Abstract are localized in the mitochondrial matrix. Prokaryotic cells


Glycolysis only utilises a small proportion of the
available ATP, which can be obtained from the glucose molecule. To become more
efficient pyruvate must be oxidised into carbon dioxide and water this is where
the citric acid is cycle vital. The citric acid cycle consists of a series of reactions,
used by all aerobic organisms, which are localized in the mitochondrial matrix.
Prokaryotic cells don’t pose mitochondria and therefore reactions occur in the
cytosol. Pyruvate is able to form different molecules which are produced
dependent on the energy requirements of the cell and the presence of oxygen, if
aerobic conditions are present pyruvate is converted into acetyl CoenzymeA
which can enter the citric acid cycle. However in the presence of anaerobic
conditions lactic acid (in animals) or ethanol is produced in plants and yeast.

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The citric acid cycle has two main roles of the cycle;
one is to increase the cell’s ATP production. The other role being to provide
the cell with precursors that can be used to build up a variety of molecules
depending on the cells needs.


Overview of the Cycle

The citric acid cycle is a key component of the
metabolic pathway used to generate energy in the form of ATP from amino acids,
fatty acids and carbohydrates. The overall pattern of the citric acid cycle is
demonstrated through figure 1.


Figure 1- Diagram showing the intermediate compounds formed
during the citric acid cycle


For the catabolism of sugars, fats and proteins a
two-carbon organic product of acetate in the form of acetylcoenzymeA is
produced which is show to enter the citric acid cycle (fig1). The production of
acetylcoenzymeA is a starting point for the citric acid cycle as it transfers
its two- carbon acetyl group and condenses with a four-carbon acceptor compound
known as oxaloacetate to yield a six-carbon compound called citrate. This is
isomerised to isocitrate. Two carbon atoms from citrate (originally donated
from oxaloacetate not acetyl-coA, as acetyl-CoA carbons become part of the
oxaloacetate carbon backbone after the first turn of the citric acid cycle) are
released via decarboxylation to form two molecules of carbon dioxide as a waste
product CoenzymeA is released and can return back to the link reaction to form
another molecule of acetyl-coA. Citrate undergoes over a series of chemical
transformations firstly it forms the five-compound ?-ketoglutarate which is oxidatively decarboxylated to form a four-carbon
compound known as succinyl CoA. In succinyl CoA the thioester bond is cleaved
to yield succinate, which is oxidised to fumarate, which is consequently
hydrated to form malate. Malate is another 4-carbon compound, which becomes
oxidised to regenerate oxaloacetate.


The citric acid itself
doesn’t generate ATP, but removes electrons to add to coenzymes to produce NADH
and FADH2 Four
oxidation-reduction reactions occur whereby three hydride ions are transferred
to three molecules of NAD+, which gives 3 molecules NADH whereas one pair of
hydrogen atoms is transferred to one molecule of FAD producing a single
molecule of FADH2.
For each acetyl group that enters the cycle. Hydrogen carriers are reduced through oxidation reactions. It is in
oxidative phosphorylation that these electrons are released in the reoxidation
of these coenzymes. This occurs as protons flow through ATP synthase to
generate ATP from ADP and an inorganic phosphate due to molecular rotation of
the enzyme.  Oxygen is the final proton
acceptor, as it is required to regenerate NAD+ and FAD to be used in the next
turn of the Krebs cycle.


From the citric acid cycle,
overall per glucose molecule two cycles and produce 4 molecules of carbon
dioxide, 6 reduced NAD, two recued FAD and two molecules of ATP are produced
directly via substrate level phosphorylation. At the end of every cycle,
four-carbon oxaloacetate has been regenerated allowing the cycle to undergo
another turn.


ATP production

The total amount of ATP
generated from just Krebs for one molecule of glucose is around 25 molecules.
Most of the energy is captured by the coenzymes NAD+ and FAD and is then
converted at a later stage to ATP from which energy transfers occur through the
relay of electrons from one substance to another through redox reactions which is
the final part of cellular respiration. This ETC releases energy so that it can
be converted to ATP through oxidative phosphorylation When the citric acid
cycle is complemented with oxidative phosphorylation it provides the majority
of the energy used by aerobic cells (95%). The Citric acid cycle has a high
efficiency, as it only requires a small quantity of molecules to generate large
quantity of NADH and FADH2. This is because oxaloacetate participates in
oxidising the acetyl group but is regenerated and hence can continue to be
utilised for further turns



The flux of metabolites within the citric acid cycle
can be regulated under different nutritional conditions to satisfy the needs of
the cell. If a build up of citric acid products and intermediates accumulate,
these compounds affect enzyme activity to greatly decrease the rate of the citric
acid reactions that occur.


