Carbohydrate metabolism Flashcards
(26 cards)
Where does carbohydrate digestion begin, and which enzyme starts it?
In the mouth, by salivary amylase
Which enzyme breaks down disaccharides at the brush border?
Disaccharidases (e.g., lactase, sucrase, maltase).
Where are monosaccharides absorbed?
In the enterocytes of the small intestine
What are the key steps of glycolysis?
Glucose → G6P → F6P → F1,6BP → G3P → 1,3BPG → 3PG → PEP → Pyruvate
What are the irreversible steps of glycolysis and their enzymes?
Hexokinase (Glucose → G6P), PFK-1 (F6P → F1,6BP), Pyruvate kinase (PEP → Pyruvate)
What is the energy yield of glycolysis under aerobic vs. anaerobic conditions?
Aerobic: 2 ATP + 2 NADH → ~7 ATP; Anaerobic: 2 ATP (NADH not used).
How is cytosolic NADH transported into mitochondria for oxidative phosphorylation
Via the glycerol-3-phosphate and malate-aspartate shuttles.
What is the Pasteur effect
Inhibition of glycolysis by oxygen (due to increased oxidative phosphorylation).
What causes lactic acidosis
Excess anaerobic glycolysis → lactate accumulation (e.g. hypoxia, mitochondrial disorders).
What is pyruvate kinase deficiency?
Enzyme defect → impaired ATP production → hemolytic anemia.
Why is glycolysis upregulated in cancer cells
Warburg effect: cancer cells prefer glycolysis even in oxygen-rich conditions.
What are the main substrates for gluconeogenesis?
Lactate (Cori cycle), alanine (glucose-alanine cycle), glycerol.
Where does gluconeogenesis occur?
Mainly in the liver (and kidney cortex in prolonged fasting)
Which enzymes bypass the irreversible steps of glycolysis?
Pyruvate carboxylase + PEPCK, F1,6-bisphosphatase, Glucose-6-phosphatase.
What is reciprocal regulation of glycolysis and gluconeogenesis?
When one pathway is activated, the other is inhibited (e.g. F2,6BP, insulin/glucagon effects).
Role of gluconeogenesis in kidney and small intestine?
Renal cortex contributes during fasting; intestine can help during metabolic stress.
What happens in pyruvate carboxylase deficiency?
Impaired gluconeogenesis → lactic acidosis, hypoglycemia, failure to thrive.
What is fructose-1,6-bisphosphatase deficiency?
Prevents glucose formation → hypoglycemia, lactic acidosis, ketosis.
What is the main function of the PPP?
Produces NADPH (for biosynthesis and glutathione regeneration) and ribose-5-phosphate (for nucleotide synthesis).
1)Where is PPP especially active?
2)What regulates the PPP?
3)What happens in G6PD deficiency?
1) Liver, adipose tissue, adrenal cortex, RBCs.
2) G6PD activity (rate-limiting step), inhibited by high NADPH.
3) No NADPH → oxidative damage in RBCs → hemolysis (triggered by fava beans, drugs).
Q: How is fructose absorbed and metabolized?
Q: What is essential fructosuria?
Q: What is hereditary fructose intolerance?
A: Absorbed by GLUT5 in enterocytes → metabolized in liver by fructokinase.
A: Fructokinase deficiency → benign fructose in urine.
A: Aldolase B deficiency → toxic accumulation of F1P → vomiting, hypoglycemia.
Q: How is galactose absorbed and metabolized?
Q: What is classic galactosemia?
Q: What is galactokinase deficiency?
Q: What is mannose used for?
A: Absorbed by SGLT1 → phosphorylated by galactokinase → converted to glucose-1-P.
A: GALT deficiency → accumulation of galactose-1-P → liver damage, cataracts.
A: Mild form → cataracts from galactitol accumulation.
A: Glycoprotein synthesis; defects cause congenital disorders of glycosylation.
Q: What are the components of glycogen?
Q: How is glycogen broken down?
Q: Which hormones regulate glycogen metabolism?
Q: What are key enzymes in glycogen synthesis and breakdown?
A: Amylose (linear α-1,4), amylopectin (branched α-1,6).
A: Phosphorylase removes glucose-1-P; debranching enzyme handles branches.
A: Insulin (activates synthesis), glucagon and epinephrine (stimulate breakdown).
A: Glycogen synthase and glycogen phosphorylase.
Q: What is Von Gierke’s disease?
Q: What is McArdle’s disease?
Q: What happens to non-degradable carbs?
A: G6Pase deficiency → severe fasting hypoglycemia.
A: Muscle phosphorylase deficiency → exercise intolerance, muscle cramps.
A: They are excreted or fermented by gut microbiota.
