Energy Production: Carbohydrate Flashcards
Outline how starch and glycogen are broken down in the body
○ Saliva contains amylase which breaks down starch and glycogen into dextrins
○ Pancreas releases amylase which breaks dextrins into monosaccharides
○ Small intestine - disaccharides attached to brush border membrane of epithelial cells
§ Lactase, sucrose, pancreatic amylase, isomaltase
Outline how monosaccharides are absorbed into the blood
• Absorption of monosaccharides - active transport by sodium dependent glucose transporter 1 (SGLT1) into intestinal epithelial cells and then via GLUT2 into blood supply
○ Cotransports 2Na and 1 glucose from apical to basolateral side
○ Uptake into cells from blood via facilitated diffusion using transport proteins (GLUT1 - GLUT5)
○ GLUT2 - kidney, liver, pancreatic beta cells, small intestine
○ GLUT 4 - adipose tissue, striated muscle (insulin regulated - high insulin increase uptake of glucose by increasing number of glucose transport proteins)
Explain the biochemical basis of lactose intolerance
• Lactose intolerance - unable to metabolise lactose
• Lactose moves into the colon where bacteria breaks it down
○ Presence of lactose in the lumen of colon increases the osmotic pressure - draw water into the lumen, causing diarrhoea
○ Colonic bacteria can produce hydrogen, CO2, methane gases from lactose - bloating and discomfort
Outline the types of lactose intolerance
• Primary lactase deficiency - absence of lactase persistence allele
○ Only occurs in adults
• Secondary lactase deficiency - caused by injury to small intestine
○ Occurs in both infants and adults
○ Generally reversible
• Congenital lactase deficiency - extremely rare, autosomal recessive defect in lactase gene
○ Cannot digest breast milk
Describe the glucose-dependency of some tissues
• Major blood sugar - glucose concentration regulated
• All tissues can metabolise glucose but some cells have an absolute requirement (can only use glucose)
○ Red blood cells (no mitochondria and nuclei)
○ Neutrophils
○ Innermost cells of kidney medulla - not much oxygen left so need glucose for energy
○ Lends of the eye - poor oxygen supply
• Uptake depends on blood glucose concentration
• CNS prefers glucose as fuel
○ Can use ketone bodies for some of energy requirements in times of starvation but needs time to adapt
State the overall equation of glycolysis
Glucose + 2Pi + 2ADP + 2NAD -> 2 pyruvate + 2ATP + 2NADH + 2H + 2H2O
Outline phase 1 of glycolysis
○ Phosphorylation of glucose to G-6-P
§ Makes glucose negatively charged - prevents passage back across plasma membrane
§ Increases reactivity of glucose to permit subsequent steps
§ Uses 2 moles ATP per mole glucose
§ Catalysed by hexokinase
§ Catalysed by glucokinase in liver and pancreas
○ Reactions 1 and 3 have a large -ve ∆G, so irreversible
○ Step 3 is the committing step - commits glucose to metabolism via glycolysis
§ Phosphofructokinase acts on this step
Outline phase 2 of glycolysis
○ Reaction 4 - Cleavage of C6 into two C3 units
§ C3 units interconvertible
○ Reaction 6 - NAD+ converted to NADH + H
§ Reducing power captured through oxidation of G3-P
§ Total NAD+ and NADH in cell is constant, therefore glycolysis would stop when all NAD+ is converted to NADH
§ Normally, NAD+ is regenerated from NADH in stage 4 of metabolism (electron transport chain)
§ RBC have no stage 3 or 4 of metabolism
□ Stage 4 needs oxygen - supply of oxygen to muscles and gut often reduced
□ Therefore, need to regenerate NAD+ through lactate dehydrogenase
○ Reaction 7 and 10 - Transfer of phosphate onto ADP to produce ATP (substrate level phosphorylation)
Reaction 10 - large -ve ∆G, therefore irreversible
Outline how phosphofructokinase regulates glycolysis
• Phosphofructokinase key regulator of glycolysis
○ Allosteric regulation (muscle)