The key regulatory
compounds that act as inhibitors are citrate, NADH and succinyl CoA which together
act to decrease the rate of activity of the citric acid cycle. With each consecutive
cycle, the key regulatory compounds increase.

There also exists a positive
regulator known as activators whose function is to increase the rate of the
citric acid cycle when the cells energy requirements are not being met. Key allosteric
activators include calcium ions, and ADP both of which signal to increase the
activity of isocitrate dehydrogenase (an enzyme which catalyses the oxidative
decarboxylation of isocitrate) and ?-ketoglutarate dehydrogenase, a vital enzyme complex,
which leads to the decarboxylation of ?-ketoglutarate both activators encourage ATP to be


During vigorous exercise there is a high glucose
consumption rate and glucose begins to be depleted from the body meanwhile
pyruvate accumulates. As long as oxygen is present, pyruvate is converted to
acetyl-CoA, which starts the citric acid cycle, and will cause the cycle to restart,
by the action of the calcium ions and ADP.


Replenishing intermediates of the citric
acid cycle

The intermediates are replenished, vital for when
there is a high level of metabolic activity like during exercise. These are
known as anaplerotic reactions as they feed the citric acid cycle with
intermediates. Several catabolic pathways converge on the Krebs cycle,
reactions that form intermediates of this cycle in order to replenish them
(especially during shortages) are called anaplerotic reactions.


Both PEP carboxylase and pyruvate
carboxylases are vital as they yield oxaloacetate from the synthesis of
acetate.  Pyruvate carboxylase catalyses
irreversible pyruvate through carboxylation is a mitochondrial protein, which
contains a biotin prosthetic group, acetyl-CoA is bound to the allosteric
binding site in order to activate bicarbonate with ATP this reaction is
demonstrated in figure 2. (Jitrapakdee
et al., 2008)

Figure 2- Activity of pyruvate carboxylase


Catabolism and anabolism

Catabolic functions are the metabolic functions that
break down molecules and anabolic functions are the metabolic processes that
build up molecules. The citric acid cycle is of amphibolic nature meaning it
has both these functions hence possess both a degradative nature and
synthesising role. Being an amphibolic pathway it breaks down carbohydrates
into smaller molecules and also produces energy in the form of ATP, which is
used to them synthesise more complex molecules from simpler ones. Different coenzymes
are used in both the catabolic and anabolic pathways, in catabolism NAD+ is
reduced to NADH acting as an oxidising agent. In comparison, in anabolism the
coenzyme NADPH is oxidised to form NADP+, acting as a reducing agent.


Early catabolism causes biological molecules such as
fatty acids, monosaccharide’s or amino acids to break down into smaller
molecules for energy. During carbohydrate catabolism, glycolysis breaks down
glucose into pyruvate where in eukaryotes it then moves into the mitochondria.
Through the link reaction, it’s converted into acetyl-CoA by decarboxylation
and enters the citric acid cycle. In protein catabolism, proteins are broken
down by proteases into their constituent amino acids. The carbon backbone of them
becomes a source of energy being converted into acetyl-CoA and entering into
the citric acid cycle. Within fat catabolism, triglycerides are hydrolysed to
break them into fatty acids and glycerol, which would be converted into glucose
via gluconeogenesis. Fatty acids are broken down in tissues producing
acetyl-CoA allowing the citric acid cycle to be initiated. Beta-oxidation of
fatty acids yields proionyl-coA which is converted into succinyl-coA and fed
into citric acid cycle.


The series of reactions citrate that covert
oxaloacetate through intermediates which reforms citrate is a further example
of anabolic function of the citric acid cycle.


The intermediates from the citric acid cycle are used
as precursors to produce fatty acids and cholesterol (from the citrate), amino
acids from the a-ketoglutarate, carbohydrates as pyruvate can be formed from
malate and glucose synthesised from oxaloacetate. The starting molecules are
all intermediates in the synthesis of other compounds during anabolic reaction