Mitochondria

Mitochondria contain an outer mitochondria membrane that serves as an interface between the organelle and the cytosol, a highly folder inner mitochondrial membrane that appears to forming proteins for its unique functions. The mitchondria are not fully autonomous, however, because nearly all of the genes that encode the enzymes used in oxidative metabolism are located in the cell nucleus. The subcompartment within the inner mitochondrial membrane is commonly referred to as the mitochondrial matrix.

Mitochondria have their own DNA which contains several genes that produce proteins essential to the mitchondrion’s role in oxidative metabolism. Thus, the mitchondrion, in many respects, acts as a cell within a cell, containing its own genetic information specifyMitochondria contain gene products encoded by mitochondrial genes situation in mitochondrial DNA (mtDNA) and by extramitochondrial (e.g., nuclear) genes not situated in the circular mitochondrial genome. While it has been estimated that a functional human mitochondrion contains on the order of 1,000-1,500 distinct proteins, the 16.5 kb mtDNA encodes 22 tRNAs, two ribosomal RNAs (12s and 16s rRNA) and only 13 polypeptides, which are enzymes of the electron transport chain (ETC), the elaborate multi-subunit complex mitochondrial assembly where, for example, respiratory oxidative phosphorylation takes place.

A eukaryotic cell does not produce brand new mitochondria each time the cell divides. Instead, the mitochondria themselves divide in two, doubling in number, and these are partitioned between the new cells. Most of the components required for mitochondrial division are encoded by genes in the nucleus and are translated into proteins by cytoplasmic ribosomes. Mitochondrial replication is, thus, impossible without nuclear participation, and mitochondria thus cannot be grown in a cell free culture.

The fact that mitochodria have their own DNA can be advantageously used for certain inherited disorders. Pronuclear transfer, first described in 2010, involves transplanting the nucleus from an egg carrying a ictochondrial NA mutation to an egg donated by an unaffected woman that hs had its nuclear genome removed. The resulting embryo inherits its parents’ nuclear DNA but the mitochondrial DNA is inherited predominantly from the donated egg. For babies with mitochondria disease casued by mutations in mitochondrial geneomes have showed no signs of mitochondrial disease thabks to pronuclear transfer.

Mitochondria specialize in the synthesis of ATP, using energy derived from electron transport and oxidative phosphorylation.

Proteins imported into the matrix of mitochondria are typically taken up from the cytosol within seconds to minutes of their release from ribosomes. Most of these proteins bound for mitochondira have a signal sequence at their N terminus which has the common feature of an amphipathic alpha helix in which positive residues are clustered on one side and uncharged hydrophobic residues are clustered on the other side.

Protein translocation across the membranes is mediated by multisubunit protein complexes. TOM complex functions as a translocase on the outer membrane and 2 TIM complexes as well as an OXA complex which function on the inner membrane. These complexes contain components that act as receptors as well as for the translocation channel.

Protein import requires ATP hydrolysis as well as an electrochemical H+ gradient across the inner membrane.

Mitochondrial precursor proteins remain unfolded in the cytosol through interactions with chaperone proteins of the hsp70 family. This prevents folding of the proteins before they engage with the TOM complex. Mitochondrial hsp70 also binds tightly to an imported protein as soon as it emerges in the matrix and is crucial for the import of the mitochondrial proteins. 2 models have been proposed to explain how this works. In the thermal ratchet model, the emerging chain slides back and forth in the TIM23 translocation channel and each time a sufficiently long portion of the chain is exposed, an hsp70 molecule binds to it thereby translocating it into the matrix.

Mitochondrial biogenesis is the crucial cellular process of creating new mitochondria, increasing mitochondrial mass, and renewing existing ones, driven by energy demands from exercise, stress, or development, and orchestrated by key regulators like PGC-1α (PGC-1alpha), which coordinates the expression of nuclear and mitochondrial DNA-encoded components to boost cellular energy (ATP) production. It’s a complex, self-renewal pathway that involves making new proteins, lipids, and DNA, adding them to existing mitochondria, and is vital for tissue adaptation and combating diseases.

–Nuclear Respiratory Factor 1 (NRF1) is crucial for mitochondrial biosynthesis, acting as a master regulator that turns on genes needed for creating new mitochondria, maintaining their health, and supporting cellular energy production, proving vital for cell survival and function, especially under stress or disease.

–Peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1alpha): is a protein that in humans is encoded by the PPARGC1A gene. PGC-1alpha is the master regulator of mitocondral biogenesis. It is also the primary regulator of liver gluconeogenesis, inducing increased gene expression for gluconeogenesis. It is a transcriptional coactivator that regulates the genes involved in energy metabolisms. It interacts with nuclear receptor PPAR-y, which permits the interaction of this protein with multiple transcription factors. Endurance exercise has been shown to activate the PGC-1alpha gene in human skeletal muscle.

Monoamine oxidase A, also known as MAO-A, is an enzyme that in humans is encoded by the MAO-A gene. This gene is one of two neighboring gene family members that encode mitochondrial enzymes which catalyze the oxidative deamination of amines, such as dopamine, norepinephrine and serotonin.

MAO-A is important in mitochondria, specifically located on the outer mitochondrial membrane, where it plays a crucial role in generating reactive oxygen species (ROS) and aldehydes as byproducts of neurotransmitter breakdown, contributing significantly to mitochondrial dysfunction, oxidative stress, and cellular damage, especially in conditions like heart failure and aging.

MAO-A (Monoamine Oxidase A) isn’t directly a building block for mitochondria but is crucial in regulating mitochondrial health, especially concerning oxidative stress and quality control, influencing mitochondrial biogenesis (creation of new mitochondria) through pathways like PGC1α, and also impacting mitophagy (clearing damaged mitochondria). High MAO-A activity generates harmful ROS (Reactive Oxygen Species) and aldehydes, leading to mitochondrial damage, while inhibiting MAO-A can be protective, suggesting it plays a vital role in mitochondrial dynamics, function, and biogenesis

Numts: DNA from mitochondria in our brain cells is also known to get integrated into DNA of the nucleus and possibly limit lifespans according to a Columbia University study earlier than people with fewer insertions. The scientists found many insertions across different grain regions, but not in blood cells. Numbts behave like jumping genes. Inherited Numts are mostly benign, but new Numts can have adverse effects.