§ Inhibited by high ATP and stimulated by high AMP
○ Hormonal regulation (liver)
§ Stimulated by insulin and inhibited by glucagon
§ Insulin aims to store glucose into glycogen
§ Glucagon aims to convert glycogen into glucose for energy
Other than phosphofructokinase, what other glycolysis regulators are there
○ Hexokinase - converts glucose to glucose-6-phosphate
§ Hexokinase lower infinity for oxygen
§ Hexokinase has end product inhibition by G6-P to regulate glycolysis
§ Glucokinase in liver - no end product inhibition as glucose constantly needed
○ Metabolic regulation - high [NADH] or low [NAD+] = high energy level signal
§ Causes product inhibition of step 6 and inhibits glycolysis
○ Pyruvate kinase - increase by high insulin: glucagon ratio
What are some important intermediates of glycolysis
○ 2,3-bisphosphoglycrate lowers affinity of oxygen to haemoglobin
○ Glycerol phosphate important to triglyceride and phospholipid biosynthesis
§ Produced in adipose and liver
§ Lipid synthesis in adipose requires glycolysis (liver can also phosphorylate glycerol directly)
State the equation and enzyme of pyruvate in anaerobic conditions
2 pyruvate + 2NADH + 2H -> 2 lactate + 2NAD
• Lactase dehydrogenase regenerates NAD with lactate as product
Explain how plasma lactate concentration is controlled
• Plasma lactate concentration determined by relative rates of production, utilisation (liver, heart, muscle) and disposal (kidney)
○ Lactate produced is transported in the circulation to the liver, heart and kidney where it is converted back to pyruvate and oxidised to CO2 or converted to glucose
○ Lactase production = rate of utilisation
Explain consequences of high plasma lactate
• Normal lactate concentration - < 1mM
• Increases in lactate can be due to: strenuous exercise, hearty eating, shock, congestive heart disease, alcohol metabolism (low lactate usage)
• Hyperlactaemia - 2-5mM, below renal threshold
○ No change in blood pH (buffering capacity)
• Lactic acidosis - above 5mM, above renal threshold
○ Blood pH lowered
• In lactic acidosis, kidney can no longer excrete lactate and could lead to renal failure and death
○ Critical marker in the acutely unwell patient
Explain the importance of the pentose phosphate pathway
○ Occurs when energy not needed - no need to produce pyruvate
○ Glucose-6-phosphate dehydrogenase is limiting enzyme
○ Produces C5-sugar ribose required in making DNA, RNA, nucleotides and coenzymes
○ NADPH produced which is required for reducing power of biosynthesis, maintenance of GSH levels and detoxification reactions
○ Doesn’t produce ATP and CO2 produced
Discuss the effects of glucose 6-phosphate dehydrogenase deficiency
○ X-linked gene defect
○ Point mutation in gene coding for glucose 6-phosphate dehydrogenase that results in reduced activity and therefore low levels of NADPH
○ Low GSH levels - protects cell against oxidative damage
○ RBC has no sources of NADPH - risk of oxidative damage
§ Haemoglobin becomes cross-linked by disulphide bonds and form Heinz bodies
§ Destruction of RBC causes haemolysis - haemolytic anaemia
Explain galactosaemia
§ Absence of galactokinase - rare, accumulation of galactose in tissues
§ Absence of uridyl transferase - accumulation of galactose and galactose 1-phosphate
§ Absence of UDP-galactose epimerase
§ Galactose accumulation - reduced to galactitol
□ Depletes some tissues of NADPH - cataracts (protein broken down) leading to blindness
§ Galactose 1-phosphate accumulation - damage to kidney, liver, brain
□ Sequestration of Pi making it unavailable for ATP synthesis
Outline the clinical importance of fructose metabolism
○ Dietary sucrose broken down by sucrase into glucose and fructose
○ Metabolised in liver to glyceraldehyde 3-phosphate
○ Essential fructosuria - fructokinase missing
