Mitochondria

A mitochondrion (; ) is a double-membrane-bound organelle found in most eukaryotic organisms. Mitochondria generate most of the cell's supply of

adenosine triphosphate

(ATP), subsequently utilized as a source of chemical energy, using the energy of oxygen released in aerobic respiration at the inner mitochondrial membrane. They were first discovered by Albert von Kölliker in 1880 in the voluntary muscles of insects. The mitochondrion is popularly nicknamed the "powerhouse of the cell", a phrase coined by Philip Siekevitz in a 1957 article of the same name.

Some cells in some multicellular organisms lack mitochondria (for example, mature mammalian

red blood cell

s). A large number of unicellular organisms, such as microsporidia, parabasalids and diplomonads, have reduced or transformed their mitochondria into other structures. One

eukaryote

, ''Monocercomonoides'', is known to have completely lost its mitochondria, and one multicellular organism, ''

Henneguya salminicola

'', is known to have retained mitochondrion-related organelles in association with a complete loss of their mitochondrial genome.

Mitochondria are commonly between 0.75 and 3  μm in area, but vary considerably in size and structure. Unless specifically stained, they are not visible. In addition to supplying cellular energy, mitochondria are involved in other tasks, such as signaling, cellular differentiation, and

cell death

, as well as maintaining control of the

cell cycle

and cell growth. Mitochondrial biogenesis is in turn temporally coordinated with these cellular processes. Mitochondria have been implicated in several human disorders and conditions, such as mitochondrial diseases, cardiac dysfunction, heart failure and

autism

.

The number of mitochondria in a cell can vary widely by

organism

, tissue, and cell type. A mature red blood cell has no mitochondria, whereas a liver cell can have more than 2000. The mitochondrion is composed of compartments that carry out specialized functions. These compartments or regions include the outer membrane, intermembrane space, inner membrane,

crista

e and matrix.

Although most of a cell's

DNA

is contained in the

cell nucleus

, the mitochondrion has

its own genome

("mitogenome") that is substantially similar to

bacteria

l genomes. Mitochondrial proteins (proteins transcribed from mitochondrial DNA) vary depending on the tissue and the species. In humans, 615 distinct types of proteins have been identified from

cardiac

mitochondria, whereas in rats, 940 proteins have been reported. The mitochondrial proteome is thought to be dynamically regulated.

Structure

Mitochondria may have a number of different shapes.
A mitochondrion contains outer and inner membranes composed of phospholipid bilayers and

protein

s. The two membranes have different properties. Because of this double-membraned organization, there are five distinct parts to a mitochondrion:
# The outer mitochondrial membrane,
# The intermembrane space (the space between the outer and inner membranes),
# The inner mitochondrial membrane,
# The

crista

e space (formed by infoldings of the inner membrane), and
# The matrix (space within the inner membrane), which is a fluid.
Mitochondria have folding to increase surface area, which in turn increases ATP (Adenosine Tri Phosphate) production.
Mitochondria stripped of their outer membrane are called mitoplasts.

Outer membrane

The outer mitochondrial membrane, which encloses the entire organelle, is 60 to 75

angstrom

s (Å) thick. It has a protein-to-phospholipid ratio similar to that of the

cell membrane

(about 1:1 by weight). It contains large numbers of integral membrane proteins called porins. A major trafficking protein is the pore-forming voltage-dependent anion channel (VDAC). The VDAC is the primary transporter of

nucleotide

s, ions and metabolites between the cytosol and the intermembrane space. It is formed as a beta barrel that spans the outer membrane, similar to that in the gram-negative bacterial membrane. Larger proteins can enter the mitochondrion if a signaling sequence at their

N-terminus

binds to a large multisubunit protein called translocase in the outer membrane, which then

actively moves

them across the membrane. Mitochondrial pro-proteins are imported through specialised translocation complexes.

