Sandbox Reserved 1625: Difference between revisions

Cytochrome ''bd'' oxidase is made up of four individual subunits.<ref name="Alexander">PMID:31723136</ref> The two major subunits, CydA and CydB, are each composed of one peripheral [https://en.wikipedia.org/wiki/Alpha_helix helix] and two bundles of four [https://en.wikipedia.org/wiki/Transmembrane_protein transmembrane] helices. The <scene name='83/832924/Cyda_subunit/6'>CydA subunit</scene> plays the most important role in the oxygen [https://en.wikipedia.org/wiki/Redox reduction reaction] as it contains the Q-loop as well as all three [https://en.wikipedia.org/wiki/Heme heme] groups. The <scene name='83/832924/Cydb_subunit/2'>CydB subunit</scene> harbors the <scene name='83/832924/Ubiquinone/3'>ubiquinone</scene> molecule which provides structural support to the subunit that mimics the three hemes found in CydA.<ref name="Safarian">PMID: 31604309</ref><ref name="Safarian2">PMID: 27126043</ref> The remaining two subunits, CydS and CydX, are both single helix structures that assist in the oxygen reduction reaction. Unique to ''E. coli'', the <scene name='83/832924/Cyds_subunit/4'>CydS subunit</scene> binds to CydA to block oxygen from directly binding to heme b₅₉₅. The <scene name='83/832924/Cydx_subunit/4'>CydX subunit</scene> promotes the assembly and stability of the oxidase complex. CydX is composed of 37 mostly hydrophilic [https://en.wikipedia.org/wiki/Amino_acid amino acid] residues, including <scene name='83/832924/Glu25/2'>Glu25</scene> that is exposed to the cytoplasm and prevents the helix from fully entering the membrane. <ref name="Alexander">PMID:31723136</ref>

Another significant structural feature of bd oxidase is the <scene name='83/832924/Q_loop/3'>Q-loop</scene> which is located between TM helices 6 and 7 of the CydA subunit.<ref name="Alexander">PMID:31723136</ref> The periplasmic Q-loop in ''E. coli'' stretches over a length of 136 amino acid residues, making it much longer than the Q-loop in [https://en.wikipedia.org/wiki/Geobacillus_thermoglucosidasius ''Geobacillus Thermodenitrificans''].<ref name="Safarian">PMID: 27126043</ref> With five helices acting as a flap covering heme b₅₅₈, the Q-loop is likely involved in [https://en.wikipedia.org/wiki/Hydroquinone quinol] binding and oxidation. The [https://en.wikipedia.org/wiki/N-terminus N-terminal end] of this Q-loop is very flexible and likely functions as the hinge that allows for quinone binding while the [https://en.wikipedia.org/wiki/C-terminus C-terminal end] is much more rigid which provides stabilization for the enzyme.<ref name="Alexander">PMID:31723136</ref>