Lipoate, activation

Lipoate-activating enzyme A protein that cataiyzes the ATP-dependent adenyiation of iipoic acid, yieiding iipoyi-AMP. 

Recently, Prasad et al. cloned a mammalian Na+-dependent multivitamin transporter (SMVT) from rat placenta [305], This transporter is very highly expressed in intestine and transports pantothenate, biotin, and lipoate [305, 306]. Additionally, it has been suggested that there are other specific transport systems for more water-soluble vitamins. Takanaga et al. [307] demonstrated that nicotinic acid is absorbed by two independent active transport mechanisms from small intestine one is a proton cotransporter and the other an anion antiporter. These nicotinic acid related transporters are capable of taking up monocarboxylic acid-like drugs such as valproic acid, salicylic acid, and penicillins [5], Also, more water-soluble transporters were discovered as Huang and Swann [308] reported the possible occurrence of high-affinity riboflavin transporter(s) on the microvillous membrane. 

Some enzymes are nonfunctional until posttranslationally modified. Examples of these enzymes include the acyl- and carboxyltransferases. While lipoate and phosphopantetheine are necessary for acyl transfer chemistry, tethered biotin is used in carboxyl transfer chemistry. Biotin and lipoate tethering occur under a similar mechanism the natural small molecule is activated with ATP to form biotinyl-AMP or lipoyl-AMP (Scheme 20). A lysine from the target protein then attacks the activated acid and transfers the group to the protein. The phosphopantetheine moiety is transferred using its own enzyme, the phosphopantetheinyltrans-ferase (PPTase). The PPTase uses a nucleophilic hydroxy-containing amino acid, serine, to attach the phosphopantetheinyl (Ppant) arm found in coenzyme A to convert the apo (inactive) carrier protein to its holo (active) form. The reaction is Mg -dependent. 

FIGURE 16-17 Biological tethers. The cofactors lipoate, biotin, and the combination of /3-mercaptoethylamine and pantothenate form long, flexible arms in the enzymes to which they are covalently bound, acting as tethers that move intermediates from one active site to the next. The group shaded pink is in each case the point of attachment of the activated intermediate to the tether. 

Mixed acid fermentations are not limited to bacteria. For example, trichomonads, parasitic flagellated protozoa, have no mitochondria. They export pyruvate into the bloodstreams of their hosts and also contain particles called hydrogenosomes which can convert pyruvate to acetate, succinate, C02, and H2.144 Hydrogenosomes are bounded by double membranes and have a common evolutionary relationship with both mitochondria and bacteria. The enzyme that catalyzes pyruvate cleavage in hydrogenosomes apparently does not contain lipoate and may be related to the pyruvate-ferredoxin oxidoreductase of clostridia (Eq. 15-35). The hydrogenosomes also contain an active hydrogenase. 

Treatment of native pig heart lipoamide dehydrogenase with cupric ion leads to loss of lipoate-linked activities and to a marked increase (10- to 30-fold) in the NADH-DCI activity. Concomitantly there is a drop of 2 in the number of titratable thiols. The action of cupric ion is catalytic 164, 155). Amperometric titration in the presence of urea before and after addition of sulfite indicates that the cupric ion-treated enzyme contains one disulfide in addition to the active center disulfide 155). Sulfite reacts with disulfides as follows ... 

A useful synthesis of racemic methyl lipoate from the key intermediate 348, prepared via a six-step reaction sequence, starts from tricarbonyl(diene)iron complex 350 (Scheme 67) <1998EJ01949>. The main goal of this practical method, based on the use of the optically active iron complex 350, was a possible stereoselective synthesis of LA and other structural analogues. 

Lipoate-protein ligase A A protein that catalyzes the ATP-dependent activation and transfer of lipoic acid to lipoyl carrier proteins. 

FAD it transfers electrons from reduced lipoate to NAD. The collection of 3 enzyme activities into one huge complex enables the product of one enzyme to be transferred to the next enzyme without loss of energy. Complex formation also increases the rate of catalysis because the substrates for E2 and E3 remain bound to the enzyme complex. 

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