Carbohydrate Digestion: Mouth

• Dietary carbohydrates mainly include polysaccharides (starch, glycogen), disaccharides (sucrose, lactose, maltose), and small amounts of monosaccharides like fructose and pentoses.
• Liquid foods bypass mouth digestion due to swallowing, while solids undergo thorough mastication.

Digestion in the Mouth

• Digestion begins in the mouth with saliva contact during chewing.
• Saliva contains salivary amylase (ptyalin), a carbohydrate-splitting enzyme.

Action of Salivary Amylase (Ptyalin)

• Ptyalin is an α-amylase that requires chloride ions for activation, with an optimal pH of 6.7.
• It hydrolyzes α-1→4 glycosidic linkages in polysaccharides like starch and glycogen, producing maltose, glucose, and maltotriose.
• Maltase III and maltase IV also exhibit sucrose activity.

Absorption of Carbohydrates

• Carbohydrate digestion completes in the small intestine, converting complex carbohydrates to monosaccharides.
• All monosaccharides are almost entirely absorbed from the small intestine.
• Glucose and galactose are absorbed rapidly, fructose and mannose at an intermediate rate, and pentoses slowly; galactose is absorbed faster than glucose.

Metabolic Pathways in Carbohydrates: Glycolysis

• Glycolysis, also known as the Embden–Meyerhof–Parnas Pathway, either begins or ends with glucose.
• Glucose is a major metabolic energy source and the primary carbohydrate form presented to body cells.
• It serves as the primary fuel for the brain and a significant energy source for specialized cells.
• Defects in glucose metabolism contribute to obesity and diabetes.
• Glycolysis is a sequence of 10 enzymatic reactions converting one glucose molecule into two pyruvate molecules, generating 2 ATP.
• This process provides free energy and prepares glucose for oxidative degradation.
• Glycolysis involves breaking down glucose to pyruvate, using released free energy to synthesize ATP from ADP and Pi.

Stages of Glycolysis

• Glycolysis consists of two stages: energy investment and energy recovery.
• Glucose enters the blood via glycogenolysis or gluconeogenesis and is transported into cells by specific carriers.
• Glycolysis enzymes are in the cytosol and convert glucose to two C3 units of pyruvate, harvesting energy to synthesize ATP.

Energy Investment (Stage I)

• Involves phosphorylation and cleavage of glucose to yield two glyceraldehyde-3-phosphate molecules, consuming 2 ATP.

Energy Recovery (Stage II)

• Converts two glyceraldehyde-3-phosphate molecules to pyruvate, generating 4 ATP.
• Glycolysis yields a net of 2 ATP per glucose molecule.

Significance of Glycolysis

• It occurs in all body cells, with enzymes in the cytosolic fraction.
• Glycolysis is the only energy source in erythrocytes.
• It can occur without (anaerobic) or with oxygen (aerobic), producing lactate or pyruvate, respectively.
• The pathway is an emergency energy source in the absence of oxygen.
• Glycolysis is a prerequisite for aerobic carbohydrate oxidation in cells with mitochondria.
• It is a major ATP synthesis pathway in tissues lacking mitochondria, such as erythrocytes, cornea, and lens.
• Glycolysis serves as a preliminary step before complete oxidation.
• Significant for ATP production in tissues with few mitochondria like testes, leukocytes, and kidney medulla.
• Provides carbon skeletons for synthesizing non-essential amino acids and the glycerol component of fat and most reactions are reversible, aiding gluconeogenesis.

Reactions of Glycolysis: Three Phases

Energy Investment Phase (Priming)

• Glucose is phosphorylated to glucose-6-phosphate by hexokinase or glucokinase, utilizing ATP.
• Hexokinase is inhibited by glucose-6-phosphate and found in most tissues.
• Glucokinase is present in the liver, phosphorylates only glucose, and is influenced by insulin.
• Phosphorylation of glucose traps it within cells, requiring it to be metabolized.
• Glucose-6-phosphate is isomerized to fructose-6-phosphate by phosphohexose isomerase, a reversible step.
• Fructose-6-phosphate is then phosphorylated to fructose-1,6-bisphosphate by phosphofructokinase (PFK), an inducible, regulatory enzyme.
• PFK is a key enzyme, and its irreversible step is the rate-limiting reaction in glycolysis; bypassed by fructose-1,6-bisphosphatase during gluconeogenesis.

Splitting Phase

• The 6-carbon fructose-1,6-bisphosphate is cleaved into glyceraldehyde-3-phosphate and dihydroxyacetone phosphate (DHAP) by aldolase, a reversible reaction.
• Dihydroxyacetone phosphate is isomerized to glyceraldehyde-3-phosphate by phosphotriose isomerase.
• Glucose is now cleaved into 2 molecules of glyceraldehyde-3-phosphate.
• The steps are called the splitting phase.

