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Learning Outcomes

• Structures and functions of carbohydrates can be described.
• How dietary carbohydrates are digested and absorbed can be described.
• Able to explain why cellulose is indigestible in the human gastrointestinal tract.
• The glucose-dependency of some tissues can be described.
• The key features of glycolysis can be described.
• The biochemical basis of the clinical conditions of lactose intolerance can be explained.
• The metabolic requirements for lactic acid/lactate generation can be understood.
• How the blood concentration of lactate is controlled can be explained.

Lecture outline

• Overview of catabolism.
• Carbohydrates.
• Stage 1 of carbohydrate catabolism, including lactose intolerance.
• Stage 2 of carbohydrate catabolism, including glycolysis and anaerobic glycolysis and lactic acid production.

Overview of Catabolism steps

• Dietary macronutrients are broken down to cellular fuel molecules during stage 1.
• Fuel molecules are transformed to metabolic intermediates during stage 2, reducing power and releasing some energy.
• The tricarboxylic acid cycle (TCA), also called the Krebs cycle or citric acid cycle (reducing power and some energy release) occurs at stage 3.
• Stage 4 is oxidative phosphorylation, converting reducing power into ATP.

Key Points About Metabolism

• Metabolism involves a series of sequential reactions in defined metabolic pathways.
• Catabolic pathways involve breakdown.
• Anabolic pathways are synthetic.
• Catabolism involves the breakdown of chemicals to release organic precursors, reducing power, and energy (ATP).

Catabolism Stage 1

• Catabolism stage 1 happens in the extracellular (GI tract).
• Carbohydrates, fats, and proteins are digested to monosaccharides, fatty acids and glycerol, and amino acids, respectively.
• During catabolism stage 1, fuel molecules are absorbed from the GI tract into circulation, but no energy is produced

Catabolism Stage 2

• Catabolism Stage 2 happens in the intracellular (cytosol & mitochondria).
• Fuel molecules are transported to tissues and are converted into various metabolites.
• Catabolism stage 2 is oxidative, so it requires H+ carriers, which are then reduced (e.g. NAD+ → NADH+ + H+), reducing power is released, and some energy as ATP is produced.

Catabolism Stage 3

• Catabolism stage 3 is known as the TCA cycle (Krebs or citric acid cycle)
• Is an Intracellular process (mitochondria)
• Acetyl CoA is oxidised to CO₂, requires NAD⁺, and FAD reducing power is released, and some energy (as GTP=ATP) is produced

Catabolism Stage 4

• Catabolism stage 4 is Electron transport and ATP synthesis also called (Oxidative Phosphorylation.
• Is an Intracellular process (mitochondria)
• NADH⁺ + H⁺ & FADH₂ is re-oxidised.
• O₂ is required (reduced to H₂O).
• Free energy from electron transport is used to synthesise large amounts of energy (ATP).

Summary of Catabolic Metabolism

• Amino Acids are turned into NH3 then Keto-acids which turns into Acetyl CoA and Urea.
• Glucose turns directly into Pyruvate then Acetyl CoA.
• Fatty Acids turn directly into Acetyl CoA.
• Alcohol turns directly into Acetyl CoA.

Body Composition and Dietary Intake

• Carbohydrate make up 1% of 70-kg males
• Carbohydrates make up 1% of 55-kg females
• 15% of overall dietary intake is Carbohydrates.
• Lipids make up 16% of 70-kg males
• Lipids make up 25% of 55-kg females
• 8% of overall dietary intake is Lipids.
• Proteins make up 16% of 70-kg males
• Proteins make up 15% of 55-kg females.
• 5% of overall dietary intake is Protein.

What are Carbohydrates?

