1. Energy Production 1 Flashcards
What are the 4 general steps of catabolism?
Stage 1
- breakdown to building block molecules
Stage 2
- breakdown to metabolic intermediates
- release of ‘reducing power’ and ‘energy’
Stage 3
- tricarboxylic acid (Kreb’s) cycle
- release of ‘reducing power’ and ‘energy’
Stage 4
- oxidative phosphorylation: conversion of ‘reducing power’ into ATP
Describe the process of stage 1 catabolism of dietary carbohydrates.
Carbohydrates are broken down into monosaccharides by:
- Saliva: amylase (starch/glycogen to dextrins)
- Pancreas: amylase (to monosaccharides)
- Small intestine: disaccharidases attached to brush border membrane of epithelial cells:
- lactase (lactose)
- sucrose (sucrose)
- pancreatic amylase (alpha1-4 bonds)
- isomaltase (alpha1-6 bonds)
What are the differences between the types of lactose intolerance.
- Primary lactase deficiency
- Cause: absence of lactase persistence allele
- Only occurs in adults
- Secondary lactase deficiency
- Cause: injury to small intestine (gastroenteritis, coeliac disease, Crohn’s disease, ulcerative colitis)
- Occurs in both infants and adults
- Generally reversible
- Congenital lactase deficiency
- Cause: autosomal recessive defect in lactase gene - cannot digest breast milk
- Occurs in both infants and adults
- Extremely rare
What are the symptoms of lactose intolerance?
Bloating/cramps Flatulence Diarrhoea Vomiting Rumbling stomach
How are monosaccharides absorbed into the blood?
- Active transport into intestinal epithelial cells by sodium-dependent glucose transporter 1 (SGLT1): co-transport of 2 Na+ for 1 sugar.
- Transport out of epithelial cells into blood via GLUT2 transporter.
How are monosaccharides absorbed from the blood into tissues? Give examples.
- Via facilitated diffusion using transport proteins (GLUT1-GLUT5).
- GLUT2: kidney, liver, pancreatic beta cells, small intestine
- GLUT4: adipose tissue, striated muscle (insulin-regulated)
What is the major blood sugar and to what is its concentration regulated?
Blood [glucose] regulated to ~5 mM
Which cell types have an absolute requirement for glucose, why?
- Red blood cells (lack mitochondria so can’t do stages 3 and 4 of metabolism)
- Neutrophils (have mitochondria but used for oxidative burst and ROS production)
- Innermost cells of kidney medulla (kidney cortex has very high oxygen requirement so not enough left for medulla)
- Lens of the eye (not vascularised as needs to be translucent, so no oxygen supply)
Which cell type has a preference for glucose as its energy source?
CNS - brain (can use ketone bodies for some of energy requirements in times of starvation but needs time to adapt)
What does glucose uptake into tissues mainly depend on?
[blood glucose] - if falls lower than Km of glucose transporter, can’t be transporter into tissues… hypoglycaemic… coma (as brain doesn’t have enough energy).
Where does glycolysis occur?
Cytosol - in all tissues
What are the functions of glycolysis?
Oxidation of glucose, producing C6 and C3 intermediates, to synthesise:
- NADH (2 per glucose)
- ATP from ADP (net gain of 2 per glucose)
What does the ‘investment’ phase of glycolysis involve?
Conversion of 2 ATP to ADP.
What are the substrate and end product of glycolysis?
Substrate = glucose (C6) Product = pyruvate (C3) x2
Why are there so many steps/enzymes in glycolysis?
- Chemistry easier in small stages
- Efficient energy conservation
- Gives versatility: allows interconnections with other pathways, allows production of useful intermediates, allows part to be used in reverse
- Allows for fine control
Which 3 enzymes of glycolysis can only catalyse irreversible reactions? Why?
- Hexokinase (glucokinase in liver): requires ATP
- Phosphofructokinase-1: requires ATP
- Pyruvate kinase: produces ATP
Reactions have large negative deltaG.
What is the main difference between hexokinase and glucokinase?
Hexokinase (in most tissues)
- glycolysis driven by need to supply energy so inhibited by products
Glucokinase (in liver)
- glycolysis driven by supply of glucose so not inhibited by products
What is the function of hexokinase/glucokinase? What does it require?
Phosphorylation of glucose to glucose-6-phosphate:
- makes glucose negatively charged… prevents passage back across plasma membrane
- Increases reactivity of glucose to permit subsequent steps
Requires 2 moles ATP per mole glucose
Which reaction commits glucose to glycolysis?
