metabolism

1. GLYCOLYSIS AND
CITRIC ACID CYCLE
Dr. Neelam H. Zaidi

2.  It is defined as sequence of reactions of
glucose to lactate & pyruvate with the production
of ATP.
 It is derived from greek word glycose -sweet or
sugar, lysis- dissolution.
 Carried out by nearly all living cells.
 Site: cytosomal fraction of the cell
Glycolysis

3.  Glycolysis occurs in the absence of oxygen
(anaerobic) or in presence of oxygen (aerobic).
 Lactate is the end product under anaerobic
condition.
 In aerobic condition, pyruvate is formed, which
is then oxidized to CO2 & H2O.
Glycolysis

4.  The conversion of glucose to pyruvate occurs in
two stages:
 The first five reactions of glycolysis correspond to
an energy-investment phase: form high energy
intermediates at the expense of ATP.
 The following reactions constitute an energy-
generation phase: form ATP.
Glycolysis

5.  Phosphorylation of glucose
Step1: Phosphorylation
ERENGY INPUT
ERENGY INPUT
ERENGY INPUT
ERENGY INPUT
Hexokinase
is regulated

6.  Hexokinase
 This enzyme is present in most cells.
 In liver glucokinase is the main hexokinase which prefers
glucose as substrate.
 It requires ATP-Mg2+ complex as substrate. Un-complexed
ATP is a potent competitive inhibitor of this enzyme.
 Hexokinase undergoes large conformational change upon
binding with glucose. It is inhibited allosterically by G6P.
Step1: Phosphorylation

7.  Conversion of glucose 6-phosphate to fructose 6-
phosphate.
Step2: Isomerization

8.  Conversion of fructose 6-phosphate to fructose 1,6-
bisphosphate
Step3: Phosphorylation
ERENGY INPUT
PFK-1 is
regulated

9.  This step is an important irreversible, rate-limiting
committed step.
 The enzyme phosphofructokinase-1 is one of the most
complex regulatory enzymes.
 ATP is an allosteric inhibitor, and fructose 2,6-
bisphosphate is an activator of this enzyme.
 ADP and AMP also activate PFK-1 whereas citrate is an
inhibitor.
Step3: Phosphorylation

10.  Fructose 1,6-bisphosphate splits into two triose phosphate
molecules.
 Cleavage of the bond between C3 & C4.
Step4: Cleavage

11.  Interconversion between two molecules.
Step5: Isomerization
(DHAP)

12.  Requires NAD+
Step6: Oxidation
((1,3-BPG)

13.  Mechanism of arsenic poisoning
 It can compete with Pi as a substrate for
glyceraldehyde 3-phosphate dehydrogenase,
forming 3-phosphoglycerate.
 By bypassing the synthesis of and phosphate
transfer from 1,3-BPG, the cell is deprived of
energy.
 Arsenate can also inhibit pyruvate dehydrogenase
complex.
Step6: Oxidation

14.  Energy output
Step7: Substrate Level
Phosphorylation

15.  Shift of phosphoryl group
Step8: Isomerization

16.  This reaction redistributes the energy within the substrate,
forming high-energy enol phosphate in PEP.
Step9: Dehydration
PEP
Fluoride inhibits
enolase.

17.  Energy output
Step10:
Substrate Level
Phosphorylation
Fructose 1,6-
bisphosphate actives PK
Feedforward
Regulation

18. Summary

19. Summary

20. Irreversible steps are
regulated:
Hexokinase/Glucokinase
Phosphofructokinase I
Pyruvate Kinase
Regulation

21.  Insulin
 Glucagon
Hormonal Regulation

22. Reduction of Pyruvate to Lactate
 Lactate is the final product of anaerobic glycolysis.
 In the cells that amount of oxygen is limited such as
RBCs, lens and cornea of eye etc.

23.  During intense exercise, lactate accumulates in
muscle, causing a drop in the intracellular pH,
potentially resulting in cramps.
 Much of this lactate eventually diffuses into the
bloodstream and can be used by the liver to make
glucose.
Lactate Formation in Muscle

24.  Elevated concentrations of lactate in the plasma. (a type
of metabolic acidosis)
 Occur when there is a collapse of the circulatory system,
such as in MI, uncontrolled hemorrhage, or in shock.
 The failure to bring adequate oxygen to the tissues results
in impaired oxidative phosphorylation and decreased ATP
synthesis.
 To survive, the cells rely on anaerobic glycolysis for
generating ATP, producing lactic acid as the end product.
Lactic Acidosis

25. Energetics

26.  Glycolysis is a major pathway for ATP synthesis
in tissues lacking mitochondria, erythrocytes,
cornea, lens etc.
 Glycolysis is very essential for brain which is
dependent on glucose for energy.
 Glycolysis is a central metabolic pathway with
many of its intermediates providing branch point to
other pathways.
 The intermediates of glycolysis are useful for the
synthesis of amino acids and fat.
Biomedical Importance of Glycolysis

27. CLINICAL ASPECTS
 Hemolytic Anaemias: inherited aldolase A &
pyruvate kinase deficiencies.
 Skeletal muscle fatigue
 Inherited pyruvate dehydrogenase deficiency-
Lactic acidosis
 Fast growing cancer cells glycolysis proceeds
at faster rate – increased acidic environment –
implication in certain types of cancer.