• Carbohydrates have the General formula: (CH2O)n
• Carbohydrates contain aldehyde: (-CHO) C=O or keto: (-C=O) group.
• Carbohydrates contain many -OH groups and are therefore hydrophilic. Because they are hydrophilic thay do not pass accross cell membranes without help
• They are also Partially oxidised and therefore Need less oxygen than fatty acids for complete oxidation

Monosaccharides

• Monosaccharides are Single sugar units (3-9 C atoms)
• Examples of Triose with 3 carbons is glyceraldehyde.
• Example of Pentose with 5 carbons is ribose.
• Examples of Hexose with 6 carbons are glucose, fructose, galactose.
• They are aldehyde-containing sugars (aldoses) e.g. glucose, galactose
• They are keto-containing sugars (ketoses) e.g. fructose

3-Carbon Monosaccharides

• 3-Carbon Monosaccharides are (Trioses) (CH2O)3
• Asymmetric C-atom → stereoisomers
• Naturally occurring forms → D isomers

D-Glucose vs L-Glucose

• D-glucose and L-glucose are isomers of glucose that are mirror images of each other.
• D-glucose is the naturally occurring form of glucose.
• L-glucose is a synthetic form of glucose.

5 or > 5 Carbon Monosaccharides

• 5 or > 5 Carbon Monosaccharides Exist mainly as ring structures
• Carbonyl groups react with alcohol group to form a ring

α- and β- D- Glucose

• The position of OH group on C1 determines whether D-glucose has α- or β- structure
• A glucose solution contains about one third of α-D glucose, two thirds of β-D-glucose and a very small amount of the straight chain D-glucose.

Polymers of Monosaccharides

• Disaccharides have 2 monosaccharides, like lactose (galactose & glucose), sucrose (fructose & glucose), and maltose (glucose & glucose).
• Oligosaccharides have 3-10 monosaccharides.
• An example of Oligosaccharides is Dextrins.
• Polysaccharides have 10 – 1000 monosaccharides, such as glycogen, starch, and cellulose (polymers of glucose monomers).

Formation of Polymers of Monosaccharides

• Polymers of Monosaccharides are Formed by condensation of monosaccharides.
• During polymer formation H₂O is eliminated and an -O-glycosidic bond is formed.

α- and β- Glycosidic Bonds

• 1,4 α: OH group was below C1
• 1,4 β: OH group was above C1

Polysaccharides facts

• Glycogen is a Polymer of glucose found in animal and a Major store of glucose in mammals (liver, skeletal muscle)
• It has α1-4 and α1-6 glycosidic bonds and is Highly branched
• Starch is a Polymer of glucose found in plants
• It is Mixture of amylose (α1-4 bonds) and amylopectin (α1-4 and α1-6 glycosidic bonds)
• Starch is Less branched than glycogen and has GI tract enzymes release glucose and maltose
• Cellulose is a Structural polymer of glucose in plants and has β1-4 glycosidic bonds
• There are No Gl enzymes to digest β1-4 bonds in cellulose, so It forms dietary fibre that is important for GI function

Polysaccharides - Glycogen

• Glycogen comprises a core protein of glycogenin surrounded by glucose units branches.
• The globular granule of glycogen may contain around 30,000 glucose units.

Catabolism of Carbohydrates: Stage 1

• Catabolism of Carbohydrates: Stage 1 happens in the Extracellular (the Gl tract)
• Polysaccharides are broken down to monosaccharides
• Monosaccharides are absorbed from Gl to blood

Digestion of Dietary Carbohydrates

• Mouth: Salivary α-amylase (α1-4 bonds) breaks down Starch and Glycogen dextrins and disaccharides
• Pancreas: Pancreatic α-amylase (α1-4 bonds)
• Small intestine: Pancreatic amylase (α1-4 bonds)
• Dextrins disaccharides
• Disaccharidases are attached to brush border of epithelial cells (enterocytes) and these comprise lactase (lactose) (β1-4 bonds), sucrase (sucrose) (α1-2 bonds), maltase (maltose) (α1-4 bonds) and isomaltase (α1-6 bonds)

Small Intestine

• The brush border is made up apical membrane extensions of epithelial cells covering villi.

Monosaccharide Transport

• Glucose, galactose and fructose are transported to enterocytes by facilitated or active transport. this happens thanks to GLUT2 (glucose transporter type 2), SGLT1 (Na+/glucose/galactose cotransporter) and GLUT5 (fructose transporter type 5).
• They are transported from enterocytes to the blood by GLUT2.
• They are then transported to target tissues via various transporters (GLUT1-14).

