Lecture 2: Glucose Homeostasis Flashcards
(41 cards)
What is glucose?
A monosaccharide, and the most important simple sugar in the body. Used for aerobic and anaerobic respiration.
Name 4 monosaccharides.
Ribose
Glucose
Fructose
Galactose
Name 3 disaccharides and their constituents.
Maltose (Glucose - Glucose)
Sucrose (Fructose - Glucose)
Lactose (Glucose - Galactose)
Name 3 polysaccharides and describe their structure.
Cellulose (B-Glucose 1,4 bonded, H bonds between chains)
Glycogen (a-Glucose 1,4 bonded, with 1,6 bonds giving it a branched structure)
Starch (Mixture of amylose and amylopectin)
(amylose - a-glucose, 1,4 bonding, consisting of long straight chains)
(amylopectin - a-glucose, 1,4 and 1,6 bonding, branched)
What is the difference between amylopectin and glycogen?
Amylopectin has less 1,6 bonds and therefore less branching, leading to a more linear structure.
Glycogen has 1,6 bonds twice as frequently, leading to branching until it is impossible anymore, giving it a more globular structure.
Amylopectin is less soluble than glycogen.
What forms of glucose can humans use and which can they not?
beta-Glucose, can’t be processed.
alpha-Glucose, can be processed.
L-Glucose, can’t be processed.
D-Glucose, can be processed.
L-Glucose, unlike D-Glucose, does not occur in nature, but can be made in laboratories.
What happens when glucose use becomes higher than the intake?
Triglycerides and proteins will get broken down and used. (fatty acids, and amino acids)
How many grams of glucose is roughly present in plasma/extracellular fluid at one time?
Around 10 grams.
What concentration of plasma glucose typically indicates hypoglycaemia?
Less than 2.2 mmol/L
What is the difference between the function of the liver and muscles in glucose use?
Glycogen stored in muscles is only used by the muscles, whereas the liver can release glucose into the rest of the body.
What are all the processes involved in glucose storage and production?
Glycogenesis - The conversion of glucose into the polysaccharide glycogen
Glycogenolysis - The breakdown of glycogen into glucose.
Gluconeogenesis - The production of glucose from other precursors
Describe how levels of glucose homeostasis reactions shift when eating and fasting.
Glucose from a meal will be circulated in the blood, resulting in a spike as Glycogenesis will convert excess glucose. Glycogenolysis will maintain glucose levels, with the ratio of glycogenolysis : gluconeogenesis decreasing the longer you wait before eating again. After extreme fasting gluconeogenesis is the principle source of glucose.
Describe the process of glycogenolysis.
Glycogen phosphorylase removes glucose-1-phosphate from branches of glycogen, adding an inorganic phosphate to the first carbon.
When the glycogen chain reaches 4 glucose residues in length, a debranching enzyme will remove 3, leaving a single 1,6 bonded glucose residue that will be hydrolysed by the secondary function of this enzyme.
Due to the frequency of chains in glycogen, the ratio of glucose-1-phosphate to glucose in glycogenolysis is 10:1.
What happens to glucose-1-phosphate after glycogenolysis?
Can be used in glycolysis to produce pyruvate, after being converted to glucose-6-phosphate by phosphoglucomutase.
Can be converted back into glucose by glucose-6-phosphatase in the liver and kidney.
What becomes the main energy source when fasting?
Ketones derived from fat become the major energy substrate.
What are the usual blood glucose levels when fasting and postprandial (after meal).
Fasting blood glucose - 4 - 5.4 mM
Postprandial can be up to 7.8 mM
What ways can glucose be formed in gluconeogenesis?
Glycogenolysis.
Glycerol can be converted to glucose by conversion to dihydroxyacetone phosphate, then to glyceraldehyde-3-phosphate.
Glucogenic amino acids can be converted to glucose by first being converted into pyruvate. Can also enter the TCA at several steps.
Lactate can be used to make glucose through conversion to pyruvate.
Acetone can be converted to pyruvate to make glucose.
What do triglycerides ultimately end up as?
The glycerol can be used to make glucose. The fatty acids can be used to produce acetyl-CoA in beta-oxidation, and in abundance, acetyl-CoA is converted by thiolase to ketone bodies.
Describe the process of glycogenesis.
- Glucose is converted to glucose 6-phosphate by hexokinase.
- Glucose 6-phosphate is converted to glucose 1-phosphate by phosphoglucomutase.
- Glucose is bonded with a UMP molecule by UDP Glucose pyrophosphorylase to make uridine diphosphate glucose. (The UMP comes from a nearby UTP molecule that loses a pyrophosphate)
- Pyrophosphate is cleaved by pyrophosphatase into 2 AMP and release energy to couple it to further steps.
- An enzyme, glycogenin, which has tyrosine residues can autoglycosylate glucose onto itself, forming primer chain of 8-12 residues with a-1,4 glycosidic bonds.
- The enzyme glycogen synthase can add further glucosyl residues with a-1,4 glycosidic bonds.
- Branching enzyme takes 5 - 8 of the 1,4 glucosyl residues and transfers them to a different part of the earlier chain, forming an a-1,6 glycosidic bond. This new chain can be added to by glycogen synthase.
How is blood glucose controlled by the body?
Through the use of hormones.
Insulin is the regulatory hormones (decreases levels of glucose by stimulating glycogenesis, promoting uptake of glucose into peripheral tissues, and suppressing hepatic production of glucose).
Counter-regulatory hormones promote an increase in blood glucose by stimulating glycogenolysis, reducing muscle cell glucose uptake, and making cells resistant to insulin.
These counter-regulatory hormones are glucagon, epinephrine, cortisol and growth hormone.
How is glucose transported?
In the small bowel and kidney, sodium/glucose co-transporters promote uptake of glucose and galactose. ATP-sodium dependent
Facilitative glucose transporters (GLUT) located on the surface of all cells, with multiple variations. 14 exist but most well studied are labelled GLUT 1 - 7. And 5 are the most important.
What are GLUT-1 transporters?
Widely distributed in the body. Highly concentrated in endothelial cells of blood-tissue barriers, important in blood brain barrier. RBCs use GLUT-1 heavily. Therefore one of the specialised roles is transporting glucose from blood to organs that have little access to small solutes via passive diffusion.
How do GLUT-1 transporters work?
The glucose will bind to the transporter and cause a conformational change that will push it through on the other side of cell membrane and release it.
GLUT-1 is insulin-independent.
What are GLUT-2 transporters?
GLUT-2 is present in plasma membrane of hepatocytes, pancreatic cells, intestinal and kidney cells, as well as the hypothalamus.
Primary tissue is the liver, and to a lesser degree, pancreatic B cells.
High Km therefore low affinity for glucose, and will only uptake when there is a very high concentration, so liver lets the rest of the body use glucose before it. B-cells will uptake glucose (that stimulates insulin release) in high concentrations therefore only releasing insulin at high blood glucose.
Insulin independent.
