Transcript Lehninger Principles of Biochemistry

David L. Nelson and Michael M. Cox
LEHNINGER
PRINCIPLES OF BIOCHEMISTRY
Sixth Edition
CHAPTER 16
The Citric Acid Cycle
© 2013 W. H. Freeman and Company
Catabolism of
proteins, fats, and
carbohydrates in
the three stages of
cellular respiration
Overall reaction catalyzed by the pyruvate
dehydrogenase (PDH) complex
The pyruvate dehydrogenase complex consists of 3 enzymes
The complex utilizes 5 cofactors for catalysis
Pyruvate Dehydrogenase Cofactors
Nicotinamide Adenine Dinucleotide (NAD+)
an electron carrier.
Vitamin required: niacin
Flavin Adenine Dinucleotide (FAD)
an electron carrier
Vitamin required: riboflavin
Coenzyme A (CoA)
an acyl carrier with a reactive thiol - often acetyl-CoA
Vitamin required: pantothenate
Thiamine Pyrophosphate (TPP)
functions in electron transfer
Vitamin required: thiamine
Lipoic Acid
has two thiols that can form a disulfide bond
Lipoic Acid is attached to a
Lysine of E2 of the PDH complex
Composition of Pyruvate Dehydrogenase Complex
In E. coli:
24 copies of dihydrolipoyl transacetylase (E2)
E2 contains covalently bound lipoate
24 copies of pyruvate dehydrogenase (E1)
E1 contains bound TPP
12 copies of dihydrolipoyl dehydrogenase (E3)
E3 contains bound FAD
The pyruvate dehydrogenase complex is large enough to
be seen with an electron microscope.
~45 nm in diameter
6 carbons
5 carbons
5 carbons
4 carbons
Enzymes of the
Citric Acid Cycle
Structure and
Mechanism of catalysis
X-ray crystal structure of citrate synthase
Oxaloacetate (orange) binds to the enzyme first. This causes a
conformational change from “open” to “closed” form.
The conformational change creates a binding site for the
second substrate acetyl-CoA (purple).
Citroyl-CoA is formed and another conformational change
puts an Asp side chain in position to cleave citroyl-CoA.
Aconitase catalyzes the transformation of citrate to isocitrate
H2O can be added to cis-aconitate in two different ways.
Isocitrate is normally formed due to the low concentration
of isocitrate, rapidly converted to a-ketoglutarate.
Aconitase has a iron-sulfur center that acts in both
substrate binding and catalysis.
Succinyl-CoA synthetase uses the energy of a thioester bond
to produce GTP from GDP and Pi.
Succinyl-CoA synthetase is phosphorylated on a His
residue during catalysis. This phosphoryl group is then
transferred to GDP (or ADP) to form GTP (or ATP).
The citric acid cycle is a central metabolic hub
Biotin in pyruvate
carboxylase is a carrier of
one carbon
groups as CO2.
CO2 is passed on to
pyruvate to form
oxaloacetate in a
carboxylation reaction.
This reaction requires
energy in the form of
ATP.
Don’t eat raw eggs!
Why?
Eggs contain a protein called avidin.
Avidin binds very tightly to biotin and prevents its
absorption in the intestine.
When eggs are cooked the avidin is denatured
just like all other proteins.
REGULATION OF
THE CITRIC ACID CYCLE
Glucose
(Gluconeogenesis)
Phosphoenolpyruvate
Fatty acids
Amino Acids
PEP
carboxykinase
CO2
CO2
The Glyoxalate
Cycle
What plants can do
and
humans cannot
The
Glyoxylate
Cycle
Net Production
Of Oxaloacetate
For Carbohydrate
Synthesis
(non-vertebrates)
In plants, enzymes of the glyoxylate cycle are in glyoxysomes