycle", during hypoxia, when the electron transport chain does not oxidize reducing equivalents, segments of this metabolic pathway remain operational but exhibit opposing directionalities. This serves the purpose of harnessing high-energy phosphates through matrix substrate-level phosphorylation in the absence of oxidative phosphorylation. In this Mini-Review, these segments are appraised, pointing to the critical importance of the α-ketoglutarate dehydrogenase complex dictating their directionalities.

Leukocyte pyruvate kinase expression is reduced in normal human pregnancy but not in pre-eclampsia

PROBLEM

Emerging evidence suggests that metabolism influences immune cell signaling and immunoregulation. To examine the immunoregulatory role of glycolysis in pregnancy, we evaluated the properties of pyruvate kinase in leukocytes from non-pregnant women and those with normal pregnancy and pre-eclampsia.

METHOD OF STUDY

We evaluated pyruvate kinase expression in lymphocytes and neutrophils from non-pregnant, pregnant, and pre-eclampsia patients using fluorescence microscopy and flow cytometry. Leukocyte pyruvate kinase activity and pyruvate concentrations were also evaluated. To study pyruvate's effect on signaling, we labeled Jurkat T cells with Ca(2+) dyes and measured cell responses in the presence of agents influencing intracellular pyruvate.

RESULTS

The expression of pyruvate kinase is reduced in lymphocytes and neutrophils from normal pregnant women in comparison with those of non-pregnant women and pre-eclampsia patients. Similarly, the activity of pyruvate kinase and the intracellular pyruvate concentration are reduced in leukocytes of normal pregnant women in comparison with non-pregnant women and women with pre-eclampsia. Using Jurkat cells as a model of leukocyte signaling, we have shown that perturbations of intracellular pyruvate influence Ca(2+) signals.

CONCLUSION

Normal pregnancy is characterized by reduced pyruvate kinase expression within lymphocytes and neutrophils. We speculate that reduced pyruvate kinase expression modifies immune cell responses due to reduced pyruvate concentrations.