Carbohydrate Metabolism [Part 1] (Viva)

Q.1 What are the various pathways by which glucose is utilized?

  1. Glycolysis followed by the tricarboxylic acid cycle.
  2. Hexose monophosphate shunt pathway.
  3. Conversion to glycogen.
  4. Conversion to galactose and then to lactose.
  5. Give rise to nonessential amino acids.
  6. Conversion to fat.

Q.2 What is glycolysis? What are rate limiting enzymes of glycolysis?

Glycolysis can be defined as oxidation of glucose or glycogen to pyruvate and/or lactate.

The rate-limiting enzymes of glycolysis are:

  • Hexokinase
  • Phosphofructokinase
  • Pyruvate kinase.

Glycolysis is sometimes also known as Embden-Meyerhof Pathway (EM pathway)

Q.3 What is the role of O2 in glycolysis?

The glycolytic pathway is unique in the sense that it can occur in presence of O2 if available (“aerobic” phase) and it can function also in absence of O2 (“anaerobic” phase).

Q.4 In which part of the cell glycolysis occurs and where the enzymes are located?

 Glycolysis occurs in the cytosol and enzymes involved are cytosolic (extramitochondrial).


Q.5 State the biomedical importance of glycolysis.

  • Provides energy.
  • Cardiac muscle has poor glycolytic activity and poor survival under conditions of ischemia.
  • Rate of glycolysis is very high in fast-growing cancer cells. Enhanced glycolysis produces more pyruvic acid than the TCA cycle can handle. Hence pyruvic acid accumulates and forms lactic acid-producing local lactic acidosis which is congenial for certain cancer therapy.
  • Inherited enzyme deficiencies like hexokinase and pyruvate kinase produce hemolytic anemia.

Q.5.1 Name the steps of glycolysis where ATP is consumed.

ATP is utilized for phosphorylations:

  • For conversion of glucose → glucose-6-P
  • For conversion of fructose-6-P → fructose 1,6-bi-P
  • Body spends two ATP molecules (-2 ATP)

Q.6 Which enzymes are required for phosphorylations?

  • For phosphorylation of glucose:
    Hexokinase/and/or glucokinase enzymes.

  • For phosphorylation of fructose-6-P:
    Phospho-fructokinase enzyme.

Q.7.State Five differences between hexokinase and glucokinase.

Hexokinase Glucokinase
Found in all tissues Found only in liver
More stable More labile
Non-specific, can phosphorylate any of the hexoses Specific only for glucose 
Km is low, hence high affinity for glucose Km is high, hence low  affinity for glucose 
Main function to make available glucose to tissues for oxidation at lower blood glucose level  Main function to clear glucose from blood after meals and at blood levels greater than 100 mg per dl 

 

Q.8 Which is the most energy-yielding step in the glycolytic pathway?

Oxidation of glyceraldehyde-3-P, in presence of O2, by the enzyme glyceraldehyde-3-P dehydrogenase which is NAD+ dependent.
2 NADH when oxidized in ETC gives 6 ATP.

Q.9 What is the end-product of glucose oxidation by glycolysis?

Pyruvic acid

Q.10 Name the inhibitors of glycolysis.

Iodoacetate and iodoacetic acid: Inhibit glyceraldehyde-3-P-dehydrogenase.

Arsenite: Inhibits phosphoglycerate kinase indirectly and no ATP is formed at the substrate level.

Fluoride: Inhibits the enzyme enolase.

Q.11 Name the tissues which solely depends for energy from glycolysis.

  • Red blood cells and,
  • Brain and nervous tissue.

Q.12 Enumerate the sources of pyruvic acid (PA) in the body.

  • By glycolysis—principal source.
  • Oxidation of LA(Lactic Acid) → PA in the presence of O2.
  • Deamination of alanine.
  • Other pyruvic acid-forming amino acids, e.g. glycine, serine, cysteine/cystine, threonine.
  • Decarboxylation of oxaloacetic acid (OAA)
  • From malic acid by malic enzyme.

Q.13 Enumerate the fate of pyruvic acid (PA) in the body.

  • Oxidative decarboxylation of PA to form acetyl CoA in presence of O2.
  • Reduction of PA → LA in the absence of O2.
  • Amination to form alanine.
  • Conversion to glucose (gluconeogenesis).
  • Conversion to malic acid
  • Formation of oxaloacetate (OAA) by “CO2-fixation reaction”.

