The document discusses glycolysis and the citric acid cycle. Glycolysis involves 10 steps that break down glucose and generate a small amount of ATP without oxygen. The citric acid cycle is a series of chemical reactions in the mitochondria that further oxidizes pyruvate from glycolysis to extract more chemical energy. It involves 8 steps that produce carbon dioxide, NADH, and FADH2 to fuel the electron transport chain for oxidative phosphorylation to generate large amounts of ATP. Both pathways are tightly regulated and provide precursors for other biological processes.

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