Dr. Basima Sadiq jaff Ph D in clinical biochemistry

DIGESTION, ABSORPTION AND TRANSPORT OF CARBOHYDRATES

Koelle, lec15, p25

Glycolysis - Enzyme mechanisms

4. Regulating a metabolic pathway Maintaining homeostasis C. Accumulating certain metabolites can have severe consequences

Figure 12.2: Transport of glucose, fructose, galactose and mannose

Figure 6-3

Protein or peptide hormone

Almost always proteins called kinases

Activation/inactivation of an enzyme; opening/closing a membrane channel; activating a transcription factor

Plasma fatty acids, ketoacids & amino acids

fatty acid & amino acid uptake

Anabolic hormone

Glycolysis

Glycogen synth.

Lipogenesis

Protein synth.

Catabolic hormone

METABOLIC FATE OF CARBOHYDRATES The major metabolic pathways of carbohydrates are : • Glycolysis:The oxidation of glucose to pyruvate and lactate. • Citric acid cycle:(Krebs cycle or tricarboxylic acid cycle) oxidation of acetyl-CoA to CO2 and water. • Gluconeogenesis:Synthesis of glucose from noncarbohydrate substances such as lactate, glycerol, glucogenic amino acids, etc. • Glycogenesis:Synthesis of glycogen from glucose. • Glycogenolysis:Breakdown of glycogen to glucose. • Hexose monophosphate Shunt(HMP Shunt): It is an alternative pathway for oxidation of glucose. Some pentoses can also be oxidized through this pathway. • Uronic acid pathway:Glucose is oxidized to glucuronic acid. • Galactose metabolism:Galactose is converted to glucose. • Fructose metabolism:Fructose is converted to glucose or metabolized in liver.

Glycolysis is regulated at 3 steps which are irreversible. These reactions are catalyzed by: 1. Hexokinase and glucokinase 2. Phosphofructokinase-I 3. Pyruvate kinase.

• Phosphofructokinase-l is activated by: – Fructose-6-phosphate (substrate) – AMP (which signals low energy state) – Fructose 2,6-bisphosphate. • Phosphofructokinase-I is inhibited by citrate,c-AMP and ATP (which signals high energy state slowing down the glycolysis). • Phosphofructokinase-I is an inducible enzyme that increases its synthesis in response to insulin and decreases in response to glucagon.

ALTERNATE FATES OF PYRUVATE A. Oxidative decarboxylation of pyruvate B. Carboxylation of pyruvate to oxaloacetate C. Reduction of pyruvate to ethanol (microorganisms)

Figure 8.24 Summary of the metabolic fates of pyruvate.

*(PDH complex) is a multimolecular aggregate of three enzymes, pyruvate dehydrogenase (PDHor E1, also called a decarboxylase), dihydrolipoyl transacetylase(E2), and dihydrolipoyl dehydrogenase(E3).

*Coenzymes:The PDH complex contains five coenzymes that act as carriers or oxidants for the intermediates of the reactions E1 requires thiamine pyro phos phate (TPP), E2 requires lipoic acid and CoA. E3 requires FAD and NAD+ .

• Pyruvate kinase is an inducible enzyme that

increases in concentration with high insulin levels and decreases with glucagon • It is activated by fructose-1, 6-bisphosphate and inactivated by ATP.

Case History An obese person came to the hospital with complaintsof polyuria, thirst, weakness and increased appetite.On investigations, he was diagnosed having diabetes mellitus. Questions a. What is the cause of diabetes mellitus? b. Give names of different types of diabetes mellitus. c. What is glucosuria? Name different types of glucosuria. d. What is the normal blood sugar level?

Case history At a village fete, a local charity group was fund raising by performing certain sideroom tests. An 8-year-old boy was found to have a blood glucose of 14.4 mmol/L. His family was concerned, and an hour later his cousin, a recently diagnosed diabetic, confirmed the hyperglycaemia with his home monitoring equipment, and found glycosuria +++. What is the significance of these findings?

CONVERSION OF PYRUVATE TO ACETYL-CoA • Pyruvate is converted to acetyl CoA by oxidative decarboxylation. This step occurs only in mitochondria. This is an irreversible reaction catalyzed by a multienzyme complex known as pyruvate dehydrogenase complex (PDH) (Figure 12.5). • The enzyme pyruvate dehydrogenase requires five coenzymes, namely thiamine pyrophosphate (TPP), lipoate, coenzyme-A, FAD and NAD+.

Citric acid cycle

Production of ATP in citric acid cycle

Amphibolic role of the citric acid cycle

Pathway of gluconeogenesis

Cori cycle or lactic acid cycle and glucose alanine cycle. The pathway of Cori cycle is shown in green and glucose alanine cycle in blue

Because muscle is incapable of synthesizing urea, most of the ammonia formed by protein catabolism is transferred to pyruvate to form alanine by transamination reaction. • Alanine enters the blood and is taken up by the liver. • In the liver, the amino groups of alanin is removed to form urea, and the resulting pyruvate is converted to glucose by gluconeogenesis which is then transported to the muscle, where it is oxidized to pyruvate. • The pyruvate acts again as the acceptor for another amino group.

Glycogenesis& Glycogeniolysis

Schematic representation of glycogenesis (mechanism of branching)

Schematic representation of glycogenolysis

(mechanism of debranching)

-During muscle contration, calcium is released from the sarcoplasmic reituclum. -It binds with calmodulin-dependent protein kinase, activating it without phosphorylation. -Protein kinase can then activate glycogen phosphorylase, causing glycogen degradation. Increased calcium activates calmodulin-dependent protein kinase with makes glycogen synthase inactive so glycogenolysis will be active, breaking down glycogen and releasing glucose to the muscle for use. This happens during exercise.