Q.14 State the irreversible steps in glycolysis.

  • Glucose-6-P → Glucose.
  • Fructose-1, 6-bi-P→ Fructose-6-P
  • Phophoenol pyruvate→Enol-pyruvate.

Q.15 What is anaplerotic reaction or anaplerosis?

  • A sudden influx of PA or acetyl CoA to the TCA cycle might seriously deplete the supplies of OAA required for the citrate synthase reaction.
  • Two reactions that are auxiliary to the TCA cycle operate to prevent this situation. These are called anaplerotic reactions (or “filling-up” reactions), and the phenomenon is called anaplerosis.

 

Q.15.1 Name the two anaplerotic reactions.

  • Conversion of PA to OAA by CO2- fixation reaction by the enzyme pyruvate carboxylase which requires biotin, ATP, Mg++ and acetyl CoA. Acetyl CoA acts as a +ve modifier; it helps the enzyme to maintain “active” conformation.
  • Conversion of PA to OAA through malic acid formation

Q.16 Name the inhibitor of lactate dehydrogenase enzyme (LDH).

Oxamate - It competitively inhibits lactate dehydrogenase (LDH) and prevents reoxidation of NADH.

Q.17 How many ATPs are produced in glycolysis in the presence of O2 (Aerobic phase)? Explain.

  • Produces 8 (Eight) ATP

Details as follows:

  • Loss:

Phosphorylation of glucose = – 1 ATP

Phosphorylation of fructose = – 1 ATP 

  • Gain:

Oxisation of glyceraldehyde-3-P = + 6 ATP 

Phosphoglycerate kinase reaction (Substrate level) = + 2 ATP

Pyruvate kinase reaction (substrate level) = + 2 ATP 

* Net gain = 10 ATP – 2 ATP = 8 ATP

Q.18 How many ATPs are produced in glycolysis in absence of O2 (Anaerobic phase)?

  • In absence of O2, NADH + H+ produced by oxidation of glyceraldehyde -3-P, cannot be oxidized in ETC NADH is converted to NAD+ in the reduction of pyruvate to lactate. Hence 6 ATP is not produced.
  • In the anaerobic phase, per molecule of glucose oxidized, 4 ATP – 2 ATP = 2 ATP will only be produced.

Q.19 What is the ATP yield under aerobic conditions?

38 ATP per molecule of glucose metabolised. i.e. glycolysis 8 ATP, 
TCA cycle 30 ATP

Q.20 What is the TCA cycle?

A TCA cycle is the final common pathway for metabolism of carbohydrates, lipids, and proteins (IIIrd phase of metabolism). It is a cyclic process and involves a sequence of compounds interrelated by oxidation-reduction and other reactions which finally produce CO2 and H2O.

TCA cycle is also known as Krebs’ cycle or citric acid cycle 

Q.21 How many ATPs are formed in the TCA cycle?

Starting From acetyl CoA - 12 ATPs

Starting From pyruvate - 15 ATPs


Q.22 State the over-all bioenergetics in complete oxidation of glucose/glycogen in glycolysis-cum-TCA cycle in presence of O2

A. Glycolysis  ATP yield per hexose unit
Glycogen → F-1, 6-bi-P  –1 ATP
Glucose → F-1, 6-bi-P  –2 ATP
Glyceraldehyde-3-P dehydrogenase (2 NADH→2 NAD+) +6 ATP
Substrate level phosphorylation:  
Phosphoglycerate kinase  +2 ATP
Pyruvate kinase  +2 ATP
Net gain in glycolysis:
For glycose = +8 ATP
For glycogen = +9 ATP 
 


B. Oxidative decarboxylation of P.A.:

PDH complex (2 NADH → 2NAD+)   +6 ATP


 C. TCA cycle:

Isocitrate dehydrogenase (2 NADH→ 2 NAD+)  +6 ATP
α-oxoglutarate dehydrogenase (2 NADH → 2 NAD+)  +6 ATP
Substrate level phosphorylation:   
Succinate thiokinase 2 (GTPor ITP) → 2 ATP +2 ATP
Succinate dehydrogenase  2 FAD.H2 → FAD + 4 ATP
Malate dehydrogenase 2 NADH → 2 NAD+ +6 ATP
Total = + 24 ATP

 

∴ Total energetics:

  • Per mole of Glucose = 24+6+8 ATP = 38 ATPs
  • Per mole of Glycogen = 25+6+9 ATP = 39 ATPs

Note: Under anaerobic conditions (in absence of O2):

  • Glucose = +2 ATPs
  • Glycogen = +3 ATPs.


