Session 1: Carbohydrates, glycolysis, pentose phosphate pathway Flashcards
(38 cards)
What are the four stages of catabolism?
- Breakdown to building block molecules
- Breakdown to metabolic intermediates
- release of ‘reducing power’
- Breakdown to metabolic intermediates
- Kreb’s cycle
- Oxidative phosphorylation
- conversion of ‘reducing power’ into energy currency, ATP
- Oxidative phosphorylation
What happens in stage 1 of carbohydrate metabolism? (3)
- Salivary amylase - starch/glycogen -> dextrins (oligosaccharides)
2A. Pancreatic amylase - monosaccharides
2B. Small intestine - disaccharidases attached to brush border of epithelial cells (lactase, sucrase, pancreatic amylase, isomaltase)
How can lactose intolerance come about?
- Primary lactase deficiency - lactase allele not expressed in adulthood
- Secondary lactase deficiency - reversible injury to small intestine
- Congenital lactase deficiency - autosomal recessive
How do monosaccharides reach the bloodstream?
Active transport
- SGLT1 (gut to intestinal epithelial cells)
- GLUT2 from intestinal epithelial to blood)
How are monosaccharides taken up by cells from blood?
Facilitated diffusion - GLUT 1-5
Where is GLUT2 found? (4)
- Kidney
- Liver
- Pancreatic beta cells
- Small intestine
Where is GLUT4 found? What is characteristic of this transport protein?
- Adipose tissue
- Striated muscle
It is insulin-regulated
What is the average blood glucose level?
~5 mM
Which cells have an absolute requirement for glucose and why?
- RBCs - no mitochondria
- Neutrophils - use mt for oxidative burst
- Innermost cells of kidney medulla
- Lens of the eye - poor oxygen supply
Describe the CNS’ (brain’s) dependency on glucose.
Prefers glucose but can use ketone bodies for some energy requirements in times.
Concentration should meet or exceed Km for transporter to move glucose across BBB
What is produced in glycolysis?
Each glucose molecule produces:
- 2 pyruvates (3C intermediates)
- 2 NADH
- 2 ATP
- 6C intermediates
Where does glycolysis take place?
Cytosol
What are the 3 main enzymes involved in glycolysis?
- Hexokinase (glucokinase in liver)
- phosphofructokinase-1
- Pyruvate kinase
What is the committing step in glycolysis?
Fructose-6-P to fructose 1,6-bis-P
What is the main regulator of glycolysis?
PFK
What regulates PFK in the muscle?
High ATP - inhibits
High AMP - stimulates
What regulates PFK in the liver?
Insulin - stimulates
Glucagon - inhibits
Name all the regulators of glycolysis
- Insulin and glucagon
- ATP and AMP
- high NADH therefore low NAD+ inhibits
- G6-P product inhibition of hexokinase
- Insulin:glucagon ratio stimulates pyruvate kinase
What is the significance of fructose 1,6-bis phosphate being broken down to DHAP (and G3P)?
DHAP important to triglyceride and phospholipid biosynthesis - forms glycerol phosphate which forms the backbone of triacylglycerol
- produced in adipose and liver
Which enzyme is involved in converting DHAP to glycerol phosphate?
Glycerol 3-phosphate dehydrogenase
Which enzyme converts 1,3-bis phosphoglycerate to 2,3-bis phosphoglycerate?
Bisphosphoglycerate mutase
What is the significance of 2,3-BPG?
Produced in RBCs to regulate affinity of Hb for O2 - BPG reduces affinity
How is NAD+ regenerated in glucose metabolism? (2)
- Stage 4 - oxidative phosphorylation
2. Lactate dehydrogenase converts pyruvate to lactate and NAD+
Which parts of the body have troubles with regeneration of NAD+ and why?
- RBCs - no Kreb’s cycle or oxidative phosphorylation because no mitochondria
- Gut and muscle - oxidative phosphorylation needs O2 and these organs get reduced O2 supply sometimes
