CARBOHYDRATE METABOLISMThe official spokesman of carbohydrate metabolism, ‘glucose’

speaks:“I burn myself to provide fuel to life!Generated through gluconeogenesis by my friends;Engaged in the synthesis of lipids, amino acids;Deranged in my duties due to diabetes mellitus.”

Glycolysis: Degradation of glucose to pyruvate (Lactate under anaerobic) generates 8 ATP.Citric acid cycle:The oxidation of acetyl CoA to CO2

Gluconeogenesis:The synthesis of glucose from non-carbohydrate precursors (amino acids, glycerol etc)Glycogenesis:The synthesis of glycogen from glucose 1Dr S Nayak

Glycogenolysis The breakdown of glycogen to glucose and then to lactate or pyruvate. Hexose monophosphate shunt: It is an alternative pathway to glycolysis and TCA cycle for the oxidation of glucose. Here the glucose is directly oxidised to CO2 and H2O.

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GLYCOLYSIS (Embden meyerhof pathway)

 Carbohydrates are the important energy source of the body.

  Glucose is the preferred source of energy for most of the body tissues.

  Brain cells derive energy mainly from glucose   Pyruvate is the end product of aerobic glycolysis   Lactate is the end product of anaerobic glycolysis Site: Cytoplasm Importance of the pathway Pathway takes place in all the cells of the body It is the Source of energy in erythrocytes

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Anaerobic glycolysis forms the major source of energy for muscle during exercise Provide carbon skeletons for the synthesis of non- essential amino acids. Most of the reactions of glycolysis are reversible The entry of glucose from ECF to cell (ICF) is under the control of insulin Glycolysis occurrence is the pre-requisite for the aerobic oxidation of carbohydrates Aerobic oxidation takes place in the cells possessing mitochondria. Glycolysis is the major pathway for ATP synthesis in tissues lacking mitochondria (erythrocytes)

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Steps of Glycolysis

Steps of Glycolysis 1.  IrreversibleGlucokinase Hexokinasea.Present in liver Present in all tissues b.Phosphorylation of Glu Phosphorylation of hexosesc.Low affinity for glucose High affinity for substrates d.Not inhibited by Glucose 6–P Inhibited by glucose 6- P•Glucose 6-P: Impermeable to the cell membrane. •Central molecule with a variety of metabolic fates; glycolysis, glycogenesis, gluconeogenesis and HMP shunt.

2. Reversible reaction3.PFK is an allosteric inducible key enzyme and the reaction is irreversible 4. Reversible, 6-carbon compound split into two 3 carbon compounds, both are reversibly convertible by an isomerase enzyme. Thus, two molecules of glyceraldehyde 3-phospahte are obtained from one molecule of glucose. Isomerase is inhibited by bromohydroxyacetone-phosphate 6Dr S Nayak

5. Reversible, end product contains a high-energy bond. Iodoacetate and arsenite inhibit the Glyceraldehyde 3- Phosphate dehydrogenase non-competitively6. The high energy of 1,3 bisphosphoglycerate is trapped to

synthesise one ATP molecule 7. Reversible reaction 8. Reversible reaction, Mg2+ act as activator Fluoride irreversibly inhibit this enzyme with the removal

of Mg2+

9. High energy of Phosphoenolpyruvate is trapped into ATP by the pyruvate kinase, irreversible reaction. Ends with pyruvate in the tissues with mitochondria (aerobic)

10. If anaerobic conditions prevail, the reoxidation of NADH formed in reaction 5 is by transfer of reducing equivalents through respiratory chain to oxygen is prevented and get reoxidised by conversion of pyruvate to lactate by LDHTissues that function under hypoxic conditions produce lactate e.g. Skeletal muscle, smooth muscle and erythrocytes

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Steps 5 and 10 are coupledGlycolysis is the major source of energy in anaerobiosisFor smooth operation of the pathway NADH is to be reconverted to NAD+.