Q.23 State the efficiency of complete oxidation of glucose.

  • One mole of glucose after complete oxidation produces = 38 ATPs
  • Total energy captured in ATP per mole of glucose oxidized= 7600 × 38 = 2,88,800 calories.
  • Oxidation of one molecule of glucose “in vitro” produces = 6,86,000 calories

∴ Hence efficiency =( 2,88,8006,86,000 )× 100 = 42%. 

Q.24 How 2,3-DPG formation takes place?

When bisphosphoglycerate mutase acts upon 1,3 bisphosphoglycerate formation of 2,3 bisphosphoglycerate taken place in erythrocytes.

Q.25 What is Rapoport-Leubering cycle or shunt (RLC or RLS)?

RLC/or RLS is a diversion in the glycolytic pathway in red blood cells. Conversion of 1, 3-BPG to 3 PG does not occur and ATP is not formed at substrate level. It forms 2, 3-BPG.

It is calculated to deplete and waste the energy needed by the RB cells 

Q.26 What is the function of 2,3 DPG?

2,3-DPG reduces the affinity of oxygen with hemoglobin so, is responsible for normal oxygen delivery to peripheral tissues.

Q.27 What is the net energy change during the formation of 2-3-DPG in glycolysis?

No net production of ATP takes place.

Q.28 Why citric acid cycle is considered the common pathway for carbohydrate, fat, and protein metabolism?

Citric acid cycle is the common pathway for the metabolism of carbohydrates, fats, and proteins since it provides the complete oxidation of acetyl CoA to carbon dioxide and water. Acetyl CoA comes from all the three metabolism:

  1. Carbohydrate Glycolysis metabolism:
  2. Fat metabolism: β-oxidation
  3. Protein Transamination metabolism: 

Hence citric acid cycle is the common pathway for the metabolisms of carbohydrate, fat and protein.

Q.29 What is the inhibitor of aconitase step in Krebs’ cycle?

Aconitase, which converts citrate to isocitrate is inhibited by fluoroacetate.

Q.30 Which step of Krebs’ cycle is inhibited by arsenite?

Arsenite inhibits the α-keto glutarate dehydrogenase complex thus impeding the conversion of α-keto glutarate to succinyl-CoA.

Q.31 What is the inhibitor of succinate dehydrogenase?

Succinate dehydrogenase is competitively inhibited by malunate and oxaloacetate.

Q.32 What is the peculiarity of succinate dehydrogenase?

It is the only enzyme of the TCA cycle which is found to the inner mitochondrial membrane, unlike others which are present in the matrix of mitochondria.

Q.33 Enumerate the vitamins which play an important role in the TCA cycle?

  1. Riboflavin.
  2. Niacin.
  3. Thiamin.
  4. Pantothenic acid

Q.34 What is oxidative decarboxylation?

Oxidation accompanied by decarboxylation is called oxidative decarboxylation.

Q.35 What is the oxidative decarboxylation product of pyruvic acid?

Pyruvic acid-- (Oxidative decarboxylation—2H,—CO2)----Acetyl CoA.

Q.36 What is substrate-level phosphorylation?

When energy is liberated without entrance to the electron transport system, it is termed as substrate-level phosphorylation.

Q.37 When substrate-level phosphorylation occurs in the TCA cycle?

When succinyl CoA is converted into succinate. One ATP is liberated by substrate-level phosphorylation.

Q.38 Can TCA cycle function in absence of O2?

TCA cycle cannot function in the absence of O2.

Q.39 Where are the enzymes of the TCA cycle located?

Enzymes of the TCA cycle are located in the mitochondrial matrix, either free or attached to the inner surface of the inner mitochondrial membrane, which facilitates the transfer of reducing equivalents to the adjacent enzymes of ETC.

