Molecular crowding defines a common origin for the Warburg effect in proliferating cells and the lactate threshold in muscle physiology

Aerobic glycolysis is a seemingly wasteful mode of ATP production that is seen both in rapidly proliferating mammalian cells and highly active contracting muscles, but whether there is a common origin for its presence in these widely different systems is unknown. To study this issue, here we develop a model of human central metabolism that incorporates a solvent capacity constraint of metabolic enzymes and mitochondria, accounting for their occupied volume densities, while assuming glucose and/or fatty acid utilization. The model demonstrates that activation of aerobic glycolysis is favored above a threshold metabolic rate in both rapidly proliferating cells and heavily contracting muscles, because it provides higher ATP yield per volume density than mitochondrial oxidative phosphorylation. In the case of muscle physiology, the model also predicts that before the lactate switch, fatty acid oxidation increases, reaches a maximum, and then decreases to zero with concomitant increase in glucose utilization, in agreement with the empirical evidence. These results are further corroborated by a larger scale model, including biosynthesis of major cell biomass components. The larger scale model also predicts that in proliferating cells the lactate switch is accompanied by activation of glutaminolysis, another distinctive feature of the Warburg effect. In conclusion, intracellular molecular crowding is a fundamental constraint for cell metabolism in both rapidly proliferating- and non-proliferating cells with high metabolic demand. Addition of this constraint to metabolic flux balance models can explain several observations of mammalian cell metabolism under steady state conditions

Cell proliferation-associated expression of a recently evolved isozyme of triosephosphate isomerase

An electrophoretically unique, thermolabile isozyme of triosephosphate isomerase (TPI; EC 5.3.1.1) accounts for 10–30% of the enzymatic activity in a range of mitotically active human cells and tissues. This type 2 form (subunit) of human TPI appears in two isozymes, an anodally migrating, relative to the constitutive TPI-1/1 homodimer, TPI-2/2 homodimer and the TPI-1/2 heterodimer with an intermediate mobility. Human cell types expressing the induced isozyme, which is the product of the same structural locus as the constitutive isozyme, include mitogen-stimulated lymphocytes, virally transformed B-lymphoblastoid cells, leukemia-derived T-lymphoblastoid cells, HeLa cells, both normal and transformed fibroblasts, and placental tissue. Extracts of nondividing or terminally differentiated human cells/tissues, such as erythrocytes, striated muscle, peripheral lymphocytes, and platelets, contain high levels of the constitutive TPI-1/1 isozyme but little or undetectable levels of the TPI-1/2 or TPI-2/2 isozyme. The cell division-associated TPI-1/2 and -2/2 isozymes are distinct in electrophoretic mobility from the deamidated forms of the constitutive isozyme. Extracts of dividing gorilla fibroblasts display an isozyme pattern identical to that of proliferating human cells, but various proliferating cells derived from the African green monkey, rabbit, and chicken express only the constitutive isozyme. Thus, expression of the cell division-associated isozyme of TPI is restricted to the hominoids, suggesting a recently evolved modification mechanism which is specifically activated in proliferating cells.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44152/1/10528_2004_Article_BF00504324.pd

The creatine kinase system and pleiotropic effects of creatine

The pleiotropic effects of creatine (Cr) are based mostly on the functions of the enzyme creatine kinase (CK) and its high-energy product phosphocreatine (PCr). Multidisciplinary studies have established molecular, cellular, organ and somatic functions of the CK/PCr system, in particular for cells and tissues with high and intermittent energy fluctuations. These studies include tissue-specific expression and subcellular localization of CK isoforms, high-resolution molecular structures and structure–function relationships, transgenic CK abrogation and reverse genetic approaches. Three energy-related physiological principles emerge, namely that the CK/PCr systems functions as (a) an immediately available temporal energy buffer, (b) a spatial energy buffer or intracellular energy transport system (the CK/PCr energy shuttle or circuit) and (c) a metabolic regulator. The CK/PCr energy shuttle connects sites of ATP production (glycolysis and mitochondrial oxidative phosphorylation) with subcellular sites of ATP utilization (ATPases). Thus, diffusion limitations of ADP and ATP are overcome by PCr/Cr shuttling, as most clearly seen in polar cells such as spermatozoa, retina photoreceptor cells and sensory hair bundles of the inner ear. The CK/PCr system relies on the close exchange of substrates and products between CK isoforms and ATP-generating or -consuming processes. Mitochondrial CK in the mitochondrial outer compartment, for example, is tightly coupled to ATP export via adenine nucleotide transporter or carrier (ANT) and thus ATP-synthesis and respiratory chain activity, releasing PCr into the cytosol. This coupling also reduces formation of reactive oxygen species (ROS) and inhibits mitochondrial permeability transition, an early event in apoptosis. Cr itself may also act as a direct and/or indirect anti-oxidant, while PCr can interact with and protect cellular membranes. Collectively, these factors may well explain the beneficial effects of Cr supplementation. The stimulating effects of Cr for muscle and bone growth and maintenance, and especially in neuroprotection, are now recognized and the first clinical studies are underway. Novel socio-economically relevant applications of Cr supplementation are emerging, e.g. for senior people, intensive care units and dialysis patients, who are notoriously Cr-depleted. Also, Cr will likely be beneficial for the healthy development of premature infants, who after separation from the placenta depend on external Cr. Cr supplementation of pregnant and lactating women, as well as of babies and infants are likely to be of benefit for child development. Last but not least, Cr harbours a global ecological potential as an additive for animal feed, replacing meat- and fish meal for animal (poultry and swine) and fish aqua farming. This may help to alleviate human starvation and at the same time prevent over-fishing of oceans

Nucleotide sequence of the pyruvate decarboxylase gene from Zymomonas mobilis.

Pyruvate decarboxylase (EC 4.1.1.1), the penultimate enzyme in the alcoholic fermentation pathway of Zymomonas mobilis, converts pyruvate to acetaldehyde and carbon dioxide. The complete nucleotide sequence of the structural gene encoding pyruvate decarboxylase from Zymomonas mobilis has been determined. The coding region is 1704 nucleotides long and encodes a polypeptide of 567 amino acids with a calculated subunit mass of 60,790 daltons. The amino acid sequence was confirmed by comparison with the amino acid sequence of a selection of tryptic fragments of the enzyme. The amino acid composition obtained from the nucleotide sequence is in good agreement with that obtained experimentally