Carbohadrate metabolism MBBS Follow respective lectures

1. CARBOHYDRATE
METABOLISM
Lecture 3
Dr. Vaneet Kaur

2. • Describe and discuss the concept of Pyruvate Dehydrogenase (PDH)
Complex and the Tricarboxylic Acid (TCA) Cycle:
• Describe the overall purpose of the pyruvate dehydrogenase (PDH)
complex, its reactants and products, its cellular localization, and its tissue
distribution.
• Compare the general structure, regulation, and required cofactors/vitamins
of the PDH complex to that of alpha-ketoglutarate dehydrogenase and the
branched-chain alpha-keto acid dehydrogenase complex.
• Describe the overall purpose of the TCA cycle, its cellular localization, and
its tissue distribution.
• Describe the reactants and products of the TCA cycle as related to the fates
of the breakdown products of carbohydrates, fatty acids, and amino acids.

3. Cori’s cycle/ Lactic acid cycle
• Process in which Glucose is converted to lactate in the muscle and
reconverted to glucose in liver
• During muscle contraction, pyruvate is reduced to lactate which would
accumulate in muscle
• This leads to muscle cramps ( normally seen during exercise)
• Significance- The body uses cori’s cycle to prevent lactate accumulation
• The lactate formed in muscle diffuses into blood from where it reaches the
liver
• It is oxidized to pyruvate which is channeled to the production of glucose
• Regenerated glucose now enters the blood & than the muscle.
• This is CORI’s cycle ( explained in the diagram in next slide)

5. Metabolic fate of Pyruvate in aerobic
conditions
• Under aerobic conditions, pyruvate is oxidized to acetyl CoA which
enters Kreb’s cycle to be oxidized to CO2 with production of energy
• Pyruvate dehydrogenase Complex
• Pyruvate is converted into acetyl CoA by a multienzyme complex
which is found in the mitochondria
• The reaction is irreversible
• Pyruvate is transported into the mitochondria via a pyruvate
transporter by symport mechanism in which one proton is
cotransported

7. • Pyruvate is decarboxylated by PDH component to hydroxyethyl TPP
• Hydroxyethyl TPP reacts with oxidized lipoamide in presence of
dihydrolipoyl transacetylase to form Acetyl lipoamide
• Acetyl lipoamide reacts with coenzyme A to form Acetyl CoA &
reduced lipoamide
• Reduced lipoamide is further oxidized to oxidized lipoamide by a FAD
containing lipoprotein in presence of dihydrolipoyl Dehydrogenase.
• Finally, the reduced flavoprotein is oxidized by NAD which transfers
the reducing equivalents to respiratory chain( explained in the
diagram in adjacent slide)

9. Cofactors associated with PDH complex
• TPP ( Thiamine pyrophosphate)
• Lipoic acid
• NAD+, FAD+
• Coenzyme A
• Magnesium ions

10. Advantage of PDH complex
• Can act sequentially
• Product of one enzyme is handed over to the next enzyme as substrate
• It increases the efficiency of the complex
• Importance of PDH complex
• Glucose is converted to acetyl CoA which is utilized for formation of fat
• It is an irreversible reaction
• Since backward reaction is not possible, so glucose cannot be synthesized
from fat
• Pyruvate may be channeled back to glucose through gluconeogenesis

11. Regulation of PDH complex
• PDH complex regulation takes place through
• End product inhibition– PDH is inhibited by increased conc of acetyl CoA
and NADH( Reduced form of NAD+)
• Covalent modification-
• Conversion of Active PDH to inactive PDH
• PDH kinase converts the active dephosphorylated form of PDH enzyme
into inactive phosphorylated form
• PDH kinase is activated by
• Rise in ATP/ADP ratio Rise in Acetyl CoA/ CoASH ratio
• Rise in NADH/NAD ratio Increased cAMP in cells

12. • PDH Phosphatase which converts the inactive form of PDH into active
form of PDH is activated by Insulin and calcium ions
• During starvation and Diabetes Mellitus, acetyl CoA and NADH are
produced by enhanced β oxidation
• These activates PDH kinase which by phosphorylation makes the PDH
inactive ( by covalent modification)
• Also, NADH and acetyl CoA inhibit the PDH complex (by feedback
/endproduct inhibition)
• So, PDH complex and hence glycolysis are inhibited under conditions
of Fatty acid oxidation

15. Biochemical importance of PDH
• Arsenic or mercuric ions complex with –SH group of lipoic acid and
inhibit PDH leading to accumulation of pyruvate
• Chronic alcoholics have thiamine deficiency as alcohol inhibits
thiamine absorption
• This leads to accumulation of pyruvate and hence lactate via LDH.
Lactic acidosis is usually seen after glucose load
• Inherited deficiency of PDH will also lead to lactic acidosis
• Dietary deficiency of thiamine will inhibit PDH and so accumulation of
pyruvate leading to lactic acidosis

16. Citric acid cycle/ Kreb’s cycle/ Tricarboxylic
acid cycle( TCA cycle)
• Site- Mitochondria
• Involves oxidation of acetyl CoA to CO2 and water
• Biomedical importance
1) Final common pathway for oxidation of carbohydrates, lipidsand
proteins.
2) Plays a major role in gluconeogenesis, transamination, deamination
and lipogenesis. So, serves to integrate a number of pathways.
3) Abnormalities of enzymes of TCA cycle are very rare in humans. If
present are associated with severe neurologic damage.

17. • Consists of eight sequential reactions
• Citrate synthase- Begins with condensation of four membered
oxaloacetate with two carbon acetyl CoA– to form six membered Citrate
molecule by an irreversible reaction
• Aconitase- Converts citrate to isocitrate by an isomerization reaction
• Isocitrate dehydrogenase- catalyses oxidative decarboxylation of isocitrate
to alpha ketoglutrate with removal of CO2
NAD+ is converted to NADH
• Alpha ketoglutrate DHG- catalyses oxidative decarboxylation of alpha
ketoglutrate to Succinyl CoA, with loss of CO2 & production of NADH(
reaction similar to that catalyzed by PDH)

18. • Succinyl CoA synthetase- catalyses conversion of succinyl CoA to succinate
with production of GTP ( an example of substrate level phosphorylation)
• Succinate DHG- Succinate is further oxidized to fumarate with reduction of
FAD to FADH2
• Note: All the enzymes of Kreb cycle are located in mitochondrial matrix
except succinate DHG which is anchored to inner mitochondrial membrane
• Fumarase- This is a hydration reaction which is reversible. Fumarate is
converted to malate
• Malate DHG- It converts malate to OAA in a reversible reaction. NAD+ is
converted to NADH

21. Important points about Kreb’s cycle
• There is no net consumption of oxaloacetate in the cycle
• There occurs complete oxidation of acetyl CoA
• This is associated with generation of energy
• Fat (acetyl CoA which is also derived from beta oxidation of fats) is burned
with the help of carbohydrates ( OAA which is formed from pyruvate by
pyruvate carboxylase ).
• There occurs no net synthesis of carbohydrates from fat ( Acetyl CoA is
converted into CO2, so cannot be used for production of glucose)
• Excess carbohydrates are converted into fat which gets deposited in
adipose tissue ( Glucose forms pyruvate by glycolysis--- Pyruvate forms
acetyl CoA by PDH complex– Acetyl CoA is the starting material for fatty
acid synthesis if it is not oxidized in kreb cycle for production of energy)