3 - Major Nutrient Groups And The Metabolic Profile

Question 0

How are carbohydrates absorbed by the intestine?

Answer 0

Net breakdown begins with amylase
Other key enzymes are located on the brush border membrane:
-GLUT-2 allows facilitated diffusion
-SGLT allows active transport for galactose and glucose, and requires a NA+/K+ transporter and ATP
-GLUT 5 allows facilitated diffusion of fructose only.

Question 1

How is dietary protein digested and what by?

Answer 1

Starts with pepsin
Pancreatic enzymes in the duodenum break down polypeptides further into free amino acids and oligopeptides, which are further digested by enzymes on the brush border.
These are absorbed into enterocytes via luminal transport proteins

Question 2

What are the 5 phases of lipid digestion / absorption?

Answer 2

1) hydrolysis of TAG to FFA and MAG within the lumen
2) large oil phase TAG droplets are emulsified partly by the stomach first to smaller emulsion droplets
3) mixed micelles are formed by bile acids and other pancreatic lipase enzymes
4) uptake of FFA and MAG via specific FAT and FATP1 transport proteins
5) they are packaged unto chylomicrons which enter the lacteal and lymphatic circulation

Question 3

What transporter allows uptake of glucose into the liver, dependant on blood glucose levels?

What is glucose then readily phosphorylated to?

Answer 3

GLUT-2

Glucose-6-phosphate

Question 4

Where does the liver get most of its energy?

Answer 4

Beta oxidation of fats or amino acid oxidation

Question 5

True or false

Is the GLUT-4 gene encoded glucose transporter insulin independent?

Answer 5

False
It is the only insulin dependant transporter, and is expressed in muscle and adipose tissue, which are the principle sites of glucose deposition in response to insulin

Question 6

What precursors does the liver create from lipids?

Answer 6

Bile salts, steroids, ketone bodies

Question 7

What is fatty acid storage:oxidation regulated by? (3)

Answer 7

Insulin:glucagon
Compartmentation (synthesis in cytosol vs oxidation in mitochondria)
Allosteric regulation (Malonyl coA)

Question 8

What does Malonyl-coA do?

Answer 8

Blocks movement of acyl-CoA into mitochondria, separating fatty acid synthesis from fatty acid breakdown, making it the first committed step in fat synthesis

Question 9

Free fatty acids are ______ into triglycerides and transported around the body in ______
________ transport dietary fat
________ transport liver synthesised fat

Answer 9

Free fatty acids are esterified into triglycerides and transported around the body in lipoproteins
Chylomicrons transport dietary fat
VLDLs transport liver synthesised fat

Question 10

What is the role of hormone-sensitive lipase?

Answer 10

Hydrolyses stored triglycerides to free fatty acids and glycerol, which enter circulation and distribute to liver and muscle for energy.
Adrenaline directly activates HSL

Question 11

What is the difference between the two different lipases in terms of ther relationship with insulin and location and function

Answer 11

LPL: lipoprotein lipase is an endothelial wall enzyme, which is activated/elevated by insulin and clears circulating triglyceride from plasma to the tissues.
HSL: an enzyme found within adipose cells which is suppressed by insulin and activated via phosphorylation by adrenaline

Question 12

What is transamination?

Answer 12

Removal of the amino group and transfer to another substrate

Eg alanine + alpha keto glutarate = pyruvate + glutamate

Question 13

How does a low protein diet reduce waste nitrogen?

Answer 13

Uses carbon skeletons from carbohydrate to synthesise non essential amino acids