The outer membrane also contains

enzyme

s involved in such diverse activities as the elongation of fatty acids,

oxidation

of

epinephrine

, and the degradation of

tryptophan

. These enzymes include monoamine oxidase,

rotenone

-insensitive NADH-cytochrome c-reductase,

kynurenine

hydroxylase and fatty acid Co-A

ligase

. Disruption of the outer membrane permits proteins in the intermembrane space to leak into the cytosol, leading to cell death. The outer mitochondrial membrane can associate with the endoplasmic reticulum (ER) membrane, in a structure called MAM (mitochondria-associated ER-membrane). This is important in the ER-mitochondria calcium signaling and is involved in the transfer of lipids between the ER and mitochondria. Outside the outer membrane are small (diameter: 60 Å) particles named sub-units of Parson.

Intermembrane space

The mitochondrial intermembrane space is the space between the outer membrane and the inner membrane. It is also known as perimitochondrial space. Because the outer membrane is freely permeable to small molecules, the concentrations of small molecules, such as ions and sugars, in the intermembrane space is the same as in the cytosol. However, large proteins must have a specific signaling sequence to be transported across the outer membrane, so the protein composition of this space is different from the protein composition of the cytosol. One

protein

that is localized to the intermembrane space in this way is

cytochrome c

.

Inner membrane

The inner mitochondrial membrane contains proteins with three types of functions:
# Those that perform the

electron transport chain

redox

reactions
#

ATP synthase

, which generates

ATP

in the matrix
# Specific transport proteins that regulate metabolite passage into and out of the mitochondrial matrix

It contains more than 151 different polypeptides, and has a very high protein-to-phospholipid ratio (more than 3:1 by weight, which is about 1 protein for 15 phospholipids). The inner membrane is home to around 1/5 of the total protein in a mitochondrion. Additionally, the inner membrane is rich in an unusual phospholipid,

cardiolipin

. This phospholipid was originally discovered in cow hearts in 1942, and is usually characteristic of mitochondrial and bacterial plasma membranes. Cardiolipin contains four fatty acids rather than two, and may help to make the inner membrane impermeable. Unlike the outer membrane, the inner membrane does not contain porins, and is highly impermeable to all molecules. Almost all ions and molecules require special membrane transporters to enter or exit the matrix. Proteins are ferried into the matrix via the translocase of the inner membrane (TIM) complex or via OXA1L. In addition, there is a membrane potential across the inner membrane, formed by the action of the

enzyme

s of the

electron transport chain

. Inner membrane fusion is mediated by the inner membrane protein OPA1.

Cristae

The inner mitochondrial membrane is compartmentalized into numerous folds called

crista

e, which expand the surface area of the inner mitochondrial membrane, enhancing its ability to produce ATP. For typical liver mitochondria, the area of the inner membrane is about five times as large as the outer membrane. This ratio is variable and mitochondria from cells that have a greater demand for ATP, such as muscle cells, contain even more cristae. Mitochondria within the same cell can have substantially different crista-density, with the ones that are required to produce more energy having much more crista-membrane surface. These folds are studded with small round bodies known as F particles or oxysomes.

Matrix

The matrix is the space enclosed by the inner membrane. It contains about 2/3 of the total proteins in a mitochondrion. The matrix is important in the production of ATP with the aid of the ATP synthase contained in the inner membrane. The matrix contains a highly concentrated mixture of hundreds of enzymes, special mitochondrial

ribosomes

,

tRNA

, and several copies of the

mitochondrial DNA

genome

. Of the enzymes, the major functions include oxidation of

pyruvate

and fatty acids, and the

citric acid cycle

. The DNA molecules are packaged into nucleoids by proteins, one of which is TFAM.

Function

The most prominent roles of mitochondria are to produce the energy currency of the cell,

ATP

(i.e., phosphorylation of

ADP

), through respiration and to regulate cellular

metabolism

. The central set of reactions involved in ATP production are collectively known as the

citric acid cycle

, or the

Krebs

cycle. However, the mitochondrion has many other functions in addition to the production of ATP.

