Download The TCA cycle

The TCA cycle
Dr. S. Picksley
H17, ext 5935
THE PROBLEM
Petrol or diesel
(hydrocarbons)
+ oxygen (O2)
+ spark
Energy + C02 + H20
Combustion releases energy very fast & explosively.
We need to release energy from food slowly and discretely,
This is done by the TCA cycle and oxidative phosphorylation.
Learning objectives
1. Understand how complex foodstuffs (carbohydrate, fat
& protein) are converted into energy (ATP).
2. The conversion of food to ATP is a two step process, in
which catabolites (breakdown products of carbohydrate,
fat & protein) are channelled into the TCA cycle and
oxidised to produce C02 and energy rich (reduced)
molecules of NADH and FADH2 (nucleoside cofactors)
and GTP.
Learning objectives cont.
3. NADH and FADH2 then transfer their energy to ATP
by electron transport / oxidative phosphorylation (the
subject of the next lecture!).
4. In effect these two processes represent
RESPIRATION - the process by which aerobic cells
obtain energy from the oxidation of food by oxygen.
5. The TCA cycle is also a starting point for some
biosynthetic pathways (anabolic metabolism, - from
simple to complex molecules).
Recommended texts
‘Instant notes in biochemistry’ Section L1
The Citric acid cycle. By Hames et al. p291-305.
‘Biochemistry’ by Campbell. p532-571.
Overview
We derive all our cellular energy from carbohydrates, fat
and proteins.
Carbohydrates, fat and proteins, are catabolised (broken
down) to a common intermediate, acetyl CoA, that enters
the TCA cycle. CoA stands for a co-enzyme A (an
essential enzyme activator).
By a series of enzyme catalysed reactions the two carbon
atoms of acetyl CoA is oxidised (loses electrons) to C02
to produce GTP (from GDP) and four pairs of electrons.
These electrons are transferred to nucleotide derived
coenzymes, NAD+ and FAD, which finally transfer them
to O2 (oxygen).
Overview cont.
Energy is produced and trapped as ATP by oxidative
phosphorylation.
Energy is also produced during the TCA cycle in the form
of GTP (which is formally equivalent to ATP).
Energy use in man
At rest we will consume half our body weight in ATP per
day! Of course we cannot store this amount of ATP.
As we consume energy, ATP --> ADP + Pi, we replace it
by oxidising food molecules, and regenerating ATP
molecules.
The production of ATP is initially carried out by enzymes
of the TCA pathway and finally by an electron transport
chain.
The TCA or Citric acid or Krebs Cycle
Its called a cycle because the acetyl CoA reacts with a
metabolite (oxaloacetate), that is regenerated by a series
of enzyme catalysed reactions.
Oxaloacetate + acetyl Coa ------> citrate
Hence the alternative name, the Citric Acid Cycle.
Citrate has three COO- groups, TriCarboxylic Acid.
The cycle takes place in the mitochodria in all
mammalian cells.
TCA cycle
Location:
occurs in the mitochondria of eukaryotic cells and in the
cytosol of bacteria.
Glycolysis
Glucose ---> Pyruvate-------> Acetyl CoA
Fats, Proteins ------> Acetyl CoA
Acetyl CoA is the form in which fuel molecules enter the
cycle.
This cycle has eight well-characterised stages.
We will only consider an overview of the cycle.
TCA cycle
Acetyl CoA (C2) + oxaloacetate (C4) --> Citric acid (C6)
Oxaloacetate (C4) + 2CO2
And then the cycle begins again.
2 carbon atoms enter and 2 leave.
Yields per cycle:
1 GTP, 3NADH, 1FADH2, CoA + 2CO2
In the text books it will describe the TCA cycle as follows:
AcetylCoA
Citrate
Isocitrate
A-ketoglutarate
Succinyl CoA
THIS LEVEL OF
DETAIL IS NOT
REQUIRED.
THE AIM IS TO
UNDERSTAND
THE PRINCIPLES.
Succinate
Fumarate
Malate
oxaloacetate
TCA cycle key points
Acetyl CoA + oxaloacetate -----> Citric acid
2 Carbons + 4 Carbons
6 Carbons
2 CO2
Yields: 1 GTP, 3 NADH, 1 FADH2, CoA + 2CO2
Acetyl CoA is oxidised to CO2
This does not involve oxygen
Oxidation of acetyl CoA
Acetyl CoA is oxidised to CO2 by the donation of
electrons (e-) along with the hydrogen (H+) or hydride
(H-)ion.
H atom = H+ + e- (electron)
Hydrogen ion or proton is H+
Hydride ion is H-, H- = H+ + 2eOxidation = loss of electrons.
Reduction = gain of electrons.
Oxidation and reduction occur side by side, as electrons
are not created or destroyed
Electrons are donated to NAD+ or FAD
NAD is nicotinamide adenine di-nucleotide.
It is a co-enzyme, i.e. a chemical that is essential for
enzyme activity but is easily dissociated from the
protein with the loss of activity.
NAD+ + 2H+ + 2e- = NADH + H+
(oxidised form)
(reduced form)
FAD is flavin adenine di-nucleotide. It is a prosthetic
group, i.e. a chemical that is essential for activity and
that is physically linked to an enzyme.
FAD + 2 H+ + 2e- = FADH2
(oxidised form)
(reduced form)
Oxidised and reduced forms of NAD+ and FAD
The TCA reaction
Acetyl CoA + 3NAD+ + 1FAD + GDP + Pi + 2H20
CoA + 2CO2 + 3NADH + 3H+ + 1FADH2 + GTP.
One complete cycle yields in energy terms:
1 GTP + 3NADH + 1FADH2.
(=ATP)
GTP is a nucleotide and an energy source just like ATP and
is equivalent to ATP.
The production of GTP during the TCA cycle is referred to
as substrate level phosphorylation, i.e. it is directly
produced from GDP + Pi, without an intermediate.
NADH & FADH2 are energy-rich carrier
molecules
NADH and FADH2 are energy-rich molecules, which
transfer their energy to ATP molecules by an indirect
route of transfering electrons via the electron transport
chain to oxygen (oxidative phosphorylation).
1 molecule of NADH ------> 2.5 molecules of ATP
(3 molecules of ATP in older textbooks)
1 molecule of FADH2 ------> 1.5 molecules of ATP
(2 molecules of ATP in older textbooks)
Regulation of the TCA cycle by
respiratory control
The flow of intermediates through the TCA cycle is
regulated by the demand for ATP.
If energy demand is high as indicated by low
[ATP]:[ADP] and [NADH]:[NAD+]
there will be a high flow of intermediates through the
TCA cycle to produce the required energy.
TCA cycle as a source of biosynthetic precursors
The TCA cycle in addition to generating energy rich
molecules also has a role in generating important
precursors for:
1. Synthesis of some amino acids
2. Synthesis of glucose
3. Synthesis of proteins
4. Synthesis of nucleic acids
5. Syntheis of fats
6. Synthesis of pophyrins (used to make haem for
haemoglobin).