Lactose Intolerance

• When undigested, lactose is passed to the large intestine so Colonic bacteria ferment lactose which produces organic acids and gases
• Lactose and organic acids increase osmotic pressure and draw in water thus causing diarrhoea
• Gases cause abdominal cramps and bloating

Lactose Intolerance - Causes

• Lactose Intolerance is caused by Loss/reduction of lactase activity, meaning lactose is not hydrolysed to glucose and galactose.
• Other causes are genetic e.g Lactase activity is high in infants but decreases in childhood in most populations (especially African and Asian). Or Nongenetic e.g. injury to the small intestine.
• NOT is an allergic reaction to lactose (no immune system involvement)

Lactose intolerance Symptoms

• abdominal pain, discomfort, bloating, diarrhea, nausea; appear in 30 to 120 minutes following consumption of lactose

Lactose intolerance Diagnosis

• Positive hydrogen breath test or a Positive stool acidity test

Lactose intolerance Management

• Decrease or elimination of the amount of lactose in diet (lactose free diet)
• Consumption of lactase-treated foods or lactase supplements

Glucose Requirements of Tissues

• Around 180g of glucose is needed per day.
• Glucose is the major fuel molecule and is metabolised by all tissues and Blood glucose is regulated (~5 mM)
• some tissues (RBC, WBC, kidney medulla, testes, lens and cornea of the eye) have an total requirement for glucose as uptake by these tissues depends on its concentration in blood at approximately 40g/day.
• The CNS (brain) prefers is approximately 140g/day.
• Some tissues need it for special functions (liver, adipose)

Catabolism of Carbohydrates: Stage 2- Glycolysis

• Glycolysis Is an Intracellular process (cytosol) which Occurs in all tissue types and is Oxidative

10- Step Glycolysis & its Phases

• Phase 1 is the Preparative ATP-consuming phase of glycolysis reactions 1 to 3. This phases uses 2 moles of ATP per 1 mole of glucose
• Phase 2 of the 10-Step Glycolysis is the ATP-generating phase which occurs in reactions 4 to 10. 4 moles of ATP per mole of glucose are synthesised.

Phase 1 of Glycolysis (Reactions 1 - 3)

• In reaction 1, Phosphorylation of glucose to glucose-6- phosphate (G-6-P) is carried out by hexokinase (glucokinase in liver). This Prevents glucose from going back through the plasma membrane and it increases' the reactivity of glucose to permit subsequent steps.
• In reaction 2, the Isomerisation of G-6-P to fructose-6-phosphate (F-6-P) by phosphoglucose isomerase occurs.
• Reaction 3: Phosphorylation of F-6-P to fructose-1,6- bis phosphate (F-1,6-bis P occurs by phosphofructokinase-1 and is a 'committing step as it is the firs step that commits glucose to glycolysi.
• Reactions 1 and 3 have -ve AG and are irreversible

Phase 2 of Glycolysis (Reactions 4 - 6)

• Reaction 4: Cleavage of (F-1,6-bis P) into two C₃ units by aldolase, forming -DHAP (dihydroxyacetone phosphate) and glyceraldehyde 3- phosphate (G-3-P)
• Reaction 5: DHAP is rapidly converted to G-3-P by triose phosphate isomerase
• Therefore 2 moles of G-3-P pass through the rest of glycolysis
• Reaction 6: REDOX reaction, an Oxidation which occurs due to aldehyde group of G-3-P to carboxyl group resulting in addition of inorganic phosphate forming 1,3-bis phosphoglycerate (1,3-BPG) catalysed by G-3-P dehydrogenase and a Reduction of NAD+ to NADH+ + H+

Phase 2 of Glycolysis (Reactions 7 - 10)

• Reaction 7: Substrate level phosphorylation where a Transfer of phosphoryl group from 1,3- BPG to ADP is catalysed by phosphoglycerate kinase to produce ATP and 3- phosphoglycerate (3-PG)
• Reaction 8: Isomerisation of 3-PG to 2-PG catalysed by phosphoglyceromutase
• Reaction 9: Dehydration of 2-PG to form phosphoenolpyruvate (PEP), catalysed by enolase
• Reaction 10: Substrate level phosphorylation, which is the Transfer of phosphoryl group from PEP to ADP to form pyruvate and ATP, catalysed by pyruvate kinase and is Largely -ve AG and is irreversible

