(8/24/10) - Mitochondria, Peroxisomes and Related Pathologies

Mitochondrial Structure
1. Describe the structure of mitochondria, distinguishing the organization and functions of their four subcompartments – outer membrane, inner membrane, intermembrane space, and matrix.

Mitochondria are a double bilayered organelles in the cytoplasm. The outer membrane separates them from the cytosol and has a relatively small surface area. The inner membrane is folded to from cristae and has a relatively high surface area; it is this membrane in which the ATPases are found (and where the electron transport change and oxidative phosphorylation takes place). The intermembrane space lies between the outer and inner membranes, and has a relatively acidic pH due to the pumping of H+ ions into this space. The matrix is the space contained by the inner membrane, and it is where the citric acid cycle (oxidative metabolism) takes place.

Mitochondrial Movement
2. Recall how mitochondria are able to relocate within cells and to concentrate locally at sites of high energy need.

Mitochondria use the mircotubule system in the cell to relocate within cells. They concentrate at sites of high energy need, such as near the cellular membrane in epithelial cells (for active transport), and near sarcomeres in muscle cells.

Origin of Mitochondria
3. Explain why mitochondria are thought to have co-evolved with an oxygen-containing atmosphere and distinguish the metabolic/energetic advantage provided to cells by mitochondria.

The mitochondria are thought to have evolved by a primitive anaerobic eukaryotic cell engulfing a primitive aerobic prokaryotic cell, thus becoming capable of aerobically metabolizing their energy. This allows the cell to produce much more energy per unit of glucose (38 ATP vs. 2 ATP).

Mitochondrial DNA
'''4. Describe what mitochondrial DNA encodes. Explain the maternal inheritance of mitochondrial DNA, the distribution of mitochondria to daughter cells during mitosis, and underlying causes of heterogeneity among mitochondria in individual cells.'''

Mitochondrial DNA encodes several NADH dehydrogenase subunits, subunits of ATP synthase, cytochrome oxidase subunits, as well as the rRNA genes that make it possible to produce ribosomes and tRNA in order to produce the protein subunits.

During the development of the egg, one daughter cell contains all the mitochondria contained in the 'mother' cell, and the sperm contributes very little mitochondria to the zygote. As such, mitochondria are passed along maternally, rather than equally between the parents.

Mitochondria have a different cell cycle (fission) than that of the larger cell, so the signal for cell mitosis does not necessarily produce mitochondria, but when mitosis occurs, the mitochondria split between the two daughter cells. mtDNA does not partition equally during mitochondrial fission, so the number of molecules in each mitochondrion is variable.

Mitochondrial Proteins
5. Recall where most mitochondrial proteins are synthesized and describe how they are accurately distributed to the four mitochondrial subcompartments.

Though there are some proteins encoded for and produced within the mitochondria, most proteins are encoded by nuclear DNA and are synthesized by free ribosomes in the cytoplasm. Chaperone proteins keep the newly synthesized protein unfolded in order to facilitate transport into the mitochondria. There are signals (mostly on the N terminus) that bind to receptors at OMM and IMM contacts and are ultimately cleaved when they reach their destination.

Mutations in Mitochondrial DNA
6. Explain the potential consequences of mutations in mitochondrial DNA with respect to mitochondrial function and to distribution of mutation-containing mitochondria between and within tissues.

Mitochondrial DNA encodes the subunits of those proteins involved in the electron transport chain and oxydative phosphorylation. If these proteins lose their function due to dysfunctional subunits, they cannot fully make the energy required for the cell. If a mitochondrion fully loses its function, it is removed from the cell, so it is only those that are partially dysfunctional will continue to proliferate in the cell. When the cells undergo several mitotic divisions, there will be a wide range of mitochondrial function, from cells that only contain the mutant phenotype to those that contain only a normal phenotype, and everything in between. This is referred to as heteroplasmy. As a result, some tissues can be affected while others are not.

Mitochondrial Disease
7. Explain why it is difficult to diagnose and treat mitochondrial diseases.

There are many mechanisms through which mutations in DNA related to mitochondria may occur: nuclear DNA could become mutated, mitochondrial DNA may become mutated, or there may be sporadic mutations that are not inherited. Due to heteroplasmy, each individual will have a different set of symptoms (different tissues will be affected). As such, diagnosis is lengthy and expensive. Also, because it can affect a wide variety of organs and tissues, each treatment must be catered to the individual, and so often relates merely to the relief of symptoms. There is no cure due to the extreme variety, and there is also no way to predict the progression of the disease.

Mitochondria and Apoptosis
8. Describe the role of mitochondria in apoptosis (programmed cell death) focusing on which mitochondrial subcompartments contribute to this process.

Cell damage induces the release of BAX and BAK, which trigger the mitochondria to release cytochrome c and other activators of apoptosis (protease activators) into the cytoplasm through the outer mitochondrial membrane from the intermembrane space. Anti-apoptotic molecules such as BCL-2 prevent this leakage by closing the gates of the mitochondrial channels.

Peroxisomal Structure
'''9. Peroxisomes constitute the cell’s other compartment devoted to oxidative metabolism. Compare and contrast the structure, DNA content, and electron transport function of peroxisomes and mitochondria. Indicate how peroxisomes handle hydrogen peroxide production and degradation.'''

Peroxisomes only have a single membrane and have an amorphous content. Peroxisomes have a very crude respiratory chain using oxygen as an electron receptor (via type II oxidases), creating hydrogen peroxide. This is then converted to water by peroxidatic enzymes or catalase. Hydrogen peroxide levels must be kept at a low level. The energy produced in this oxidation is dissipated as heat.

Peroxisomal Function
10. Recall the key functions of peroxisomes

Peroxisomes are used in the oxidation of amino acids, purines, very long chain fatty acyl-CoAs, lactic acid, and ethanol. They also function in the first two steps of the synthesis of plasmalogens, the main phospholipids found in myelin, and help synthesize lipid derivatives such as platelet activating factor.

They are mostly responsible for very long chain (C24 and C26) fatty acids (VLCFAs), bile acid precursors that cannot be processed by mitochondria.

Function of the ER
11. Identify the functions of the endoplasmic reticulum and import of peroxisomal proteins from the cytoplasm in peroxisomal biogenesis.

There is no DNA contained in peroxisomes and they bud as precursors from specialized sites in the ER—the key membrane proteins Pex-19 and Pex-3 come from the ER. They are similar to mitochondria in that many proteins are imported post-translationally from the cytosol and have targeting signals at the end of the amino acid chain (on either the C or N terminus).

Peroxisomal Diseases
12. Describe the types of peroxisomal diseases and recall how they demonstrate the essential function of peroxisomes in cells.

Incidence is 1:20,000. There are two groups: single protein/enzyme deficiencies with normal structure (where the peroxisomes lack some essential function), and organelle biogenesis deficiency with abnormal structure and multiple functions missing.

More common single protein disorder is X-linked adrenoleukodystrophy, which is most severe and earliest onset in males, causes neurological problems, and is resultant from the defective uptake of VLCFAs. It is diagnosed by high serum levels of VLCFAs; accumulated VLCFAs induce a high inflammatory response.

Zellwger syndrom results from the defect in peroxisomal biogenesis. It is fatal soon after birth, there is multiple congenital defects, and extensive neuropathology (both hepatic and renal dysfunction). Peroxisomes are absent, so cells accumulate VLCFAs and bile acids precursors. Catalase is found in cytosol, so membrane ghosts appear.