Mitochondrial matrix
In a mitochondrion, the matrix is the space within the inner membrane. It can also be referred as the mitochondrial fluid. The word "matrix" stems from the fact that this space is viscous, compared to the relatively aqueous cytoplasm. The mitochondrial matrix contains the mitochondrial DNA, ribosomes, soluble enzymes, small organic molecules, nucleotide cofactors, and inorganic ions. The enzymes in the matrix facilitate reactions responsible for the production of ATP, such as the citric acid cycle, oxidative phosphorylation, oxidation of pyruvate, and the beta oxidation of fatty acids.
The composition of the matrix based on its structures and contents produce an environment that allows the anabolic and catabolic pathways to proceed favorably. The electron transport chain and enzymes in the matrix play a large role in the citric acid cycle and oxidative phosphorylation. The citric acid cycle produces NADH and FADH2 through oxidation that will be reduced in oxidative phosphorylation to produce ATP.
The cytosolic, intermembrane space, compartment has a higher aqueous:protein content of around 3.8 μL/mg protein relative to that occurring in mitochondrial matrix where such levels typically are near 0.8 μL/mg protein. It is not known how mitochondria maintain osmotic balance across the inner mitochondrial membrane, although the membrane contains aquaporins that are believed to be conduits for regulated water transport. Mitochondrial matrix has a pH of about 7.8, which is higher than the pH of the intermembrane space of the mitochondria, which is around 7.0–7.4. Mitochondrial DNA was discovered by Nash and Margit in 1963. One to many double stranded mainly circular DNA is present in mitochondrial matrix. Mitochondrial DNA is 1% of total DNA of a cell. It is rich in guanine and cytosine content, and in humans is maternally derived. Mitochondria of mammals have 55S ribosomes.
Composition
Metabolites
The matrix is host to a wide variety of metabolites involved in processes within the matrix. The citric acid cycle involves acyl-CoA, pyruvate, acetyl-CoA, citrate, isocitrate, α-ketoglutarate, succinyl-CoA, fumarate, succinate, L-malate, and oxaloacetate. Malonyl-CoA is also present, where it serves as the two-carbon donor for mitochondrial fatty acid synthesis and as a donor for lysine malonylation. The urea cycle makes use of L-ornithine, carbamoyl phosphate, and L-citrulline. The electron transport chain oxidizes coenzymes NADH and FADH2. Protein synthesis makes use of mitochondrial DNA, RNA, and tRNA. Regulation of processes makes use of ions. Additional metabolites present in the matrix are CO2, H2O, O2, ATP, ADP, and Pi.Enzymes
Enzymes from processes that take place in the matrix. The citric acid cycle is facilitated by pyruvate dehydrogenase, citrate synthase, aconitase, isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, succinyl-CoA synthetase, fumarase, and malate dehydrogenase. The urea cycle is facilitated by carbamoyl phosphate synthetase I and ornithine transcarbamylase. β-Oxidation uses pyruvate carboxylase, acyl-CoA dehydrogenase, and β-ketothiolase. Mitochondrial fatty acid synthesis is facilitated by at least six enzymes, including malonyl-CoA:ACP transacylase, 3-ketoacyl-ACP synthase, 3-ketoacyl reductase, 3-hydroxyacyl-ACP dehydratase 2, and trans-2-enoyl-ACP reductase, together with a mitochondrial acyl carrier protein. This enzyme system is organized as individual, matrix-soluble proteins, in contrast to the single, multi-domain enzyme FASN of cytosolic fatty acid synthesis. Amino acid production is facilitated by transaminases. Amino acid metabolism is mediated by proteases, such as presequence protease.Non-enzymatic proteins
Members of the superfamily of LYRM proteins – with the exception of LYRM3 and LYRM6, which are embedded in mitochondrial Complex I – are primarily soluble mitochondrial matrix proteins. They are involved in the assembly of electron transport chain complexes and mitochondrial ribosomes, as well as in iron–sulfur cluster biogenesis and the function of the electron-transfer flavoprotein.Inner membrane components
The inner membrane is a phospholipid bilayer that contains the complexes of oxidative phosphorylation. which contains the electron transport chain that is found on the cristae of the inner membrane and consists of four protein complexes and ATP synthase. These complexes are complex I, complex II, complex III, and complex IV.Inner membrane control over matrix composition
The electron transport chain is responsible for establishing a pH and electrochemical gradient that facilitates the production of ATP through the pumping of protons. The gradient also provides control of the concentration of ions such as Ca2+ driven by the mitochondrial membrane potential. The membrane only allows nonpolar molecules such as CO2 and O2 and small non charged polar molecules such as H2O to enter the matrix. Molecules enter and exit the mitochondrial matrix through transport proteins and ion transporters. Molecules are then able to leave the mitochondria through porin. These attributed characteristics allow for control over concentrations of ions and metabolites necessary for regulation and determines the rate of ATP production.Processes
Citric acid cycle
Following glycolysis, the citric acid cycle is activated by the production of acetyl-CoA. The oxidation of pyruvate by pyruvate dehydrogenase in the matrix produces CO2, acetyl-CoA, and NADH. Beta oxidation of fatty acids serves as an alternate catabolic pathway that produces acetyl-CoA, NADH, and FADH2. The production of acetyl-CoA begins the citric acid cycle while the co-enzymes produced are used in the electron transport chain.All of the enzymes for the citric acid cycle are in the matrix except for succinate dehydrogenase which is on the inner membrane and is part of protein complex II in the electron transport chain. The cycle produces coenzymes NADH and FADH2 through the oxidation of carbons in two cycles. The oxidation of NADH and FADH2 produces GTP from succinyl-CoA synthetase.