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The α-ketoglutarate dehydrogenase is multienzymatic complex made up of three different types of enzymes, responsible for the conversion of α-ketoglutarate in to succinyl CoA. This is an important step in the citric acid cycle as it can be used to regulate energy production. The citric acid cycle is a central point in metabolism, linked to processes which are not part of energy production. α-ketoglutarate is a gateway to one of these processes. It allows amino acids (making up proteins) to enter the citric acid cycle and produce energy; this is a reversable reaction, so sugars which enter the cycle can leave it to make amino acids.
Brief info. on the enzyme complex α-ketoglutarate
Without all the scientific explaination this enzyme complex does the following :
- Provides a key step in the citric acid cycle (part of energy production.)
- Provides a way of controlling how fast the citric acid cycle goes.
- Links the citric acid cycle to amino acid (protein) synthesis and breakdown.
- Links sugar metabolism to amino acid metabolism.
Metabolism within the citric acid cycle
Succinyl CoA] goes on to be used in the production of
GTP and then continuing through the rest of the citric acid cycle. I have researched several topics surrounding this
complex which I shall show below. To do this I have used both library resources in several
Universities along with the
internet.
The α-ketoglutarate dehydrogenase complex catalyses the overall conversion of α-ketoglutarate (2-oxoglutarate) to succinyl-CoA and
CO2. As stated in the ncbi site (2000) the alpha-ketoglutarate dehydrogenase is also known as
oxoglutarate decarboxylase, but the official name of the enzyme is 2-oxoglutarate dehydrogenase (E.C. number is 1.2.4.2). The names suffixes are given due to the type of reactions catalysed being
decarboxylation and
oxidation reactions.
Structure
Co-factor’s used with the alpha ketoglutarate complex (Voet and Voet 1990) are :
- Thiamin pyrophosphate (TPP), bound to E1 and decarboxylates pyruvate, yielding a hydroxyethyl-TTP carbanion. and the prosthetic group is lipoic acid.
- Lipoic acid] which is covalently linked to a Lys on E2 (lipoamide), which accepts the hydroxyethyl carbanion from TPP as an acetyl group.
- Coenzyme A (CoA) which is a substrate for E2 and accepts the acetyl group from lipoamide.
- Flavin adenine dinucleotide (FAD) is bound to E3 sub-unit which is reduced by lipoamide.
- Nicotinamide adenine dinucleotide (NAD+) which is a substrate for E3 and is reduced by FADH2.
Similarity to Pyruvate dehydrogense (also in citric acid cycle)
Oxidative
decarboxylation of both a-ketoglutarate and pyruvate dehydrogenases, share the
co-factors listed above. The oxidative decarboxylation of α-ketoglutarate is
catalysed by an
enzyme complex that is structurally similar to the
pyruvate dehydrogenase complex. This is due to the two complexes being
homologous enzyme structures. The α-ketoglutarate dehydrogenase complex contains multiple copies of 3 enzymatic components which are the α-ketoglutarate dehydrogenase component (
E1), a transsuccinylase component (
E2) and a dihydrolipoyl dehydrogenase component (
E3). The
genes for this complex are found on
chromosome 7.
E2 is bound directly to both E1 and E3, but E1 and E3 are not bound to each other except through E2. This means that the transsuccinylase component (E2) is the core component of the complex. The dihydrolipoyl dehydrogenase is identical to it’s equivalent component in pyruvate dehydrogenase complex.
The
chemical aspect of the co-enzymatic action of a-lipoic acid is to mediate the transfer of
electrons and activate
acyl groups, resulting from the
decarboxylation and
oxidation of a-ketoglutarate within the complex. In this process, lipoic acid is itself
transiently reduced to dihydrolipoic acid; this reduced form is the acceptor of the activated succinyl groups. It’s
dual role of electron and acyl-group acceptor enables lipoic acid to act as a
shuttle and couple the two processes.
Below is the overall metabolic reaction in the citric acid cycle which the α-ketoglutarate complex is responsible for:
α-ketoglutarate + NAD+ + CoA ↔ succinyl CoA + CO2 + NADH
Linking Amino acid pathways to the Citric acid cycle
a-ketoglutarate is an
intermediate of the
citric acid cycle. It can also be transported out of the
mitochondria and in to the
cytosol where it is also a precursor for other
essential molecules e.g.
amino acids (Stryer 1994).
A common element in all protein bound amino acids is the α-amino group. Directly or indirectly, the α-amino group are derived from ammonia by way of L-glutamine. Ammonia can be incorporated directly into α-ketoglutarate to make glutamate. This allows reduced nitrogen to be incorporated into organic molecules. Glutamate serves as the precursor of glutamine], proline and arginine. The amino acids which can be produced this way are known as the glutamate family.
