alpha ketoglutarate Dehydrogenase Enzyme Complex

Reaction catalyzed
	The enzyme α-ketoglutarate dehydrogenase catalyzes this conversion. This conversion is quite complex and requires FIVE vitamin derived cofactors to complete. This is very similar to that for the pyruvate dehydrogenase enzyme complex.. in fact the mechanism and overall enzyme structures are very similar... the only difference - this one catalyses the decarboxylation of α-ketoglutarate instead of pyruvate.
Reaction Types	thiamine dependent aldol reaction Redox (lipoc acid) Group transfer (succinyl group from liopic acid to CoA) Redox (flavin) Redox (NAD⁺)
Cofactors/Cosubstrates	Thiamine pyrophosphate (derived from thiamine) flavin adenine dinucleotide (derived from riboflavin) lipoic acid (derived from lipoic acid) pantothenic acid as part of Coenzyme-A (CoA) nicotinamide adenine dinucleotide NAD⁺ (derived from niacin)
Pathway Involvement	Citric Acid Cycle (also amino acid synthesis - put an amine on the ketone and it is Glutamate)

Rationale

The detailed mechanism of α-ketoglutarate dehydrogenase is beyond the scope of this course but suffice it to say that this reaction requires the concerted efforts of three enzymes all held together in a giant complex to catalyze the conversion.

Three different types of chemistries are utilized overall:

An aldol cleavage (to break one C-C bond In this is a special case, this reaction requires thiamine because it is breaking the C-C bond ADJACENT to the ketone of pyruvate)
an oxidation which requires:
1. Lipoic Acid (Contains two sulfurs that oxidize/reduce just like the cysteine disulfides that we dicussed in module 2)
2. Flavin Adenine Dinuleotide (FAD)
3. NAD⁺
A Group transfer Reaction. (transfer the two carbon acetyl group - which is covalently bound to the enzyme - to Coenzyme A.

The carbon that is cleaved off is removed as CO₂. This is the the last carbon lost as CO₂ in the complete oxidation steps of glucose. Please recall that for every glucose that started glycolysis there are two acetyl-CoA. Thus six carbons have so far been completely oxidized. (2 at pyrvate dehydrogenase and 2 at isocitrate dehydrogenase and 2 here)

Notice also the production of one NADH for each α-ketoglutarate oxidized as well.

Difference in Subsequent reaction

There is a differnce in how the product of this reaction vs. that of Pyruvate dehydrogenase is used. Recall that the product of Pyruvate Dehydrogenase is acetyl-CoA which is then used to add the acetate to oxaloacetate in an aldol reaction. For this enzyme the product is succinyl-CoA. Both are coenzyme A thioester adducts. However, in this case instead of using the thioester to make a carbon-carbon bond it will be used in the next reaction to generate an ATP equivalent... GTP in this case.

Reversibility of α-ketoglutarate Dehydrogenase

I made a big deal in the first module about all reactions being reversible. While this one is theortically reversible, for practical purposes it only runs toward succnyl-CoA. There are a couple very important reasons that combine to make this the case.

The ΔG^o' = -33.4 kJ/Mole (the reaction lies toward succinyl-CoA and CO₂ by a factor of ~640,000)
The concetration of all the products never build up...ever. While theoretically NADH and succinyl-CoA could build up sufficiently ... what happens to CO₂?

α-ketoglutarate dehydrogenase is a large multi-enzyme complex organized to provide easy transfer of substrates to each of three different active sites. One cofactor, lipoic acid, is covalently bound to one of the enzymes yet must have access to all three. The active portion of lipoic acid is one a long arm that is in turn covalently linked via an amide bond to a lysine. This combination gives it a long reach. The three differnt enzymes are:

α-ketoglutarate Decarboxylase: Contains Thiamine and performs the aldol cleavage reaction of pyruvate to succinate and CO₂.
succinate Transferase: Contains Lipoic Acid and Transfers the succinate from Thiamine to Coenzyme A.

Lipoic Acid Dehydrogenase: Contains the FAD, as the acetate is transferred Lipoic acid is reduced. It must be re-oxidized at the FAD which in turn reduced NAD⁺

Note: the reaction mechanisms between this enzymes and pyruvate dehydrogenase are so similar, I did not even bother to redraw this set of pictures... just replace pyruvate with α-ketoglutarate and acetate with succinate. The cofactors that are permanent to each active site (TPP, Lipoic acid. FAD) are colored according to atom type. The cosubstrates that come and go with each turn-over event are colored orange. The enzyme names are in green.

Two models of the mammalian aKGDH organization. (a) Cut-away model of a fully assembled PDH. Lipoic acid dehydrogenase molecules (red) are bound inside the pentagonal dodecahedron core formed by 60 inner domains of Acyl transferase (AT) (green). Pyruvate devarboxylase (yellow) is bound to the AT-core through the inner linkers of AT (blue) (Zhou et al., 2001). (b) A model of the inner core formed by AT. Twelve inner domains of BP (red) replace 12 inner domains of AT and bind with 48 inner domains of AT (blue) to form a 60-mer pentagonal dodecahedron (Hiromasa et al., 2004).Lioubov G Korotchkina, SUNY at Buffalo, Buffalo, New York, USA Published online: January 2006 DOI: 10.1038/npg.els.0003943 http://www.els.net/WileyCDA/ElsArticle/refId-a0003943.html

No JMOL demonstration for this enzyme.

Aerobic Glucose Metabolism

α-ketoglutarate Dehydrogenase Enzyme Complex

Difference in Subsequent reaction

Reversibility of α-ketoglutarate Dehydrogenase