The formation of lactate allows the regeneration of NAD+

which can be reused by glyceraldehyde 3-P Dh. So that glycolysis proceeds even in the absence of oxygen to supply ATP. Fate of pyruvate depends on the presence or absence oxygen in the cells.The occurrence of un-interrupted glycolysis is very important in skeletal muscle during strenuous exercise.Brain, retina, renal medulla and GI tract derive their energy from glycolysis. Glycolysis in the erythrocytes leads to lactate production, since the mitochondria, the centres for oxidation are absent

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Energetics of glycolysis: Energy consuming steps are 1 and 3 Hexokinase and phosphofructokinasecatalysed reactions = - 2 ATP Energy yielding steps are: Step 5Catalysed by Glyceraldehyde –3-PDHOxidative phosphorylation:NADH x 2 = 3ATP x 2 = 6 ATP Substrate level phosphorylation Steps 6 and 9Catalysed by Phosphoglycerate kinase & Pyruvate kinase 2ATP x 2 = 4 ATPTotal ATP in aerobic glycolysis =10 ATP – 2 ATP = 8 ATP/ glucose Anaerobic glycolysis: 1 and 3 = 2 ATP used Steps 6 and 9 2 ATP x 2 = 4 – 2 = 2 ATP

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                     Shuttle pathways:• If the cytosolic NADH uses malate-aspartate shuttle, 3 ATP are produced. If it uses Glycerol phosphate shuttle produces 2 ATP. Regulation• Insulin favours glycolysis by activating key glycolytic enzymes like glucokinase, phosphofructokinase(PFK) & pyruvate kinase • Glucose–6 P, inhibits hexokinase. This enzyme prevents the accumulation of glucose 6-phosphate.

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PFK-1 is an inducible enzyme that increases its synthesis in response to insulin and decreases in response to glucagon

Pyruvate kinase is an inducible enzyme that increases its synthesis in response to insulin and decreases in response to glucagon

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Role of Fructose 2, 6-bisphosphate: It is the most regulatory factor for controlling PFK and ultimately glycolysis in the liver.

The function of synthesis and degradation of F2, 6-BP is brought out by a single enzyme (with two active sites), which is referred to as bifunctional or Tandem enzyme. The activity of these enzymes is controlled by covalent

modification, which in turn regulated by Cyclic AMP. cAMP brings about the phosphorylation of the tandem enzyme, resulting in inactivation of active site

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Regulation by PFK-2

There will be no stimulation when Fru-2, 6 bisphosphate decreases, with low glucose the PFK1 remains inactive.

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Pasteur effectThe inhibition of glycolysis by oxygen When anaerobic yeast exposed to air, the glucose utilisation decreases.In the aerobic condition the levels of glycolytic intermediates from fructose 1, 6 bisphosphate onwards decreases while the earlier intermediates accumulate. The Pasteur effect is due to the inhibition of PFK. Citrate and ATP inhibition explains the Pasteur effect.

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Rapoport Leubering Cycle (BPG Shunt)

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The kinase reaction is bypassed in the erythrocytesNo energy is trapped from 2, 3 BPGThe BPG when combines with Hb, reduces the affinity of Hb towards oxygen. In the presence of 2, 3 BPG oxyhemoglobin unload oxygen more easily in tissues Therefore the 2, 3 BPG increases in hypoxic condition 15 to 25% of the lactate formed goes through this pathway In hexokinase deficiency phosphorylation does not takes place further. So 2,3 BPG decreases. Then affinity to hemoglobin increases

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                Fate of pyruvateUnder aerobic condition pyruvate is transported into mitochondria via pyruvate transporter

Pyruvate dehydrogenase Complex Pyruvate Acetyl CoA NAD+ NADH+ H+

TPP, FAD, Lipoic acid, CoASHLack of TPP leads to accumulation of pyruvate In thiamine deficient alcoholics, pyruvate converted to lactate and it leads to lactic acidosis

Lactic acidosis is caused by:Inherited deficiency of PDH Inability to reoxidise NADH in the electron transport chainExcessive NADH production, e.g., ethanol intoxication Pyruvate dehydrogenase inhibited by arsenic and mercuric ions 17Dr S Nayak

Ref: Essentials of Biochemistry, Dr S Nayak18Dr S Nayak