citric acid cycle (CAC) is often described as “The Hub of the Metabolic Wheel”
as several metabolic processes converge with the CAC. The cycle is part of the
final pathway of oxidative metabolism, and therefore is involved in producing
the majority of ATP in most mammals. The cycle occurs entirely within the
mitochondria where it produces reduced coenzymes to be oxidised in the electron
transport chain and also participates in several important synthetic reactions
involving glucose, amino acids and heam. Thus, the cycle is not a closed circle
and this description is accurate.
The CAC, discovered by Sir Hans
Krebs in 1937, is a metabolic process by which electron carriers such as the
coenzyme NADH and the cofactor FADH2 are produced for oxidation and
energy yield, and produces energy in the form of GTP which is readily converted
into ATP using nucleoside-diphosphate kinase. Intermediates in the cycle can be
either completely oxidised to CO2 and H2O, or starting as
precursors for biosynthetic pathways.
The CAC consists of 8 enzyme-controlled steps: Citrate synthase
catalyses the condensation of acetyl CoA, the 2-carbon product of glycolysis,
with oxaloacetate, a 4-carbon CAC intermediate, to produce citrate, a 6-carbon intermediate.
Aconitase catalyses the two-step sequential dehydration and hydration of
citrate to isocitrate, another 6-carbon intermediate. Isocitrate dehydrogenase
catalyses the two-step oxidative decarboxylation of isocitrate to
?-ketoglutarate, a 5-carbon intermediate. The first reaction is accompanied by
the reduction of NAD+ to NADH and the second reaction involves the
elimination of a carboxylate group as carbon dioxide. ?-ketoglutarate
dehydrogenase catalyses the oxidative decarboxylation of ?-ketoglutarate to
succinyl-CoA, a 4-carbon intermediate. This reaction is also accompanied by the
reduction of NAD+ to NADH and the elimination of a carboxylate group
as carbon dioxide. Succinyl-CoA synthetase catalyses the substrate-level
phosphorylation of succinyl-CoA to succinate, another 4-carbon intermediate.
This is accompanied by the phosphorylation of ADP to ATP. Succinate
dehydrogenase catalyses the dehydrogenation of succinate to fumarate, another
4-cabon intermediate. This is facilitated by the reduction of FAD to FH2
and the elimination of CoA. Fumarase catalyses the hydration of fumarate to
malate, another 4-carbon intermediate. Malate dehydrogenase catalyses the
dehydrogenation of malate to oxaloacetate. This is accompanied with the
reduction of NAD+ to NADH.
In summary, for every molecule of acetyl-CoA; two molecules of
carbon dioxide are released, substrate-level phosphorylation produces one GDP
and four reduced cofactor molecules.
LINKS TO METABOLIC PATHWAYS: CELLULAR
is produced from the breakdown of glucose into pyruvate which is then actively
transported into the mitochondria where it is decarboxylated by pyruvate
dehydrogenase in glycolysis.
reduced electron carriers generated in the CAC participate in oxidative
phosphorylation, a metabolic pathway that leads to the production of ATP. NADH
and FADH2 enter the electron transport chain where both electron
carriers are oxidised by membrane-bound complexes. NADH transfers electrons to
complex I (NADH coenzyme reductase) and FADH2 transfers electron to
complex II (succinate dehydrogenase). Coezyme Q (ubiquinone) receives electrons
from these complexes and passes the electrons to complex III (Hirst, J. 2005).
Complex I and III are proton pumps which each translocate four protons from the
matrix to the inter membrane space, generating the pH gradient and an
electrical potential (Hunte et al, 2003). As protons flow back down this potential
energy gradient, they pass through membrane-extrinsic ATP synthase, driving the
phosphorylation of ADP to ATP via ATP synthase (Dimroth et al, 1998). For every
one molecule of NADH, three molecules of ATP are formed, and for every one
molecule of QH2, two molecules of ATP are formed.