28. Pyruvate to Ethanol Conversion
-----interesting
 Alcohol fermentation.
Only in yeast, bacteria
(some). NOT in human

31.  The Citric acid cycle (Tricarboxylic acid cycle [TCA
cycle] or Krebs cycle) plays several roles in
metabolism.
 Final common pathway for oxidation of fuel
molecules such as carbohydrates, amino acids,
and fatty acids.
 Provides energy: ATP.
 Aerobic pathway, O2 is required.
 Occurs totally in the mitochondria.
Citric Acid Cycle

32.  Not closed cycle.
 Supplies intermediates for the synthesis of glucose,
amino acids, heme, pyrimidine, fatty acids, ketone
bodies and sterols.
 Generates NADH,FADH2,CO2
Citric Acid Cycle

33. CITRIC ACID
CYCLE

34.  Oxidative decarboxylation of pyruvate by the
pyruvate dehydrogenase(PDH) complex.
 Connecting link between glycolysis and TCA cycle.
 The PDH requires five coenzymes:
 Thiamine pyrophosphate (TPP)---B1
 CoA---B5
 Flavin adenine dinucleotide (FAD)---B2
 Nicotinamide adenine dinucleotide (NAD+)---B3
 Lipoic acid
Formation of Acetyl CoA

35. Formation of Acetyl CoA
ATP, NADH, acetyl
CoA inhibit PDH
Pyruvate, AMP, Ca2+
activate PDH

36.  The function of CoA is to accept and carry acetyl groups.
CoA

37.  Most common biochemical cause of congenital
lactic acidosis.
 Pyruvate is converted to lactate via LDH.
 Brain, which relies on the TCA cycle for energy and
is particularly sensitive to acidosis.
 Symptoms: neurodegeneration; muscle spasticity;
early death.
 Dietary restriction of carbohydrate and
supplementation with thiamine.
Pyruvate Dehydrogenase Deficiency

38.  Arsenite forms a stable
complex with the thiol (–SH)
groups of lipoic acid, making
IT unavailable.
 Pyruvate dehydrogenase
inhibited.
 Pyruvate and lactate
accumulate.
 Neurologic disturbances
(brain) and death.
Arsenic Poisoning

39.  Condensation: with C-C bond formation.
 Rate limiting step
 Regulated by substrate availability(OAA) and
product inhibition(citrate).
Step1: Condensation
Tricarboxylic Acid
ATP, NADH,
succinyl CoA
inhibit
ADP
activate

40. Step2: Isomerization
Fluoroacetate
inhibit aconitase
Fluoroacetate, a plant toxin,
used as pesticide.

41.  Irreversible, rate limiting step.
 Produces CO2 and NADH.
Fluoroacetate
inhibit aconitase
Step3: Oxidative Decarboxylation
isocitrate dehydrogenase isocitrate dehydrogenase
ADP, Ca2+ activate;
ATP,NADH inhibit it.
aa metabolism

42.  Requires coenzymes: TPP, FAD,NAD+, CoA,
lipoic acid.
Step4: Oxidative Decarboxylation
succinyl-CoA, NADH inhibit
it; Ca2+ activates it.
(high-energy thioester)
Fatty acids, aa

43.  Cleaves high energy thioester bond.
 Yields GTP, which can be converted to ATP.
Step5:Substrate Level Phosphorylation
(succinate thiokinase)

44.  The only enzyme in TCA embedded in the inner
mitochondrial membrane, component of ETC.
Step6: Oxidation
Fumarate is also produced in urea
cycle, purine synthesis.

45. Step7: Hydration

46.  Final step in TCA cycle.
 Regenerates OAA.
Step8: Oxidation
aa

47. Summary
2C

48.  Calculation:
 One turn of TCA cycle can generate
 3 NADH
 1 FADH2
 1 GTP
 (2CO2)
 1 glucose can produce 2 pyruvate, then
undergoes 2 turns of TCA cycle.
 So the total energy that can yield is 20 ATP per
molecule of glucose.
Energetics
10ATP

49. Regulation

50.  Amphibolic: serves in both catabolism and anabolism.
 All the metabolites in TCA cycle needs to be replenished.
TCA Cycle Is Amphibolic Pathway

51.  Complete oxidation of acetyl CoA.
 ATP generation.
 Final common oxidative pathway.
 Integration of major metabolic pathways.
 Fat is burned on the wick of carbohydrates.
 Excess carbohydrates are converted as neutral
fat.
 No net synthesis of carbohydrates from fat.
 Carbon skeleton of amino acids finally enter the
TCAcycle.
Significance

52. BIOMEDICAL IMPORTANCE


Final common pathway for oxidation of
carbohydrates, lipids & proteins.
Supplies intermediates in gluconeogenesis,
transamination, deamination & lipogenesis.

Vitamins play a key role in this cycle
Eg; Riboflavin – FAD
Niacin – NAD
Thiamine –TPP
Pantothenic acid – CoA.

54.  De Meirleir L: Defects of pyruvate metabolism and the Krebs
cycle. J Child Neurol 2002;(suppl 3):3S26.
 Rama Rao KV, Norenberg MD: Brain energy metabolism
and mitochondrial dysfunction in acute and chronic hepatic
encephalopathy. Neurochem Int 2012;60:697.
 Lalau JD: Lactic acidosis induced by metformin: incidence,
management and prevention. Drug Saf 2010;33:727.
 https://www.slideshare.net/arijabuhaniyeh/chapter-16-the-
citric-acid-cycle-biochemistry.
 Kim J-W, Dang CV: Multifaceted roles of glycolytic enzymes.
Trends Biochem Sci 2005;30:142.
References