Mitochondria mechanism

Brini, M. Ca2+ signaling in mitochondria mechanism and role in physiology and pathology. Cell Calcium 34 399 105,2003. 

Mitochondria have their own DNA (mtDNA) and genetic continuity. This DNA only encodes 13 peptide subunits synthesized in the matrix that are components of complexes I, III, IV, and V of the respiratory chain. Most mitochondrial proteins are synthesized on cytoplasmic ribosomes and imported by specific mechanisms to their specific locations in the mitochondrion (see below). 

ALASl. This repression-derepression mechanism is depicted diagrammatically in Figure 32-9. Thus, the rate of synthesis of ALASl increases greatly in the absence of heme and is diminished in its ptesence. The turnover rate of ALASl in rat liver is normally rapid (half-life about 1 hour), a common feature of an enzyme catalyzing a rate-limiting reaction. Heme also affects translation of the enzyme and its transfer from the cytosol to the mitochondrion. 

Homeostatic mechanisms also allow animals to control their intracellular pH very strictly. In humans for example, blood pH (usually taken as a reliable but indirect measure of cellular pH) is 7.4 0.04. At 37 °C cytosolic pH is actually slightly lower at about 7.0 but different compartments within the eukaryotic cells may have quite different pH, for example, lysosomes have an internal pH of about 5 the inside of a mitochondrion is more alkaline than the outside whilst the inside of a phagosome in a white blood cell is more acidic than its surrounding cytosol, both situations arising due to proton pumping across a membrane. 

The control mechanisms operating on the TCA cycle are similar to those described for glycolysis above, that is allosteric and covalent. As might be predicted, it is the concentration of acetyl-CoA and the ATP-to-ADP and NADH-to-NAD+ ratios which are crucially important as these indicate energy status within each mitochondrion and implicitly therefore the energy status of the whole cell. High concentrations of ATP or NADH slow down the cycle, an effect which is partly mediated by covalent modification. 

Oxaloacetate is returned to the mitochondrion, by the shuttle mechanism described in Chapter 9 (Figure 9.17). 

Figure 11.3 Mechanism of transfer of acetyl-CoA out of the mitochondrion. In the mitochondrion, acetyl-CoA reacts with oxaloacetate to form citrate, which is transported across the mitochondrial inner membrane. In the cytosol, citrate is split to re-form citrate and oxaloacetate, catalysed by citrate lyase. It has been shown that inhibition of citrate lyase inhibits fatty acid synthesis.

The compartmentalization of energy generation provides a mechanism for the increased efficiency of high energy bond transfer to form the ultimate cellular fuel ATP, and has been the driving force behind the evolution of the mitochondrion. This viewpoint is supported by studies of the mitochondrial proteome, which have demonstrated that proteins of eubacterial origin predominantly... 

The transport of calcium into the mitochondrion can lower external Ca2+ to levels of 1 to 0.1 jumole/1. This has, therefore, been interpreted as a basic mechanism in maintaining intracellular calcium at these levels. Only about 3 % of the calcium which passively diffuses into the cell is expelled by a calcium pump into the plasma membrane, whereas the remaining 97 % is sequestered into the mitochondria. This occurs because both processes have similar rate constants, but the total mitochondrial surface is some 30 times larger than that of the plasma membrane. This argument presupposes, however, that the calcium which enters the cell is equally available to both sets of membranes. 

The mechanism by which the mineral leaves the mitochondrion is only one of the problems of this theory. The mineral in the mitochondrion exists in association with the fluid contents. Thus, unless this water is in some structural form with abnormal solubilities, the mineral must be saturating the fluid, and solubility products apply. It follows that the mitochondrial calcium and phosphate concentrations must be similar to those of the extracellular fluids, i. e. calcium must be concentrated thousandfold to overcome the low intracellular values. 

Mitochondria arise by division and growth of preexisting mitochondria. Because they synthesize only a few proteins and RNA molecules, they must import many proteins and other materials from the cytoplasm. A mitochondrion contains at least 100 proteins that are encoded by nuclear genes.50,50a The mechanisms by which proteins are taken up by mitochondria are complex and varied. Many of the newly synthesized proteins carry, at the N terminus, presequences that contain mitochondrial targeting signals51-53 (Chapter 10). These amino acid sequences often lead the protein to associate with receptor proteins on the outer mitochondrial membrane and subsequently to be taken up by the mitochondria. While the targeting sequences are usually at the N terminus of a polypeptide, they are quite often internal. The N-terminal sequences are usually removed by action of the mitochondrial processing peptidase (MPP) in... 

Calcium levels are believed to be controlled in part at least by the uptake and release of Ca2+ from mitochondria.172"174 The capacity of mitochondria for Ca2+ seems to be more than sufficient to allow the buffering of Ca2+ at low cytosol levels. Mitochondria take up Ca2+ by an energy-dependent process either by respiration or ATP hydrolysis. It is now agreed that Ca2+ enters in response to the negative-inside membrane potential developed across the inner membrane of the mitochondrion during respiration. The uptake of Ca2+ is compensated for by extrusion of two H+ from the matrix, and is mediated by a transport protein. Previous suggestions for a Ca2+-phosphate symport are now discounted. The possible alkalization of the mitochondrial matrix is normally prevented by the influx of H+ coupled to the influx of phosphate on the H - PCV symporter (Figure 10). This explains why uptake of Ca2+ is stimulated by phosphate. Other cations can also be taken up by the same mechanism. 

Cells must ensure that each newly synthesized protein is sorted to its correct location where it can carry out the appropriate function. This process is called protein targeting. In a eukaryotic cell, the protein may be destined to stay in the cytosol, for example an enzyme involved in glycolysis (see Topic J3). Alternatively it may need to be targeted to an organelle (such as a mitochondrion, lysosome, peroxisome, chloroplast or the nucleus) or be inserted into the plasma membrane or exported out of the cell. In bacteria such as E. coli, the protein may stay in the cytosol, be inserted into the plasma membrane or the outer membrane, be sent to the space between these two membranes (the periplasmic space) or be exported from the cell. In both prokaryotes and eukaryotes, if a protein is destined for the cytosol, it is made on free ribosomes in the cytosol and released directly into the cytosol. If it is destined for other final locations, specific protein-targeting mechanisms are involved. 

Oxidative phosphorylation is ATP synthesis linked to the oxidation of NADH and FADH2 by electron transport through the respiratory chain. This occurs via a mechanism originally proposed as the chemiosmotic hypothesis. Energy liberated by electron transport is used to pump H+ ions out of the mitochondrion to create an electrochemical proton (H+) gradient. The protons flow back into the mitochondrion through the ATP synthase located in the inner mitochondrial membrane, and this drives ATP synthesis. Approximately three ATP molecules are synthesized per NADH oxidized and approximately two ATPs are synthesized per FADH2 oxidized. 

Answer Pyruvate dehydrogenase is located in the mitochondrion, and glyceraldehyde 3-phosphate dehydrogenase in the cytosol. Because the mitochondrial and cytosolic pools of NAD are separated by the inner mitochondrial membrane, the enzymes do not compete for the same NAD pool. However, reducing equivalents are transferred from one nicotinamide coenzyme pool to the other via shuttle mechanisms (see Problem 21). 

OAA by pyruvate carboxylase (EC 6.4.1.1), thereby completing the net transport of the C2 unit (acetate) from the mitochondrion to the cytosol with the added advantage of having converted a reducing equivalent as NADH + H+ to NADPH + H+. This mechanism of C2 transport provides up to 50% of the NADPH + H+ for fatty acid synthesis in nonruminants. 

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