Energy conversion

A dominant role for the mitochondria is the production of ATP, as reflected by the large number of proteins in the inner membrane for this task. This is done by oxidizing the major products of

glucose

:

pyruvate

, and NADH, which are produced in the cytosol. This type of

cellular respiration

, known as aerobic respiration, is dependent on the presence of

oxygen

, which provides most of the energy released. When oxygen is limited, the glycolytic products will be metabolized by anaerobic fermentation, a process that is independent of the mitochondria. The production of ATP from glucose and oxygen has an approximately 13-times higher yield during aerobic respiration compared to fermentation. Plant mitochondria can also produce a limited amount of ATP either by breaking the sugar produced during photosynthesis or without oxygen by using the alternate substrate

nitrite

. ATP crosses out through the inner membrane with the help of a specific protein, and across the outer membrane via porins. ADP returns via the same route.

Pyruvate and the citric acid cycle

Pyruvate

molecules produced by

glycolysis

are

actively transported

across the inner mitochondrial membrane, and into the matrix where they can either be

oxidized

and combined with coenzyme A to form CO,

acetyl-CoA

, and NADH, or they can be carboxylated (by pyruvate carboxylase) to form oxaloacetate. This latter reaction "fills up" the amount of oxaloacetate in the citric acid cycle and is therefore an anaplerotic reaction, increasing the cycle's capacity to metabolize acetyl-CoA when the tissue's energy needs (e.g., in

muscle

) are suddenly increased by activity.

In the citric acid cycle, all the intermediates (e.g. citrate,

iso-citrate

,

alpha-ketoglutarate

, succinate,

fumarate

, malate and oxaloacetate) are regenerated during each turn of the cycle. Adding more of any of these intermediates to the mitochondrion therefore means that the additional amount is retained within the cycle, increasing all the other intermediates as one is converted into the other. Hence, the addition of any one of them to the cycle has an anaplerotic effect, and its removal has a cataplerotic effect. These anaplerotic and cataplerotic reactions will, during the course of the cycle, increase or decrease the amount of oxaloacetate available to combine with acetyl-CoA to form citric acid. This in turn increases or decreases the rate of

ATP

production by the mitochondrion, and thus the availability of ATP to the cell.

Acetyl-CoA, on the other hand, derived from pyruvate oxidation, or from the beta-oxidation of fatty acids, is the only fuel to enter the citric acid cycle. With each turn of the cycle one molecule of acetyl-CoA is consumed for every molecule of oxaloacetate present in the mitochondrial matrix, and is never regenerated. It is the oxidation of the acetate portion of acetyl-CoA that produces CO and water, with the energy thus released captured in the form of ATP.

In the liver, the carboxylation of cytosolic pyruvate into intra-mitochondrial oxaloacetate is an early step in the

gluconeogenic

pathway, which converts

lactate

and de-aminated

alanine

into glucose, under the influence of high levels of

glucagon

and/or

epinephrine

in the blood. Here, the addition of oxaloacetate to the mitochondrion does not have a net anaplerotic effect, as another citric acid cycle intermediate (malate) is immediately removed from the mitochondrion to be converted to cytosolic oxaloacetate, and ultimately to glucose, in a process that is almost the reverse of

glycolysis

.

The enzymes of the citric acid cycle are located in the mitochondrial matrix, with the exception of

succinate dehydrogenase

, which is bound to the inner mitochondrial membrane as part of Complex II. The citric acid cycle oxidizes the acetyl-CoA to carbon dioxide, and, in the process, produces reduced cofactors (three molecules of NADH and one molecule of

FADH

) that are a source of electrons for the

electron transport chain

, and a molecule of GTP (which is readily converted to an ATP).

NADH and FADH: the electron transport chain

The electrons from NADH and FADH are transferred to oxygen (O), an energy-rich molecule, and hydrogen (protons) in several steps via the electron transport chain. NADH and FADH molecules are produced within the matrix via the citric acid cycle but are also produced in the cytoplasm by

glycolysis

. Reducing equivalents from the cytoplasm can be imported via the

malate-aspartate shuttle

system of

antiporter

proteins or feed into the electron transport chain using a glycerol phosphate shuttle. Protein complexes in the inner membrane (NADH dehydrogenase (ubiquinone), cytochrome c reductase

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