Coenzyme Q Ubiquinone

First detected in lipid extracts of mitochondria and identified as a quinone, coenzyme Q (CoQ) was so named to signify its cofactor role in oxidation reactions. A second group of investigators discovered a cofactor that had ubiquitous occurrence in oxidative processes, which they named ubiquinone. In time, CoQ and ubiquinone were found to be the same compound. Early studies on the electron transport chain of mitochondria showed at least three complexes required CoQ and recognized its essential role in the electron transfer process overall. CoQ can be synthesized in man from tyrosine in a rather complex synthesis.

Reactivity CoQ and its reduced form CoQH2 are designed to handle electron pairs in transit in oxidation-reduction reactions. A third form, semiquinone (CoQH.), exists as a stable radical and is capable of a one-electron transfer (Figure 12). Because of its ability to deal with electrons on a single or paired base, CoQ takes part in electron transport chains where one- and two-electron transfers are essential. Its lipid nature allows the cofactor to bind firmly to

ch2 ch—ch2-ch2-ch2-ch2-coo- —-v ch2 ch—ch2-ch2-ch2-ch2-coo-

Figure 10 Lipoic acid in its reduced (sulfhydryl) and oxidized (disulfide) forms. The carboxyl group on the end is attached to a lysine group on the enzyme thus forming a swinging arm that is designed to transverse remote donor-acceptor sites across the surface of the enzyme.

ch3 OH

Acyl-CoA

Carnitine

c=o Acyl carnitine |

Acyl-CoA Carnitine

HS-CoA

Acyl carnitine

Acylcarnitine transferase II

Carnitine

Acylcarnitine transferase I

Acyl carnitine

HS-CoA

Acyl-CoA

Figure 11 Carnitine-dependent transfer of fatty acyl groups. Two juxtaposed membrane transferase enzymes, acylcarnitine transferase I,II, are designed to transport a fatty acyl carnitine complex into the mitochondria and return the carnitine for additional reactions. Note that the acyl group is transferred to the carnitine from CoA and returned to CoA inside the mitochondria.

H3CO

H3CO

H3CO

H3CO

Coenzyme Q (CoQ) or ubiquinone (oxidized or quinone form)

H3CO

H3CO

H3CO R

CoQH • (radical or semiquinone form)

H3CO R

OH CoQH2

(reduced or hydroquinone form)

Figure 12 Coenzyme Q (ubiquinone) as an electron carrier. The coenzyme is a mobile electron carrier that moves between protein complexes in the mitochondria membrane. Shown are single (semiquinone) and dual (hydroquinone) forms that permit one- and two-electron transport, respectively.

the mitochondria inner membrane. Besides being a prominent carrier of electrons in the electron transport chain of mitochondria, CoQ is known to be a source and mediator of protons that are pumped across the inner mitochondria membrane to form the high-energy proton gradient associated with oxi-dative phosphorylation.

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