§ Fructose in urine - no clinical signs
○ Fructose intolerance - aldolase missing
§ Fructose 1-P accumulates in liver - liver damage
§ Treatment - remove fructose from diet
What happens to malnourished people when they are suddenly given lots of protein
• If protein supplied in excess of body’s immediate requirement
○ Protein cannot be broken down into ammonia and then urea to be excreted
○ Leads to liver damage - build up of toxins
Explain the key role of pyruvate dehydrogenase in glucose metabolism
• Pyruvate needs to convert to acetyl CoA
• Catalysed by pyruvate dehydrogenase
○ Oxidises pyruvate through the reduction of NAD+
○ Large multi-enzyme complex (5 enzymes)
• Bond of carboxyl group combines with CoA to create acetyl CoA of high energy state
• Occurs in the mitochondrial matrix - pyruvate transported from cytoplasm across mitochondrial membrane
• Different enzyme activities require various cofactors - B-vitamins provide these factors, so reaction is sensitive to vitamin B1 deficiency
• Reaction is irreversible, so is a key regulatory step
• High energy substrates will switch PDH off (inhibited) - phosphorylation of pyruvate dehydrogenase
○ Acetyl CoA, NADH, ATP, citrate
• Low energy substrates switch the enzyme on - dephosphorylation
○ Insulin - lots of glucose so need energy for biosynthesis
• PDH deficiency leads to lactic acidosis due to pyruvate conversion to lactate
Discuss the role of TCA cycle in metabolism
• Occurs in the mitochondria
• 2 cycles for every glucose entering glycolysis
○ Produces 6 NADH, 2 FADH2, 2 GTP, 4 CO2
• 2 irreversible reactions - conversion of isocitrate (C6) and α-ketoglutarate (C5)
○ Both oxidative steps (NADH produced) with CO2 released
• From C5 to C4 - some energy comes from CoA
• Succinyl-CoA coverts GDP to GTP
• Cycle is catalytic as no net synthesis or degradation in intermediates through pathway - for acetyl CoA cycle
• Does not function in absence of oxygen
• Both a catabolic and anabolic pathway - supplies biosynthetic processes
Explain how the TCA cycle is regulated
• Isocitrate dehydrogenase catalyzes isocitrate (C6) to α-ketoglutarate (C5)
○ Activated by high ADP concentration
○ Inhibited by high NADH, ATP concentration
• α-ketoglutarate dehydrogenase catalyses α-ketoglutarate to succinyl-CoA (C4)
Describe the processes in the electron transport chain
• Occurs in inner membrane of mitochondria
○ Outer porous membrane and impermeable inner membrane, creating an intermembrane space and mitochondrial matrix
○ NADH passes into proton translocating complex, which takes two electrons and passes 2 protons from matrix into the intermembrane space
○ Passes through 3 translocating complexes where electrons give up their energy to move 6 proteins overall
○ ~30% of energy used to move protons across membrane (lot of energy released as heat)
○ Oxygen acts as a final electron acceptor, converting oxygen to water
• Creates both a concentration and electrical gradient
○ Proton concentration gradient (membrane potential) across inner mitochondrial membrane = proton motive force (pmf)
○ However inner membrane impermeable so for hydrogen to go across membrane, it needs to go through a carrier (ATP synthase)
• Electrons in NADH have more energy than in FADH2 - NADH uses 3 PTC, FADH2 uses 2
• The more reducing force (greater pmf), the more ATP is produced
○ 2 moles of NADH synthesises 5 moles of ATP
○ 2 moles of FADH2 synthesises 3 moles of ATP
Outline the role ATP plays in regulating electron transport chain
• When ATP concentration is high and thus ADP concentration is low, no substrate for ATP synthase which stops inward flow of proton
○ Concentration of proton in intermitochondrial space increases
○ Prevents further proton pumping - stops electron transport chain
○ [H] outside increases to a level that prevents more protons being pumped