Q.40 Succinyl CoA is an intermediate in the TCA cycle. How it is formed?

Succinyl CoA is formed by oxidative decarboxylation of α-oxoglutarate by “α-oxoglutarate dehydrogenase complex” which requires TPP, lipoic acid, CoA-SH, FAD, NAD+, and Mg++ ions as coenzymes/ cofactors 

Q.41 State the inhibitors of the TCA cycle.

  • Fluoroacetate: Inhibitor of “aconitase” and allows citrate to accumulate.
  • Arsentie: inhibits “α-oxoglutarate dehydrogenase” enzyme complex and allows accumulation of α−oxoglutarate (α-keto glutarate)
  • Malonate/OAA: inhibits succinate dehydrogenase by competitive inhibition and allows accumulation of succinate.

Q.42 What is the importance of OAA in the TCA cycle?

  • It is required to start the cycle
  • A small quantity is necessary.
  • At the end of the cycle, OAA is regenerated by oxidation of malate by malate dehydrogenase.
  • Thus OAA acts catalytically to restart the cycle again.

Q.43 What will happen to the TCA cycle if OAA is not available?

In the absence of OAA, the TCA cycle will not operate. Acetyl CoA will accumulate and will be diverted to form ketone bodies and biosynthesis of FA and cholesterol.

Q.44 Why TCA cycle is said to be amphibolic in nature?

  • TCA cycle has a dual role:

Catalytic role:
The acetyl CoA produced by the metabolism of carbohydrates, lipids, and proteins are completely oxidized to produce CO2, H2O, and energy.

Anabolic role (Synthetic role):
Intermediates of the TCA cycle are utilized for the synthesis of various biologically important compounds in the body,
e.g.
– Synthesis of non-essential amino acid.
– Formation of glucose (gluconeogenesis)
– FA synthesis – Synthesis of cholesterol and steroids.
– Heme synthesis.

Q.45 What is the enzyme of the above oxidative decarboxylation reaction?

Enzyme involved is the pyruvate dehydrogenase complex. This enzyme complex consists of three enzymes:

  1. Pyruvate dehydrogenase.
  2. Dihydrolipoyl transacetylase.
  3. Dihydrolipoyl dehydrogenase.

Q.46 What are the cofactors of the above reaction?

  1. Thiamine pyrophosphate (TPP).
  2. Lipoic acid.
  3. Coenzyme A (CoA-SH).
  4. Flavin adenine dinucleotide (FAD).
  5. Nicotinamide adenine dinucleotide (NAD+).
  6. Mg++ ions.

Q.47 What is the active form of PDH?

“Active” forms of PDH is the dephosphorylated form.
Insulin stimulates phosphatase enzyme and converts ‘inactive’ → to ‘active’ form by dephosphorylation.

Q.48 What is the inactive form of the PDH enzyme?

“Inactive” form is the Phosphorylated form catalyzed by the enzyme “PDH kinase”.
Following favor the formation of “inactive” form:

  • Rise in ATP/ADP ratio
  • Rise in NADH/NAD+ ratio
  • Acetyl CoA/CoA. SH ratio
  • Increased cyclic AMP level in cells

Q.49 Pyruvic acid is formed in the cytosol by glycolysis but oxidative decarboxylation takes place in mitochondrion. Pyruvic acid is impermeable to the mitochondrial membrane. How it is done?

Pyruvic acid formed in the cytosol is not permeable to the mitochondrial membrane, it is transported to mitochondrion by a specific transport protein.

Q.50 What is the importance or metabolic significance of Hexose monophosphate shunt pathway?

  1. Provides NADPH which is used in fatty acid synthesis.
  2. Provides pentose sugar: which is the building block of nucleic acid.

Q.51 Acetyl CoA is formed inside mitochondria but the fatty acid synthesis (de Novo) from acetyl CoA occurs in the cytosol. Acetyl CoA is not permeable to the mitochondrial membrane. How acetyl CoA made available in the cytosol?

Acetyl CoA is transported out in the form of citrate, an intermediate of the TCA cycle, to cytosol, as citrate is readily permeable to the mitochondrial membrane.

In the cytosol, citrate is cleaved by the enzyme citrate cleavage enzyme (ATP-citrate lyase) to acetyl CoA and OAA, so that acetyl CoA can be used for FA synthesis.