ATP Synthesis and Reducing Power Release

• Glucose + 2 Pi + 2 ADP + 2 NAD+ → 2 pyruvate + 2 ATP + 2 NADH+ + 2 H+ + 2 H₂O
• Reactions 1 & 3 use 2 moles of ATP per mole of glucose to initiate pathway
• Reactions 7 & 10 produce 4 moles of ATP per mole of glucose
• Overall process results in a net of 2 ATP per mole of glucose

Anaerobic Glycolysis

• Anaerobic glycolysis is the transformation of glucose to lactate when limited amounts of oxygen are available so It is a means of energy production in cells that cannot produce adequate energy through oxidative phosphorylation.
• Anaerobic glycolysis occurs naturally during exercise and in certain disease states.

Anaerobic Glycolysis

• Under anaerobic conditions, pyruvate does not undergo oxidative phosphorylation in mitochondria but instead, the cytosolic enzyme lactate dehydrogenase converts pyruvate to lactate.
• Although lactate itself is not utilized by the cell as a direct energy source, the reaction allows for the regeneration of NAD+ from NADH, which acts as an oxidizing cofactor necessary to maintain glucose flow through glycolysis
• Anaerobic glycolysis produces 2 ATP per glucose molecule thus providing a direct means of producing energy in the absence of oxygen

Anaerobic Glycolysis

• Regeneration of NAD+ during anaerobic glycolysis (step 11)
• 2 NADH+ + 2 H+ + 2 pyruvate ↔ 2 NAD+ + 2 lactate (Lactate dehydrogenase)
• Overall reaction of the 11- steps of anaerobic glycolysis: Glucose + 2 Pi + 2 ADP → 2 lactate + 2 ATP + 2 H2O

Where Does Lactate Go?

• Lactate is released into blood and is metabolised by liver, heart and kidney
• NADH+ + H+ : NAD+ ratio in liver and heart is lower than in exercising muscle

Cori Cycle (Lactic Acid Cycle)

• The Cori cycle is the cycling of lactate and glucose between peripheral tissues (muscle) and liver. Lactic released into blood is taken up by liver, oxidised to pyruvate which is then converted to glucose (gluconeogenesis) and released back into the blood
• The rate of lactate production = the rate of its utilisation
• Lactate utilisation is impaired in Liver disease, Vitamin deficiencies - e.g. thiamine, High alcohol intake, or Enzyme deficiencies.

Lactate Production rates

• Without exercise: 40-50 g/day are produced by RBC, skin, brain, skeletal muscle, and the G.I. trac
• During Strenuous exercise (includes hearty eating!) - production is usually around 30 g/5 min, increasing Plasma levels X10 in 2 - 5 mins then Back to normal by 90 min without exercise.
• Also due to pathological situations involving hypoxia caused by SHOCK e.g. (insufficient blood flow to maintain tissue perfusion) caused by eg SEPSIS (Septic shock due to infection), or CONGESTIVE HEART DISEASE (failure of heart to pump blood effectively) or ARTERIAL DISEASE (localised poor perfusion e.g. intermittent claudication)

Elevations of Plasma Lactate Concentration

• Plasma lactate concentration is determined by relative rates of production which increases during exercise and cardiac arrest, utilisation which is in effect during alcohol intake, and disposal by kidneys.
• Hyperlactatemia: 2 - 5 mM in blood, is Below renal threshold and has No change in blood pH when concentration is normally constant <1 mM.
• Lactic acidosis which is Above 5 mM in blood is caused by the Above renal threshold and results in a lowered Blood pH

Lactic acidosis causes

• Lactic acidosis can occur under normoxic conditions, resulting from Utilisation being impaired in severe liver disease), or Disposal being impaired in severe kidney failure
• Can be caused by side effects of certain drug and toxins or rare congenital deficiencies of specific enzymes required to metabolize lactate

Summary of Glycolysis

• Central pathway of carbohydrate catabolism which Occurs in all tissues (cytosolic)
• Oxidation of 1x glucose generates 2x pyruvate. Exergonic pathway
• 2x ATP used, 4x ATP produced and 2x are the net result.
• Releases 'reducing power'
• The only catabolic pathway that can operate anaerobically and Produces intermediates for specific cell functions or anabolic processes Feeds into Stage 3 of catabolism (TCA cycle)