BCH 202 Carbohydrate Metabolism Flashcards
The principal sites of carbohydrate digestion are the
mouth and the small intestine
Enzyme that begins digestion of starch in the mouth
salivary alpha amylase.
What are the enzymes that hydrolyze disaccharides into monosaccharides in the small intestine
sucrase, maltase, isomaltase and lactase.
What are the mechanisms of carbohydrate absorption?
•1. Active transport against a concentration gradient, i.e. from a low glucose concentration to a higher concentration.
•2. Facilitative transport, with concentration gradient i.e. from a higher concentration to a lower one.
The process of transport of carbohydrates is mediated by what?
Transporters
SGluT-1
SGluT-2
GLUT-1
GLUT-2
GLUT-4
Location and properties of SGlut
Intestine and kidney
Co-transport from lumen into cell
Location and properties of GLUT1
RBC, Kidney, brain, retina and placenta
Glucose uptake in most of the cells
Location and properties of GLUT2
Intestinal cells, liver, beta cells of the pancreas
Low affinity, glucose uptake in liver, glucose sensor in beta cells
Location and properties of GLUT3
Neurons, brain
High affinity, glucose into brain cells
Location and properties of GLUT4
Skeletal, heart
Insulin-mediated muscle, adipose tissue glucose uptake
How does insulin increase globose uptake
Insulin induces the movement of intracellular GluT4 molecules to the cell surface and thus increases glucose uptake.
How does type 2 diabetes mellitus affect absorption of glucose?
In type 2 diabetes mellitus, membrane GluT4 is reduced, leading to insulin resistance in muscle and fat cells. In diabetes, entry of glucose into muscle is only half of normal cells.
Normal fasting plasma glucose level
70-110mg/dl
plasma glucose level after a carb heavy meal
Under 150mg/dl
What is glycolysis?
Glycolysis is the sequence of reactions that converts glucose into pyruvate in the presence of oxygen (aerobic) or lactate in the absence of oxygen (anaerobic) with the production of ATP.
Glycolysis is also known as
Embden Meyerhof pathway.
Site of glycolysis reaction
Cytoplasm of cells
Enumerate the significance of glycolysis
It is the only pathway that is taking place in all the cells of the body
•Glycolysis is the only source of energy in erythrocytes
•The glycolytic pathway may be considered as the preliminary step before complete oxidation of glucose
•In strenuous exercise, when muscle tissue lacks enough oxygen, anaerobic glycolysis forms the major source of energy for muscles.
SGlut stands for
Sodium glucose link transporter
What is diabetes mellitus?
diabetes mellitus, disorder of carbohydrate metabolism characterized by impaired ability of the body to produce or respond to insulin and thereby maintain proper levels of sugar (glucose) in the blood.
Difference between hexokinase and glucokinase
Hexokinase
Present in extra-hepatic tissue
High affinity for its substrate glucose (low Km)
Inhibited by its product glucose-6-phosphate
Its function is to ensure supply of glucose for the tissues irrespective of blood glucose concentration
Its activity is not affected by insulin
Catalyze the phosphorylation of other hexoses like fructose galactose, etc.
Glucokinase
Present in liver
Low affinity for its substrate glucose (high Km)
No inhibition by its product glucose-6-phosphate
Its function is to remove glucose from the blood when the blood glucose level increases (following meal)
It is an inducible enzyme that increases its synthesis in response to insulin
Specific for glucose
Briefly describe the 9 stages of glycolysis
- Glucose is phosphorylated to glucose-6-phosphate by hexokinase or glucokinase and ATP is required as a phosphate donor. Hexokinase and glucokinase are isoenzymes.
- Conversion of glucose-6-phosphate to fructose-6-phosphate by an enzyme phosphohexose isomerase which involve isomerization and is freely reversible reaction.
- Fructose-6-phosphate to fructose 1, 6-bisphosphate, second phosphorylation reaction requiring ATP catalyzed by an enzyme phosphofructokinase-I. This step is irreversible under physiological conditions. Phosphofructokinase-I is regulatory enzyme of glycolysis.
- Fructose-1,6-bisphosphate is cleaved by aldolase to two three carbon compounds, glyceraldehyde-3-phosphate and dihydroxy acetone phosphate (DHAP)
DHAP is isomerized to glyceraldehyde-3-phosphate by the enzyme phosphotriose isomerase, so that, 2-molecules of glyceraldehyde-3-phosphate are formed from one molecule of glucose. - Oxidation of glyceraldehyde-3-phosphate to 1,3- bisphosphoglycerate by glyceraldehyde-3-phosphate dehydrogenase, is a NAD dependent reversible reaction. The reducing equivalents NADH+ H+ formed, are re-oxidized by electron transport chain, to generate 3 ATP molecules per NADH+H+.
- 1,3-bisphosphoglycerate to 3-phosphoglycerate is catalyzed by phosphoglycerate kinase. This is the step in glycolysis that generates ATP at substrate level phosphorylation. Since two molecules of glyceraldehyde-3-phosphate are formed per molecule of glucose undergoing glycolysis, two molecules of ATP are generated at this stage per molecule of glucose.
- 3-phosphoglycerate to 2-phosphoglycerate is a reversible reaction catalyzed by phosphoglycerate mutase.
- 2-phosphoglycerate to phosphoenol pyruvate. This reaction is catalyzed by enolase. Enolase is inhibited by fluoride, a property that can be used
when it is required to prevent glycolysis in blood prior to the estimation of glucose. - Phosphoenol pyruvate to pyruvate is an irreversible reaction catalyzed by pyruvate kinase. This is the second step in glycolysis that generates ATP at substrate-level phosphorylation. Enol pyruvate formed in this reaction is converted spontaneously to the keto form of pyruvate.
Under aerobic condition, pyruvate is taken up into mitochondria and after conversion to acetyl-CoA, it is then oxidized to CO2 and H2O by citric acid cycle.
What is substrate-level phosphorylation?
Substrate level phosphorylation is a reaction process that involves the trapping of energy directly from the substrate, without the help of the complicated electron transport chain reactions. When energy is trapped by oxidation of reducing equivalents such as NADH, it is called oxidative phosphorylation.
Briefly describe Anaerobic glycolysis
In anaerobic condition, pyruvate is reduced to lactate by lactate dehydrogenase (LDH). LDH has 5 isoenzymes. The cardiac isoenzymes will be increased in myocardial infarction.
In red blood cells, there are no mitochondria. Hence RBCs derive energy only through glycolysis, where the end product is lactic acid.