Energy Generation Phase

• Glyceraldehyde-3-phosphate is dehydrogenated and simultaneously phosphorylated to 1,3-bisphosphoglycerate with the help of NAD⁺ by glyceraldehyde-3-phosphate dehydrogenase.
• The product contains a high-energy bond, and the reaction is reversible.
• The enzyme brings about both oxidation, where the hydrogens are transferred to NAD⁺ and phosphorylation of the substrate adding one phosphate moiety.
• Phosphoglycerate kinase acts on 1,3-bisphosphoglycerate, synthesizing ATP and forming 3-phosphoglycerate through substrate-level phosphorylation.
• 3-phosphoglycerate is isomerized to 2-phosphoglycerate by phosphoglucomutase, shifting the phosphate group.
• 2-phosphoglycerate is converted to phosphoenolpyruvate by enolase, removing a water molecule.
• Enolase requires Mg²⁺ and is irreversibly inhibited by fluoride, stopping glycolysis; fluoride is added to blood samples for sugar estimation.
• Phosphoenolpyruvate (PEP) is dephosphorylated to pyruvate by pyruvate kinase, generating ATP.
• Pyruvate kinase is a key glycolytic enzyme and this step is irreversible.
• In anaerobic conditions, pyruvate is reduced to lactate by lactate dehydrogenase (LDH).
• Under aerobic conditions, pyruvate enters the citric acid cycle for complete oxidation.
• The end product of anaerobic glycolysis is lactate.

Cori's Cycle (Lactic Acid Cycle)

• Glucose is converted to lactate in muscle, then reconverted to glucose in the liver.
• Pyruvate is reduced to lactic acid in actively contracting muscle, potentially leading to muscle cramps.
• Lactic acid from muscle diffuses into the blood, reaches the liver, and is oxidized to pyruvate.
• Pyruvate is then channeled into gluconeogenesis and the regenerated glucose enters the blood and then the muscle.

Significance of Cori's Cycle

• The lactate produced in the muscle is efficiently reutilized by the body.
• During exercise, high lactate production is utilized by the liver to produce glucose, requiring significant ATP and increasing oxygen consumption, explaining the oxygen debt after vigorous exercise.

Regulatory Enzymes of Glycolysis

• These include glucokinase/hexokinase, phosphofructokinase, and pyruvate kinase.

Glucokinase/Hexokinase

• Phosphorylate glucose, regulated by feedback inhibition (hexokinase by glucose-6-phosphate) and activation by insulin (glucokinase).
• Glucokinase in the liver has a high Km for glucose and acts when glucose supply is adequate for storage.
• Hexokinase, with low Km, phosphorylates glucose even at lower concentrations, supplying the brain, cardiac, and skeletal muscle; it makes glucose available to the brain and muscles when the supply is limited.

Phosphofructokinase (PFK)

• It is the most important rate-limiting enzyme, inhibited by ATP and citrate, and activated by AMP.

Pyruvate Kinase

• Catalyzes an irreversible step and is a regulatory enzyme; glycolysis is inhibited when energy is plentiful.
• Insulin increases and glucagon inhibits its activity; pyruvate kinase is inactive in the phosphorylated state.

Metabolic Fate of Pyruvate

• In aerobic conditions, pyruvate is converted to acetyl CoA and enters the TCA cycle, generating ATP.
• Pyruvate is generated and transported into the mitochondria by a pyruvate transporter.

Pyruvate Dehydrogenase Complex

• Inside mitochondria, pyruvate is oxidatively decarboxylated to acetyl CoA by pyruvate dehydrogenase (PDH).
• PDH requires five co-enzymes (TPP, CoA, FAD, NAD⁺, Lipoamide) and three apo-enzymes.

Pyruvate Dehydrogenase Complex Enzymes

• Pyruvate Dehydrogenase (Enzyme 1): Catalyzes oxidative decarboxylation, requiring TPP; thiamine is essential for pyruvate utilization.
• Dihydro Lipoyl Trans Acetylase (Enzyme 2): Oxidizes the hydroxyethyl group to form an acetyl group and transfers it from TPP to lipoamide.
• Dihydro Lipoyl Dehydrogenase (Enzyme 3): Oxidizes lipoamide, regenerating TPP, Lipoamide, and FAD.

Fate of Pyruvic Acid

• Under aerobic conditions, complete oxidation of pyruvate carbon atoms to CO₂ is mediated by the citric acid cycle and in anaerobic metabolism, pyruvate is metabolized to a lesser extent to regenerate NAD⁺
• Pyruvic acid can be aminated to form alanine, converted to glucose during gluconeogenesis, or converted to malic acid, vital for gluconeogenesis.
• It can also be converted directly to oxaloacetic acid by CO₂ fixation.

Metabolic Fate of Pyruvate: Fermentation

Homolactic Fermentation

• In muscle, during vigorous activity with high ATP demand and low oxygen.
• ATP is synthesized largely via anaerobic glycolysis.
• Lactate dehydrogenase (LDH) catalyzes NADH oxidation by pyruvate, yielding NAD+ and lactate.
• The lactate dehydrogenase reaction is freely reversible.
• During pyruvate reduction by LDH, a hydride ion is transferred from C4 of NADH to C2 of pyruvate.
• Lactate can be either exported from the cell or converted back to pyruvate. Lactate does get sent from muscles to the liver where it is used to synthesize glucose.
• High concentrations of lactate doesn't cause muscle fatigue and soreness, but the accumulation of glycolytically generated acid does.

Alcoholic Fermentation

• Under anaerobic conditions in yeast, and NAD+ for glycolysis is regenerated.
• The conversion of pyruvate to ethanol and CO₂ is catalyzed by yeast; not present in animals.
• Acetaldehyde is reduced to ethanol by NADH, catalyzed by alcohol dehydrogenase, regenerating NAD+ for the GAPDH of glycolysis.