Hormonal regulation of glycogenolysis

Allosteric regulation of glycogenesis and glycogenolysis

Glycogen storage diseases

1-Case History A chronically cranky, irritable and lethargic baby girl has an extended abdomen, resulting from an enlarged liver and was diagnosed of having Von Gierke’s disease. Questions a. Which enzyme is deficient in Von Gierke’s disease? b. Name the pathways where the enzyme is required. c. Give manifestations of the disorder. 2-Case History A 28-year-old man has complained of chronic leg muscle pains and cramps during exercise. This patient suffers from McArdle syndrome. Questions a. What is McArdle syndrome? To metabolism of which biomolecule is it related? b. What is the cause of this syndrome? c. Name different types of disorders related to the concerned biomolecule

Pentose phosphate pathway, where, TPP: Thiamine pyrophosphate

1-Case History A chronically cranky, irritable and lethargic baby girl has an extended abdomen, resulting from an enlarged liver and was diagnosed of having Von Gierke’s disease. Questions a. Which enzyme is deficient in Von Gierke’s disease? b. Name the pathways where the enzyme is required. c. Give manifestations of the disorder. 2-Case History A 28-year-old man has complained of chronic leg muscle pains and cramps during exercise. This patient suffers from McArdle syndrome. Questions a. What is McArdle syndrome? To metabolism of which biomolecule is it related? b. What is the cause of this syndrome? c. Name different types of disorders related to the concerned biomolecule

Oxidative non-reversible phase

-generates NAPDH

-Glucose 6-p undergoes dehydrogenation and decarboxylation to give a pentose, ribulose 5-p, which is converted to its isomer, D-ribose 5-p.

-

Non-oxidative reversible phase

-ribose 5‐P is converted back to Glucose 6-p by a series of reactions involving especially two enzymes

1. Transketolase :Transfer of the 2‐C fragment

2. Transaldolase :Transfer of the 3‐C fragment

Significance of Pentose Phosphate Pathway • The pentoses (ribose-5-phosphate) required for the biosynthesis of nucteotide and nucleic acids . • It provides a route for the interconversion of pentoses and hexoses. •NADPH is required for the biosynthesis of fatty acids, cholesterol, steroid hormones and neurotransmitters. • It is required for oxidation-reduction reactions involved in detoxification, e.g.for detoxification of drugs by microsomal cytochrome P450mono-oxygenaseand for reduction of oxidized glutathione. • In RBC, NADPH is required to maintain the level of reduced glutathione.The reduced glutathione protects the RBC membrane from toxic effect of H2O2. • NADPH is necessary for phagocytosis carried out by white blood cell.

Regulation of Pentose Phosphate Pathway • The first step in the pathway, catalyzed by glucose-6-phosphate dehydrogenase (G-6-PD) is the rate limiting step. • The activity of this enzyme is regulated by cellular concentration of NADPH. NADPH is a competitive inhibitor of the enzyme G-6-PD. • An increased concentration of NADPH decreases the activity of G-6-PD, for example: • Under well-fed condition, the level of NADPH decreases and pentose phosphate pathway is stimulated. • in starvation and diabetes, the level of NADPH is high and inhibits the pathway. • Insulin , It enhances the pathway by inducing the enzyme G-6-PD and 6-phosphogluconolactone dehydrogenase.

Table 12.5: Difference between glycolysis and pentose phosphate pathway

Figure 12.17: Role of NADPH in disposal of H2O2

It includes oxidation of glucose to It is an alternative oxidative pathway for glucose that doesn’t lead to ATP generation. 1. Glucuronic acid 2. Ascorbic acid It occurs mainly in the liver cytoplasm.

Significance of Uronic Acid Pathway • The uronic acid pathway is a source of UDPglucuronate. • UDP-glucuronate is a precursor in biosynthesis of proteoglycans (glycosaminoglycans) and glycoproteins. • UDP-glucuronate is involved in detoxification reactions that occur in liver. Many naturally occurring waste substances (like bilirubin and steroid hormones) as well as many drugs (like morphine, methanol, salicylic acid, etc.) are eliminated from the body by conjugating with UDP-glucuronate. • The uronic acid pathway is a source of UDP-glucose, which is used for glycogen formation. • The uronic acid pathway provides a mechanism by which dietary D-xylulose can enter the central metabolic pathway.

Conjugation to less polar compounds as bilirubin, steroids and some drugs making them more water soluble (detoxicated) .

Synthesis of glycosaminoglycans (mucopolysaccharide) as heparin, hyaluronic acid.

In plants and some animals (not Human) glucuronic acid serves as a precursor of L-ascorbic acid.

The uronic acid pathway also provides a mechanism by which dietary D-xylulose enter the central pathway.

Figure 12.19: Metabolic significance of UDP-glucuronate

Figure 12.21: Metabolic pathway of fructose and galactose

Figure 12.20: Pathway for conversion of galactose to glucose in the liver

The Effect of Hyperglycemia on Sorbitol Metabolism Because insulin is not required for entry of glucose into the cells such as retina, lens and nerve, large amounts of glucose may enter these cells during hyperglycemia, e.g. in uncontrolled diabetes. • Elevated intracellular glucose concentrations cause increase in the amount of sorbitol and therefore accumulates inside the cell. • This causes osmotic damage, leading to cataract formation, peripheral neuropathy, retinopathy and nephropathy. Drugs inhibiting aldose reductase improve the peripheral nerve function in diabetes.

Figure 12.22: Sorbitol or polyol pathway

Figure 12.24: Role of insulin in regulation of blood glucose level

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