Conversion of fructose-6-P to fructose 1,6-bis-P by phosphofructokinase-1 (key control enzyme)
What is substrate level phosphorylation?
Transfer of Pi to ADP to produce ATP
How can glycolysis be used to detect cancer?
Rate of glycolysis up to 200x greater in cancer - can measure uptake of FDG (radioactive modified hexokinase substrate) and image with positron emission tomography.
What is the key regulator of glycolysis and how does this occur?
Phosphofructokinase-1: converts fructose 6-P to fructose 1,6-bisP using ATP.
1) allosteric regulation (muscle):
- inhibited by high ATP
- stimulated by high AMP
2) hormonal regulation (liver):
- stimulated by insulin
- inhibited by glucagon
Apart from PFK, how is glycolysis regulated?
- Hexokinase (not glucokinase):
- product inhibition by G6-P.
- Metabolic regulation:
- high [NADH] or low [NAD+] (= high energy level signal)
- causes product inhibition of step 6 and inhibits glycolysis.
- Pyruvate kinase:
- increased by high insulin:glucagon ratio
Name 2 important intermediates of glycolysis.
- 1,3-bis phosphoglycerate
- converted to 2,3-BPG by bisphosphoglycerate mutase.
- 2,3-BPG produced in RBCs, regulator of Hb O2 affinity (promotes release)
- DHAP (C3) (interconvertible with glyceraldehyde 3-P)
- converted to glycerol phosphate by glycerol 3-P dehydrogenase (NADH to NAD+)
- important to triglyceride and phospholipid biosynthesis
- produced in adipose and liver (lipid synthesis in adipose requires glycolysis)
Why is NAD+ regeneration important for glycolysis and how is this achieved?
- 2 moles of NADH produced per mole of glucose so pathway requires NAD+. Total NAD+ and NADH in cell is constant so glycolysis would stop when all NAD+ is converted to NADH.
- NAD+ normally regenerated from NADH in stage 4 of metabolism.
- Some tissues such as RBCs have no stage 3 or 4 (as these require O2) so use lactate dehydrogenase to regenerate NAD+.
How is NADH converted to NAD+ in RBCs or under low O2 conditions?
Pyruvate is converted to lactate by lactate dehydrogenase (LDH):
NADH + H+ + pyruvate NAD+ + lactate
How can lactate be used to provide energy?
- Lactate taken up by myocytes… LDH converts it to pyruvate (NAD+ to NADH + H+)… oxidation to produce energy + CO2.
- Lactate taken up by liver and kidney… LDH converts it to pyruvate… gluconeogenesis to glucose… returned to tissue for glycolysis.
How can the conversion of lactate to pyruvate be impaired in disease?
- Impaired in liver disease
- vitamin deficiency (thiamine)
- alcohol NAD+ -> NADH
- enzyme deficiencies
What is plasma lactate concentration determined by?
Relative rates of:
1- production
2- utilisation (liver, heart, muscle)
3- disposal (kidney)
What is the normal [plasma lactate], and what is hyperlactaemia and lactic acidosis?
- Concentration normally constant <1 mM
- Hyperlactaemia: 2-5 mM but below renal threshold (can still be excreted by kidneys) so no change in blood pH (buffering capacity).
- Lactic acidosis: >5 mM so above renal threshold and blood pH is lowered - critical marker in the acutely unwell patient.
What is fructose, where is it metabolised, by which major enzymes?
- Fructose = fruit monosaccharide (found as glucose-fructose disaccharide).
- Metabolised in liver.
- By fructokinase and aldolase (produces glyceraldehyde and DHAP).
What is the difference between essential fructosuria and fructose intolerance?
- Essential fructosuria = deficient fructokinase. Fructose in urine but no clinical signs.
- Fructose intolerance = deficient aldolase. Fructose-1-P accumulates in liver leading to liver damage.
Treatment = remove fructose from diet.
Name an important alternative pathway to glycolysis. What is its substrate?
Pentose phosphate pathway, starts from glucose-6-P
What is the function of the pentose phosphate pathway?
- Important source of NADPH required for:
- reducing power in biosynthesis of fatty acids and steroids
- maintenance of GSH levels (glutathione - important antioxidant)
- detoxification reactions
- Produces C5-sugar ribose required for synthesis of:
- nucleotides
- DNA and RNA
- coenzymes
Does the pentose phosphate pathway produce ATP and CO2?
- No ATP
- Yes CO2
What is the rate limiting enzyme in the pentose phosphate pathway?
Glucose 6-phosphate dehydrogenase
