Download PG1005 Lecture 12 Kreb`s Citric Acid Cycle

PG1005
Lecture 12
Kreb’s Citric Acid Cycle
Dr. Neil Docherty
My Teaching Objectives
•  To discuss the transition from pyruvate generation in the
cytosol to the establishment of electron harvesting reactions
in the mitochondrial matrix
•  To revise the general mechanisms of glucose uptake.
•  To describe the enzymatic reactions occurring at each step
of Kreb’s Citric Acid Cycle (KCAC).
(substrates, enzymes, products, reaction types)
•  To highlight the existence of checkpoints in the KCAC
which permit physiological supervision of flux through the
process
Glycolysis to KCAC
How we got here
•  A sequence of 10 reactions based on using glucose as a
substrate to generate two molecules of pyruvate.
•  Anaerobic
•  For each molecule of glucose a net production of 2 ATP occurs.
Lactic acid fermentation
Oxidative phosphorylation
Gluconeogenesis
Adapted from Figure 2-10 Human Physiology Cells to Systems (7th Ed.) Sherwood L. p245
Pyruvate dehydrogenase Complex
Aim: Generation of 2 carbon intermediate for subsequent
Generation of citrate (6C)
3 enzyme complex, 5 co-factors
(Thiamine pyrophosphate TPP, lipoamide, FAD, CoA, NAD)
Hydroxyethyl TPP
DECARBOXYLATION
Lipoamide
pyruvate
By-Products
CO2
NADH
OXIDA
TION
=enzyme
OXIDATION
acetyllipoamide
Italics=substrate/co-factor
CAPS=reaction type
Dihydrolipoamide
N.B. Do not try to learn the
steps on this slide “off by heart”
Cycle Basics
•  Harvest high energy electrons and transfer them to
electron carriers for use in oxidative phosphorylation
•  Involves the generation of a 6 carbon tricarboxylic
acid (citric acid) from oxaloacetate (C4) and acetyl
CoA (C2-derived from pyruvate).
•  Sequential oxidation of two carbon units generates
CO2, reduced electron intermediates, 1 molecule of
guanosine triphosphate (GTP) and regenerates
oxaloacetate
•  High energy phosphate group transfer from GTP to
ADP generates ATP (x2 per molecule of glucose)
Location of Kreb’s Cycle
Definition as a cycle
is due to regeneration
of oxaloacetate
Adapted from Fig. 2.12 Human Physiology Cells to Systems (7th Ed.) Sherwood L.
Citrate Synthase
Aim: Generation of 6 carbon intermediate for subsequent
coupling of electron transfer to decarboxylation
1)  Oxaloacetate condenses with acetyl CoA to form citryl
CoA
2)  Hydrolysis of Citryl CoA yields citrate and Co A
CONDENSATION
Binding order
Oxaloacetate-Acetyl-CoA
Enclosure of
active site
HYDROLYSIS
Release order
CoA-Citrate
Citrate Isomerase (aconitase)
•  Aim: Rearrangement of hydroxyl group to facilitate subsequent
decarboxylation reactions
Dehydration
Hydration
Isocitrate Dehydrogenase
•  Aim: Generation of ALPHA KETOGLUTARATE (5C)
and NADH
OXIDATION-REDUCTION
DECARBOXYLATION
Alpha ketoglutarate dehydrogenase
•  Aim: Generation of succinyl CoA (4C) and NADH
•  Similar to pyruvate dehydrogenase
(i.e. 3 enzymes, co-factors, addition of CoA, loss of 1C and
gain of NADH)
high energy
thioester bond
Succinyl CoA synthetase
•  Aim: Transfer of energy stored in Succinyl CoA to drive GTP formation
and generation of succinate (4C)
•  Succinyl CoA is phosphorylated while in alpha subunit of enzyme
releasing CoA
•  Subsequently, the attached phosphoryl group is transferred to GDP in
beta subunit of enzyme
SUBSTRATE LEVEL PHOSPHORYLATION
Succinate to Oxaloacetate
Aim -Oxaloacetate synthesis
-Further synthesis of reduced intermediates
OXIDATION
Succinate
dehydrogenase
HYDRATION
fumarase
OXIDATION
malate
dehydrogenase
Net Reaction of Cycle (Per Acetyl CoA)
Acetyl CoA + 3NAD + FAD + GDP + Pi + 2H2O
2CO2 + 3NADH + FADH2 + GTP + CoA
Control Points in Cycle
Pyruvate Dehydrogenase
•  i) inhibited by high acetyl CoA, ATP and NADH
ii) Phosphorylative (kinase) inactivation of enzyme when ATP/
ADP ratio is high
iii) Phosphorylation inhibited by ADP and pyruvate
Isocitrate Dehydrogenase
•  Allosteric activation/inhibition by ADP/ATP
Alpha ketoglutarate dehydrogenase
•  Inhibited by high Sucinyl CoA, ATP and NADH
Your Learning From Today
Should focus on being able to;
•  Outline how pyruvate generated in glycolysis is fed into the
KCAC chain of enzymes in the mitochondrial matrix
•  Relate the events occurring in the KCAC to a meaningful
biological imperative, that is the harvesting of high energy
electrons for use in driving ATP synthesis
•  List and describe the enzymatic reactions occurring at
each step of Kreb’s Citric Acid Cycle (KCAC).
(substrates, enzymes, products, reaction types)
•  Discuss allosteric checkpoints in the KCAC