Q.52 In glycolysis, NADH is produced in the cytosol, but it is oxidized in ETC in mitochondria to produce ATP. NADH is not permeable to the mitochondrial membrane. Explain how it is achieved?

NADH produced in the cytosol by glycolysis transfers the reducing equivalents through the mitochondrial membrane via substrate pairs linked by suitable dehydrogenases by “shuttle systems”. Two such shuttle systems are:

– Glycerophosphate shuttle.

– Malate shuttle.

Q.53 Show schematically glycerophosphate shuttle.

 Note: α-glycero-P-dehydrogenase in mitochondrion is Fp-dependent.
Hence produces 2 ATP per mole of glucose oxidized. Hence in this 36 ATP is produced per mole of glucose oxidized.

Q.54 Show schematically malate shuttle.

Note: Malate shuttle is commonly used by the body. Use of malate shuttle forms 38 ATP.

Q.55 Name the tissues where the hexose monophosphate shunt pathway is active?

HMP shunt pathway is active in the liver, adipose tissue, mammary gland, adrenal cortex, testis, etc.

Q.56 What are the clinical problems do G-6-PD deficient persons have to face?

The patients with G-6-PD deficiency when given antimalarials like primaquine, they develop hemolysis.

Q.57 Why is it so?

G-6-PD is responsible for the maintenance of glutathione in reduced state. Moreover, when primaquine is given it leads to the generation of more free radicals. These two factors contribute to hemolysis.

Q.58 Name two keto acids involved in carbohydrate metabolism.

  1. Pyruvic acid
  2. α-keto-glutaric acid

Q.59 Name two reactions catalyzed by kinases.

Reactions catalyzed by kinases:

  1. Glucose → Glucose -6-PO4 .
  2. Fructose-6-PO4 → Fructose-1, 6-diPO4.

 Q.60 Name two reactions catalyzed by mutases.

Reactions catalyzed by mutases:

  1. 3-Phosphoglycerate →2-Phosphoglycerate.
  2. Glucose-6-PO4 → Glucose 1-PO4.

Q.61 What is glycogenesis?

It is the formation of glycogen from glucose in the body.

Q.62 Show schematically the steps of glycogenesis.

 Q.63 What happens to liberated UDP by glycogen synthase action?

UDP is converted to UTP again by the enzyme nucleoside diphosphokinase and is reutilized again for UDPG formation.

Q.64 What is the primer? How the first primer formed?

The first “primer” originally supposed to be synthesized on a protein backbone which is a process similar to the synthesis of other glycoproteins.

A pre-existing glycogen molecule or “primer” is a must, so that UDPG can add the glucose molecule to the outer end of a chain.

Q.65 How much ATP is spent by the body to add one glucose unit to the outer chain?

  • In each addition of glucose unit, 2 ATP molecules are expended by the body: 

– One ATP is utilized in phosphorylation of glucose→ Glucose-6-P.

– Another ATP is used to convert UDP to UTP again.

Q.66 What is key and rate-limiting enzyme in glycogenesis?

Glycogen synthase (synthetase) enzyme

Q.67 What are the active and inactive forms of the enzyme glycogen synthase?

  • Glycogen synthase enzyme occurs as ‘active’ GS ‘α’ or ‘inactive’ GS ‘β’ forms and both are interconvertible.
  • GS ‘α’→GS ‘β’ by phosphorylation which is modulated by cyclic-AMP dependent protein kinase and glycogenesis is stopped.
  • GS ‘β’ → GS ‘α’ is formed by dephosphorylation catalyzed by the enzyme “protein phosphatase-1” when glycogenesis starts.

Q.68 What is UDPG? What are its functions?

UDPG is uridine diphosphate glucose. It is an intermediate in glycogenesis. It is activated glucose which adds one glucose unit to a chain in ‘primer’ molecule, by α 1 → 4 glycosidic linkage.

  • Other functions:
    – It is formed as an intermediate in the uronic acid pathway required for the formation of D-glucuronic acid.
    – It is also required for the synthesis of lactose from galactose in the lactating mammary gland.

Q.69 State the factors that bring about stimulation and inhibition of glycogenesis.