LINKS TO METABOLIC PATHWAYS: ANABOLIC
discussed before, the CAC does not just catabolise molecules to meet cellular
energetic needs, the cycle is also an important source of precursors for
biochemical anabolic pathways, where intermediates leave the cycle to be
converted primarily to glucose, fatty acids, or non-essential amino acids.
citrate can be removed from the CAC and transported across the inner
mitochondrial membrane to the cytosol. There it is cleaved by ATP citrate-(pro-S)-lyase
into acetyl-CoA and oxaloacetate. This cytosolic acetyl-CoA can then be used
for fatty acid synthesis.
malate can be removed from the CAC and transported across the inner
mitochondrial membrane to the cytosol. There malate dehydrogenase catalyses the
conversion of malate to oxaloacetate, which can also be removed directly from
the CAC. Phosphoenolpyruvatecarboxykinase then catalyses the conversion of
oxaloacetate into phosphoenolpyruvic acid which can be transported into the
cytosol and converted into glucose.
catalyses both the transamination of oxaloacetate to aspartate and
?-ketoglutarate to glutamate. Both aspartate and glutamate can be converted
into other amino acids and purines (Tornheim and Ruderman, 2011).
reacts with glycine to produce D-aminolevulinic acid which initiates the
synthesis of porphyrin (Avissar et al, 1989).
Though oxaloacetate is regenerated in the cycle, half of the
intermediates on which the cycle depends are drawn off into metabolic pathways
leading to the production of important metabolites such as fatty acids, amino
acids or porphyrins as described previously. If any of these intermediates are
diverted, the cycle is broken and can no longer function. Production of energy
can only commence again once diverted intermediates or subsequent intermediates
are replenished by anaplerotic reactions.
protein catabolism, proteases break down proteins into their amino acids. Once
de-aminated, the glucogenic amino acids (the amino acids that can only be
removed from the cycle ultimately through the gluconeogenic pathway via malate)
are able to be converted into CAC intermediates dependent on their structure
(Owen et al, 2002). See figure X.
fat catabolism, water hydrolyses triglycerides into fatty acids and glycerol.
Fatty acids with an odd number of methylene bridges can undergo beta-oxidation
to produce propionyl-CoA, which is then converted into succinyl-CoA (Houten et
are 4 major reactions classed as anaplerotic
can be replenished through three different routes: (1) the carboxylation of
pyruvate catabolised by pyruvate carboxylase, (2) the transamination of
aspartate catalysed by aspartate transaminase. ?-ketoglutarate is replenished
by the conversion of glutamate catalysed by glutamate-dehydrogenase. Succinyl-CoA
can be replenished by either (1) the ?-Oxidation of fatty acids catalysed by
methylmalonyl-CoA mutase or (2) the degradation of the amino acids isoleucine,
methionine, threonine and valine.
In regard to the CAC, most control
of energy metabolism is a response to demand for ATP rather than substrate
seen in Figure Y, three steps regulate the CAC. Isocitrate dehydrogenase is
allosterically stimulated by ADP which increases the enzyme’s affinity for
substrates and inhibited by ATP. At large concentrations, NADH displaces the
NAD+ molecules its formed from to inhibit this reaction. ?-ketoglutarate
dehydrogenase catalyses the rate-determining step of the CAC. The enzyme is
inhibited by succinyl CoA and NADH and by vast levels of ATP.
exercise, muscles contract leading to the activation of the ryanodine receptor
which leads to the release of Ca2+ from SR stores (Cheng et al, 1998). A
transporter alters mitochondrial Ca2+ to reflect change in cytoplasmic
concentration. Ca2+ in the matrix activates; pyruvate dehydrogenase, isocitrate
dehydrogenase and alpha-ketoglutarate dehydrogenase (Glancy and Balaban, 2012).
The activation of these enzymes increases the reactions they catalyse and so
increases the reaction of the whole cycle, ultimately leading to a larger
production of ATP.
starvation, initially the body converts protein to glucose. As starvation
continues, the body preserves muscle. Muscle uses fat as fuel leading to a build-up
of acetyl CoA. This build up inhibits the conversion of pyruvate to acetyl CoA,
meaning the whole cycle reaction rate decreases.