Todas las temperaturas son demasiado altas

Energy metabolism takes places in multiple compartments, separated by lipid bilayer membranes. First, glucose must cross the cytoplasmic membrane to undergo glycolysis in the cytosol. The products from glycolysis (pyruvate and NADH, in the form of reducing equivalents) are transported into the mitochondria. This exchange of molecules across the membrane is mediated by membrane transporters.

Glucose Transporters

Glucose transporters mediate the influx

of glucose across the cytosolic membrane. Generally speaking, there are two types of glucose transporters: GLUT and SGLT. The GLUT transporters are uniporters for facilitated transport, allowing glucose to move along its concentration gradient. They have twelve transmembrane regions and intracellular carboxyl and amino termini. According to common sequence motifs, they are divided into three subclasses. GLUT1 is ubiquitous, found in almost all cells. It can transport glucose and galactose in a concentration- dependent manner that is described by Michalis- Menten kinetics. The K_m for glucose is very low (1 – 2 mM). At the glucose concentration used in culture medium, the flux of GLUT1 is at its maximum. In some cells, GLUT1 is under the regulation of the transcription factor HIF-1 (hypoxia inducible factor). Under hypoxic conditions, the expression of GLUT1 is up regulated to increase the uptake rate of glucose. The K_m of GLUT1 for galactose is rather high. When galactose is used as the only sugar, even at a concentration of 25 mM, the uptake rate is so low that only a little lactate is produced. A few other notable GLUT transporters are: insulin responsive GLUT4 and fructose transporting GLUT5. In addition to GLUT1, cells in culture and in different tissues may express other GLUT transporters at different proportions. The expression of different transporters will give them different responses to the concentration of glucose or other sugars.

Two Main Types of Glucose Transporters

• GLUT transporters mediate facilitative diffusion across plasma membrane .

• SGLT, the sodium dependent glucose co-transporters are expressed primarily in small intestinal absorptive cells or renal proximal tubular cells . They use Na+_-K+

Lactate Transport

Lactate and pyruvate are transported by monocarboxylate transporters. These transporters exist in two forms: one on the cytoplasmic membrane and one on the mitochondrial inner membrane. Lactate and pyruvate are both negatively charged. Their movement across the cellular membrane will cause a charge unbalance and create an electric potential, unless measures are taken to counter that imbalance. The monocarboxylate transporters (MCT), which are responsible for their transport, are a family of co-transporters that couple the transport of lactate or pyruvate to the transport of a hydrogen ion in the same direction to neutralize the charge transfer. MCT is thus a symporter; its mechanism of transport is facilitated diffusion.

Lactate transport is enhanced by a large difference in lactate concentration between intracellular and extracellular environments. pH also affect the flux of lactate through MCT, however, whether the

Fig. 3.10: Monocarboxylate transporter for lactate and pyruvate

Table 1: Glucose Transporters Tissue

Expression Affinity

Class 1

GLUT1 ubiquitous glucose, K _m = 1 - 2 mM GLUT2 Liver, pancreas, _{intestine, kidney} glucose, K m = 16 - 20 mM

glucosamine K _m = 0.8 mM GLUT3 brain, neurons glucose, K _m = 0.8 mM GLUT4 heart, muscle, _adipose glucose, K _m = 5 mM

Class II

GLUT5 intestine, testis fructose K _m = 10 - 13 mM GLUT7 intestine, testis glucose, Km = 0.3 mM

fructose K _m = 0.1 mM GLUT9 kidney, liver fructose K _m = ? mM GLUT11 heart, muscle fructose K _m = ? mM

Class III

GLUT6 brain, spleen, _leukocytes glucose, K _m = 5 mM GLUT8 testis, brain, liver glucose, K _m = 6 mM GLUT10 liver, pancreas glucose, K _m = 0.3 mM GLUT12 heart, muscle, _prostate not well known