If blood sugar levels and body energy stores are low, proteins can be broken down to produce energy via the citric acid cycle. This can happen by reversing the amino acid production pathway, started with α-ketoglutarate. This newly synthesised α-ketoglutarate would then enter the citric acid cycle through the a-ketoglutarate complex.
Controlling the Citric acid cycle
The
control of alpha-ketodehydrogenase complex is very
important to it’s metabolic role, which is controlled by the
energy supply and demand. When the energy supply is high, it is advantageous to allow the concentration of a-ketoglutarate to rise, allowing the formation of
glutamate, providing a release valve for thecitric acid cycle. This is achieved by
negative regulation in response to
NADH. This metabolic control is designed to keep a high NADH / NAD
+ ratio, which is maintained by the combination action of
glycolysis and the citric acid cycle in converting 10 molecules of NAD
+ to NADH per molecule of
glucose oxidised. This control is provided by
regulation of several control points in
glycolysis and the
citric acid cycle. These
control points are enzymes which are effected by metabolic intermediates e.g. NAD
+. In the citric acid cycle the enzymes are pyruvate dehydrogenase, citrate synthase, isocitrate and a-ketoglutarate dehydrogenase.
NADH and succinyl CoA can be directed to produce NAD+ and CoA, direction of the reaction is dictated by the concentration of reactants Vs. products. This allows control of the enzyme complex, via product inhibition as NADH and succinyl CoA compete with NAD+ and CoA for binding sites on thier respective enzymes. They also drive the reversible E2 and E3 reactions backwards. When concentrations of NADH and succinyl CoA are high, the reversible reactions catalysed by E2 and E3 are driven backwards so inhibiting further formation of acetyl CoA. E1 is not reversable, so cannot accept more α-ketoglutarate till it’s binding sites are emptied, so raising α-ketoglutarate concentration promoting amino acid synthesis.
Essential Enzymes are Conserved throughout Evolution
The
E. coli α-ketoglutarate dehydrogenase complex has been shown by Mukherjee et. al. (1965) to have a
molecular weight of 2.3 million. The transsuccinylase has a molecular weight of about one million, made up of eight
morphological subunits arranged in a
cube-like structure. From the
electron micrograph the
molecules of a-ketoglutarate dehydrogenase and flavoprotein appear to be distributed in a
regular manner on the surface of the transsuccinylase. The a-ketoglutarate dehydrogenase complexes from
pig heart and
beef kidney has a molecular weight of about 2.7 million, these are made up of three enzymes analogous to those in the E.coli complex. Both
mammalian and
bacterial a-ketoglutarate complexes are very similar and look almost identical under the
electron microscope. Neither the mammalian or bacterial complex’s are inhibited by
ATP.
Researching the Complex
The three-dimensional solution structure of a 51-residue
synthetic peptide comprising the E3 binding domain in the E2 core of the 2-oxoglutarate dehydrogenase multienzyme complex of
Escherichia coli has been determined by
nuclear magnetic resonance spectroscopy (NMR) and hybrid distance geometry-dynamical simulated annealing calculations. A total of 56
simulated annealing structures were
calculated to find the mean co-ordinate positions for residues 12-48 of the synthetic peptide showing
backbone atoms between
residues 14 - 48, several side chains and a loop. The
solution structure of the E3-binding domain was shown to consist of two parallel
helices (residues 14-23 and 40-48), a short extended strand (24-26), a five-residue helical-like turn, and an irregular (and more disordered) loop (residues 31-39)(Robien
et.al 2000). This report presents the first structure of an E3-binding domain from a 2-oxo acid dehydrogenase complex.
Robien et.al.(2000) Stated that “multienzyme complexes are aggregates of enzymes that catalyse two or more steps in a metabolic sequence. Each component contains definite amounts of each enzyme and the component proteins are organised in a specific and regular way.” They also found they are not associated with lipids or nucleic acids and do not usually contain structural proteins which have no enzymic function. This makes these complexes more simple than membrane bound enzyme clusters; hence more detail is known about the complexes as they are simpler to study.
The construction of multienzyme complexes provides opportunities for increased specificity and efficiency and for modes of control that would not be possible by confining structurally independent enzymes within the same compartment of the cell.
Summary
As has been shown above, the α-ketoglutarate dehydrogenase complex is an
important enzyme complex which is not only part of the
citric acid cycle, but is also part of the
metabolic rate control within the citric acid cycle. This key complex is also involved in the
production of many of the 20 amino acids needed by the body. There is a lot known about the structure and mechanisms of the complex and the enzyme complex, however the structure of the E3 enzyme is not as well checked as the other sub groups. Most of what is known about a-ketoglutarate dehydrogenase complex was found by comparing it to the better known pyruvate dehydrogenase complex, also through the isolation of auxotrophs (mutants). The mechanism by which the enzymes carry out there task (although there is a proposed mechanism), is not entirely certain and needs more research on it.
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