Carbohydrates Flashcards
What are examples of monosaccharides?
Glucose, galactose and fructose
What are examples of polysaccharides?
Maltose, sucrose abd lactose
Where are the 3 areas that digestion occurs?
Mouth, duodenum and jejunum
Does digestion occur in the stomach?
Nope
How are carbohydrates digested in the mouth?
Salivary amylase enzyme hydrolyses alpha (1->4) bonds of starch
How are carbohydrates digested in the duodenum?
Pancreatic amylase enzyme hydrolyses alpha (1->4) bonds of starch
How are carbohydrates digested in the jejunum?
Enzymes hydrolyse alpha (1->6) bonds, which removes glucose and hydrolyses sucrose and lactose
What are the 3 products of digestion?
Glucose, galactose and fructose
How is glucose absorbed?
Glucose is absorbed when it travels between the intestinal lumen into blood via epithelial cells. It is driven through the lumen by a high EC Na+ concentration which is maintained by an ATP-driven Na+ pump
How is galactose absorbed?
Though concentration gradients
How is fructose absorbed?
Its absorbed through the lumen into blood by binding to the channel protein GLUT5, where it moves down its concentration gradients
What is an isoenzyme?
An enzyme that catalyses the same reaction but has different Km’s and Vmax’s
What is the connection between hexokinase and glucokinase
They are isoenzymes
What are the differences between hexokinase and glucokinase?
Hexokinase- low Km and Vmax, can bind to glucose when [glucose] is low
Glucokinase- high Km and Vmax, can only bind to glucose when [glucose] is high
When does glucokinase bind to glucose?
Quickly after a meal and traps as much as possible in the liver
Where is glucokinase found?
In the liver
What is glycogen?
It is a polymer of glucose linked by alpha(1->4) subunits with alpha(1->6) branches every 8 to 12 residues. It is mostly found in liver and skeletal muscle
How is glycogen produced?
- Glycogenin covalently binds glucose from UDP-glucose to form chains of approx. 8 sub-units
- Glycogen synthase extends the chains
- Glycogen-branching enzymes break the chain down to form 1-6 branching points
How is glycogen degraded?
Glycogen phosphorylase moves glucose subunits one at time as G-6-P from the non-reducing ends of the branches
What happens to glycogen in skeletal muscle?
It is not converted into glucose in the blood as there is no G-6-phosphatse so it enters glycolysis
What happens to glycogen in the liver?
It is converted into G-6-P then glucose in the blood as a result of a drop in [BG]
What is the function of glycolysis?
It is a metabolic pathway that saves some Ep from glucose as ATP via substrate level phosphorylation
What is the net gain in glycolysis?
2x ATP, 2xNADH and 2x pyruvate molecules
What happens to the pyruvate molecules produces in glycolysis?
It depends on what the body needs
What 2 substances are involved in dehydrogenase catalysed reactions?
Lactate (lactate -> pyruvate) and pyruvate (pyruvate -> acetyl CoA)
How is lactate converted into pyruvate?
By NAD+ during gluconeogenesis
When is blood lactate used up?
During exercise
What is the fate of blood lactate?
Gluconeogenesis converts lactate -> glucose in the liver due to a lack of supply of oxygen for oxidative phosphorylation to produce ATP so ATP is produced via substrate level phosphorylation which also produces lactate
What is the Cori cycle?
Cycle between glycolysis in the tissues and gluconeogenesis in the liver. It is a metabolic pathway in which lactate produced by anaerobic glycolysis in the muscles moves to the liver and is converted to glucose
What is the fate of absorbed galactose?
It joins glycolysis after through the conversion of G-1-P by UDP
What is the fate of absorbed fructose?
It joins glycolysis by using ATP to form F-1-P then glyceraldehyde which then uses another ATP molecule to form glyceraldehyde-3-P
What are the precursors of gluconeogenesis?
Lactate, amino acids and glycerol
What are the functions of gluconeogenesis?
Converts lactate into glucose in the liver
Is gluconeogenesis an exact reversal of glycolysis?
Nope, as 3/10 of the reactions in glycolysis are not reversible (not feasible) as they have a -ve delta G
What is the process of gluconeogenesis?
The 3 reactions that are’t feasible are bypassed by enzymes that catalyse 4 different irreversible reactions
What is the pentose phosphate pathway?
A metabolic pathway that produces NADPH and pentose sugars which are needed for nucleic acid synthesis and for the metabolism of the small amount of pentose sugars in the diet
What are the 2 phases of the pentose phosphate pathway?
Oxidative phase and a non-oxidative phase
What is the oxidative phase of the pentose phosphate pathway?
It is irreversible, generates NADPH and converts G-6-P -> pentose phosphate
What is the non-oxidative phase of the pentose phosphate pathway?
It is reversible, interconverts G-6-P and pentose phosphate and forms many carbon sugars
