Carbohydrates 2 Flashcards
What carbohydrates do we get from our diets?
Starch, Glycogen, cellulose and hemicellulose, oligosaccharides, disaccharides and monosaccharides
Describe the absorption of glucose
Glucose and Na+ from the intestinal lumen passes through a sodium-glucose transporter protein on the apical surface of the epithelial cell.
How is a low [Na+] maintained in the epithelial cells of the intestinal lumen?
A sodium-potassium pump on the basal surface of the epithelial cells transports more Na+ out than goes in.
Which other monosaccharide is absorbed in a similar way to glucose?
Galactose
What is the name of the transporter protein responsible for the passive diffusion of fructose into the wall of the intestine
GLUT5
What is the function of cellulose/hemicellulose if it can’t be digested?
Increases faecal bulk and decreases transit time
Many western diets are deficient in what type of carbohydrate?
Oligosaccharides (small polymers typically between 2 and 10 subunits large)
Disaccharide deficiencies can be genetic or result from what?
- Severe intestinal infection
- Other inflammation of the gut lining
- Drugs injuring the gut wall
- Surgical removal of the intestine
Disaccharide deficiencies are characterised by what symptoms?
Abdominal distention (swelling) and cramps
How would a doctor diagnose a disaccharide deficiency?
Testing enzyme activity of lactase, maltase and sucrase
What happens to glucose once its absorbed into the blood?
Goes through hepatic portal vein and into liver where it is phosphorylated into G6P. This stops it from leaving the cell as it isn’t recognised by transporters.
Which enzymes are involved in phosphorylation of glucose?
Glucokinase (liver) and hexokinase (other tissues)
Whats does it mean for an enzyme to have a high VMAX?
Its an efficient enzyme
What does it mean for an enzyme to have a low KM?
High affinity for a substrate
What happens when glycogen is mobilised in the liver?
Glycogen is converted to G6P which then is converted into glucose by the action of Glucose-6-phosphotase to be released into the blood
What key difference is there in mobilisation of glycogen in skeletal muscles compared to into the liver?
Skeletal muscle has no Glucose-6-phosphatase therefore G6P is ‘trapped’ in cell and is respired
Describe the first step in the synthesis of glycogen
- Glycogenin covalently binds glucose from uracil-diphosphate (UDP-glucose) to form chains of approx. 8 glucose monomers
- Glycogen synthase then extends the Glucose chains
Describe step two of glycogen synthesis
The long chains formed in step one are broken by glycogen-branching enzyme and reattached at the 6th carbon position forming 1,6-glycosidic bonds (branches)
Describe step one of glycogen mobilisation
Glucose monomers are removed one at a time from the non-reducing ends - relinquishing Glucose-1-phosphate (G1P) - catalysed by phosphorylase
Describe step two of glycogen mobilisation
Transferase enzynes remove sets of three glucose residues from branches and adds it to the nearest non-reducing end, producing a linear chain.
Describe step 3 of glycogen mobilisation
Glucosidase activity then removes the 1,6-glycosidic bond of the branching glucose residue - releasing a free glucose molecule
What is Von Gierke’s disease?
Liver (and kidney, intestine) glucose-6-phophatase deficiency
What are the symptoms of Von Gierke’s disease?
High [liver glycogen]
Low [blood glucose] - fasting hypoglycaemia
What is the treatment for Von Gierke’s disease?
Regular carbohydrate feeding - little and often every 3-4 hours
What is McArdle’s Disease?
Skeletal muscle phosphorylase deficiency
What are the symptoms of McArdle’s Disease?
High [muscle glycogen]
Weakness and cramps after exercise
No increase in [blood glucose] after exercise
What is the treatment for McArdle’s disease?
Avoid strenuous activity or make use of second wind (brief anaerobic exercise followed by aerobic exercise fuelled by oxidative phosphorylation of fatty acids)