The second type of glucose transporter, SGLT, is a co-transporter with Na+_{. It transports two sodium}

ions and one glucose molecule into the cell. The Na+ concentration is low intracellularly but is high in the medium and in body fluid. The large sodium concentration difference and negative electric potential across the cytoplasmic membrane gives rise to a high propensity of Na+ _{to enter the cell.}

Thus, the chemical potential energy of the sodium gradient and electric potential is used to drive the uptake of glucose against a concentration gradient. SGLT transport is abundant in intestinal epithelial cells and is responsible for moving glucose from the gut into the intestinal epithelial cells. The glucose is then exported into the blood stream on the other side of the cellular barrier.

effect is enhancing or retarding is dependent on the direction of the proton gradient. MCT allows for lactate transport in both directions, for excretion as well as uptake. Keeping medium pH at a lower level reduces lactate production during rapid growth period, but enhanced lactate consumption in the stationary phase.

Cells in culture typically channel about 1/20 of the carbons from their glucose intake to the TCA cycle and oxidize them to CO₂. The molar flux of pyruvate into the mitochondria is thus about 1/10 of that of the glucose consumption rate. Each mole of pyruvate entering the TCA cycle via acetyl CoA generates about 15 moles of ATP. These are exported to the cytosol and require the import of equal moles of ADP and PO₄3- _{for their synthesis. Each mole of pyruvate}

generated from glycolysis or lactate consumption is also accompanied by one mole of NADH, whose reducing equivalent is transferred into the mitochondria through the malate-aspartate shuttle. Additionally, cells in culture consume glutamine at a high rate (approximately 1/5 to 1/10 of glucose in molar ratio). Nearly half of the glutamine enters the TCA via α-ketoglutarate. CO₂ produced in the TCA cycle is then exported out of the mitochondria. Besides these major species, many other molecules (including amino acids and nucleotides) are transported into the mitochondria for DNA, RNA, and protein synthesis. As will be described later, the precursor for fatty acid and cholesterol synthesis, acetyl CoA, is generated in the mitochondria, while fatty acid and cholesterol synthesis occurs outside the mitochondria. Acetyl CoA is very reactive and does not get transported directly across the inner membrane of the mitochondria. Rather, it is transported out of the mitochondria as citrate. After cleaving off acetyl CoA, the remaining four carbons are returned to the mitochondria as pyruvate or malate. Thus, the citrate and malate flux across the mitochondrial membrane is also substantial to sustain lipid and cholesterol synthesis. The transport across the mitochondrial inner

Transport Across Mitochondria

• The chemical potential energy generated by pyruvate oxidation/TCA cycle, i.e. NADH and FADH₂, is not necessarily all converted to ATP . The process can be decoupled to generate heat instead of ATP, as occurs in hibernating mammals.

• The mitochondrion is also the main site of molecular interconversion and degradation of amino acids and the main source of acetyl CoA . The excess glutamine consumed enters the TCA cycle through α-ketoglutarate. For some cells, asparagine acts as a sink (in addition to alanine), which is formed through oxaloacetate and aspartic acid.

membrane is dynamic and complex. Many compounds crossing the membrane are charged, yet their transport must not perturb the proton and electric potential gradient. The transport across the mitochondrial membrane must be tightly regulated. Our understanding of its regulation is still rather limited.

Under physiological conditions, the flux of glycolysis and the TCA cycle is not controlled by one or a small number of “rate-limiting” enzymes. Glucose flux is the result of mutual constraints of many enzymes in the pathway, through their feed-forward and feedback inhibition and activation. A large number of pathways are highly inter-connected and crosstalk with each other through shared common substrates or regulators. In mammals, different tissues serve different metabolic roles to maintain the overall homeostatic state of the organism. The partition of metabolic roles is largely accomplished by giving different tissues a different set of isozymes. Different isozyme sets allow cells to respond to environmental fluctuations or cellular cues differently.

In document Autor LUIS FERNANDO CASTRILLON MADRIGAL (página 76-0)