  • Stimulators:
    – Insulin
    – Glucocoriclids
    – High concentration of glucose.
  • Inhibitors:

– Increase concentration of glycogen (by “feedback” inhibition).
– Increased cyclic AMP in the cells, which can be brought about by hormones viz.,

  • Epinephrine,
  • Norepinephrine,
  • Glucagon,
  • Thyroid hormones

Q.70 How insulin increases glycogenesis?

Insulin directly stimulates the enzyme protein phosphatase-1 thus brings about dephosphorylation of glycogen synthetase and forms “active” glycogen synthase, GS ‘α’, and increases glycogenesis.

Q.71 How does increased cyclic AMP level in cells inhibits glycogenesis?

Increased cyclic AMP in the cells converts inactive protein kinase (C2R2) → active protein kinase (C2). 

Active protein kinase (C2) has the following two effects:

– Brings about phosphorylation of ‘GS’ enzyme with the help of ATP and thus GS ‘α’→ GS ‘β’ inhibiting glycogenesis.

– Also converts a protein factor inhibitor 1 (inactive) and phosphorylates it to form “active inhibitor 1-P”, which in turn inhibits protein phosphatase-1, so that conversion of inactive GS ‘β’→to active GS ‘α’ does not occur thus inhibiting glycogenesis.

Q.72 What do you know about branching and debranching enzymes?

Branching enzymes produce a branching in the molecule by establishing an α (1,6) glycosidic linkages. 
Debranching enzymes produce a cleavage of α-(1,6) linkages.

Q.73 What is glycogenolysis?

The breakdown of glycogen to glucose is called glycogenolysis.

Q.74 Which is the key and rate-limiting enzyme in glycogenolysis?

Phosphorylase enzyme.


Q.74.1 What are the active and inactive forms of liver phosphorylase?

Active phosphorylase is the phosphorylated form “active” phosphophosphorylase ‘α’.

Inactive phosphorylase is the dephosphorylated form “inactive” dephosphophosphorylase ‘β’.

Q.75 What are the basic differences between liver and muscle phosphorylase?

  • There is no cleavage of structure with liver phosphorylase as compared to muscle phosphorylase.
  • Four molecules of pyridoxal-P are required for the activity of muscle phosphorylase, not so with liver phosphorylase.
  • Muscle phosphorylase is not affected by glucagon (no receptor on muscle).

Q.76 Name the hormones which bring about glycogenolysis through increased cyclic AMP levels in cells.

  • Catecholamines-epinephrine and norepinephrine
  • Glucagon, and
  • Thyroid hormones

Q.77 What is the product of phosphorylase activity on glycogen molecule?

Active phosphorylase in presence of inorganic Pi brings about phosphorolytic cleavage of α 1→4 glycosidic bond from the outermost chain of the glycogen molecule and glucose is released as glucose-1-P and not free glucose.

Q.78 What is the fate of glucose-1-P released by phosphorylase activity.

Glucose-1-P is converted to glucose-6-P by the enzyme phosphoglucomutase.

In the liver and kidney (not in muscle), glucose-6-P is acted upon by the enzyme glucose-6-phosphatase and free glucose is formed.

Q.79 Why glucose is not formed in muscle from glycogen break-down?

In muscle, glucose-6-phosphatase enzyme is absent, hence glucose-6-P enters the glycolytic cycle and forms pyruvates and lactates.

Q.80 How does glucagon action differ from catecholamines in glycogenolysis?

Catecholamines cause the breakdown of the liver as well as muscle glycogen.

But glucagon breaks down only liver glycogen and not muscle glycogen (as receptor for glucagon is not present in muscle).

Q.81 What is calmodulin?

Calmodulin is a Ca++ dependent regulatory protein that is specific for calcium. It is a flexible protein, with 4 binding sites for Ca distributed in 4 domains.

Q.82 What is gluconeogenesis. Name few gluconeogenic substances.

The process of formation of glucose from non-carbohydrate substances is called gluconeogenesis.

The gluconeogenic substances are pyruvic acid, propionate, lactic acid, glycerol, and amino acids.

Q.83 Name the rate-limiting enzymes of gluconeogenesis.

