Sandbox Reserved 1626: Difference between revisions

== Overview ==

The mitochondrial calcium uniporter (MCU) complex is the main source of entry for [https://en.wikipedia.org/wiki/Calcium calcium] ions into the [https://en.wikipedia.org/wiki/Mitochondrial_matrix mitochondrial matrix] from the [https://en.wikipedia.org/wiki/Mitochondrion#Intermembrane_space intermembrane space].  MCU channels exist in most [https://en.wikipedia.org/wiki/Eukaryote eukaryotic] life, but activity is regulated differently in each [https://en.wikipedia.org/wiki/Clade clade].<ref name="Baradaran">PMID:29995857</ref> The precise identity of the MCU wasn't discovered until 2011 and was discovered using a combination of [https://en.wikipedia.org/wiki/Nuclear_magnetic_resonance_spectroscopy NMR spectroscopy], [https://en.wikipedia.org/wiki/Transmission_electron_cryomicroscopy cryo-electron microscopy], and [https://en.wikipedia.org/wiki/X-ray_crystallography x-ray crystallography].<ref name="Woods">PMID:31869674</ref> Identification of the structure was difficult because it has no apparent sequence similarity to other ion channels.<ref name="Baradaran"/> However, like other ion channels, it is incredibly selective and efficient.  The MCU has the ability to only allow calcium ions into the mitochondrial matrix at a rate of 5,000,000 ions per second even though [https://en.wikipedia.org/wiki/Potassium potassium] ions are over 100,000 times more abundant in the intermembrane space.<ref name="Baradaran"/>  

===Selectivity Filter===

The selectivity filter of the MCU is composed by many acidic amino acids near the narrow mouth of the channel which leads to high affinity for calcium ([https://en.wikipedia.org/wiki/Dissociation_constant dissociation constant] of less than 2nM).<ref name="Baradaran"/> The arrangement of the highly conserved WDXXEP [https://en.wikipedia.org/wiki/Sequence_motif motif] in the TM2 helices form a ring in the pore to which calcium ions are attracted.<ref name="Baradaran"/> The structure in the animation is the MCU of [http://www.mycobank.org/BioloMICS.aspx?Table=Mycobank&Rec=511257 ''Cyphellophora europaea''] so every amino acid named here specifically is that of ''C. europaea'', but most of these residues are highly conserved across all species, though residue number may change. Though not part of the WDXXEP motif, Asp221 is present at the mouth of the MCU and serves to congregate positively charged calcium ions at the entrance of the channel.<ref name="Baradaran"/> The WDXXEP motif consists of Trp224 at the N-terminal end, Asp225, Glu228, and Pro229.<ref name="Baradaran"/> Trp224 and Pro229 pack against each other and are oriented towards the pore, but only serve to stabilize Asp225 and Glu228, not interact with calcium ions.<ref name="Baradaran"/><ref name="Fan"/> The X residues (Val226 and Met227 in this case) face away from the pore and are exposed to the membrane.<ref name="Baradaran"/> The negatively charged side chains of Asp225 and Glu228 point towards the pore and form rings of radius 2.5Å and 1Å, respectively.<ref name="Baradaran"/> It's a combination of these radii and charges that account for the selectivity of the MCU. For example, potassium has an [https://en.wikipedia.org/wiki/Ionic_radius ionic radius] of 1.38Å which is much larger than the 1.00Å ionic radius of calcium.<ref name="Baradaran"/> Additionally, even though sodium ions have a similar ionic radius, the +2 charge on calcium is better matched to coordination with the glutamate residues.<ref name="Baradaran"/>

==Regulation and Inhibition==

The most well-known and commonly used inhibitor of the MCU is [https://en.wikipedia.org/wiki/Ruthenium_red ruthenium red] (RuRed).<ref name="Woods"/>  RuRed is a trinuclear, oxo-bridged complex that effectively inhibits calcium uptake without affecting mitochondrial respiration or calcium efflux.<ref name="Woods"/>  The disadvantage of ruthenium red is its challenging purification.<ref name="Woods"/> Interestingly, an impure version of RuRed, termed [https://en.wikipedia.org/wiki/Ru360 Ru360], was found to be the active component of RuRed and thus another good inhibitor of the MCU.<ref name="Woods"/> Ru360 is a binuclear, oxo-bridged complex with a similar structure to that of RuRed.<ref name="Woods"/> The only flaw with Ru360 was that it showed low cell permeability, so Ru265 was developed and had twice the cell permeability of Ru360.<ref name="Woods"/> Ru265 possesses two bridged Ru centers bridged by a nitride ligand.<ref name="Woods"/>