Four rate-limiting enzymes are:

  • Pyruvate carboxylase (mitochondrial).
  • Phospho-enol pyruvate carboxykinase (cytosol).
  • Fructose-1, 6-biphosphatase (cytosol).
  • Glucose-6-phosphatase (cytosol)

Q.84 Name the irreversible steps in gluconeogenesis and enzymes used to circumvent the irreversible steps.

Irreversible steps
(Energy barrier)
 Enzymes used to circumvent
Pyruvate → Phosphoenol pyruvate Pyruvate carboxylase  (Mitochondrial)—conversion of PA to OAA by CO2 fixation reaction.
  Phosphoenol pyruvate carboxykinase (cytosol) converts OAA to Phosphoenol pyruvate
Fructose-1-5-bi-P → fructose-6-P (cytosol) Fructose-1, 6-bi-phosphatase (cytosol)
Glucose-6-P →glucose Glucose-6-phosphatase (cytosol)

 

Q.85 Name the tissues where gluconeogenesis occurs and name one disease and one condition in which gluconeogenesis is significantly enhanced.

  • Principally occurs in the liver (85%) and kidney (15%).
  • Uncontrolled diabetes mellitus and prolonged starvation.

 Q.86 State how glucose is formed from glycerol?

  • Glycerol is phosphorylated in presence of the enzyme “Glycerol kinase” and ATP to form α-Glycero P.
  • α-Glycero-P is converted to Di-OHacetone-P by dehydrogenase and NAD+.
  • Di-OH-acetone-P and glyceraldehyde-3- P forms fructose, 1-6, biphosphate which by reversal of glycolysis form glucose.

Q.87 Mention the sources of propionyl-CoA in humans.

  • From catabolism of L-methionine.
  • Catabolism of isoleucine.
  • Oxidation of odd-chain FA.
  • Synthesis of bile acids.
  • Non-oxidative deamination of L-threonine.

Q.88 Show schematically how glucose is formed from propionic acid.

 

Q.89 What is the role of hormones in gluconeogenesis?

Glucagon: Increases gluconeogenesis from lactic acid (LA) and amino acids.

Glucocorticoids: stimulate gluconeogenesis by increasing protein catabolism in the peripheral tissues and increasing hepatic uptake of amino acids. It increases the activity of transaminases and other key enzymes concerned in gluconeogenesis.

Q.90 What are the enzymes concerned with the reversal of glycolysis, i.e. gluconeogenesis?

  1. Pyruvate carboxylase.
  2. Phosphoenol pyruvate carboxykinase.
  3. Fructose 1,6 -diphosphatase.
  4. Glucose-6-phosphatase.

Q.91 How pyruvate is converted to glucose?

Pyruvate is converted to glucose by gluconeogenesis

Reaction is given as:

Pyruvate → Oxaloacetate
Oxaloacetate → Phosphoenol pyruvate
Phosphoenolpyruvate → Glucose

Q.92 What is the Cori cycle?

Lactic acid produced in the muscle reaches the liver through blood where it is converted to glucose by gluconeogenesis, which again becomes a source of energy for utilization. This process continues and is called the Cori cycle.

Q.93 Can muscle glycogen be the source of blood glucose?

Muscle glycogen cannot be a source of blood glucose, because it lacks an enzyme glucose 6-phosphatase.

Q.94 Give the breakdown products of glycogen.

Glycogen ——(Phosphorylase)--→ Glucose-1-PO4 

Glucose-I-PO4——(Mutase)—→ Glucose-6-PO4 

Glucose-6-PO4 ——G-6 Pase——→ Glucose

Q.95 Name the six classical types of GSDs and indicate the enzyme deficiencies.

Type/Name Enzyme deficiency
Type I-von Gierke’s disease Glucose-6-Pase
Type II-Pompe’s disease Acid maltase
Type III-Forbe’s (Limit dextrinosis) Debranching enzyme disease
Type IV-Andersen’s (Amylopectinosis) Branching enzyme. disease
Type V-McArdle’s disease Muscle phosphorylase
Type VI-Her’s disease Liver phosphorylase

 

Q.96 Explain von Gierke’s disease.

In von Gierke’s disease, there is a deficiency of Glucose-6-phosphatase.

Q.97 Explain Mc Ardle’s disease.

In Mc Ardle’s disease, there is a deficiency of muscle phosphorylase.