Lecture 32

Question 1

What is fat stored as?

Answer 1

Triacylglycerols, fat droplets in adipose tissues

Question 2

What happens to excess glucose?

Answer 2

It is converted to TAGs in the liver, adipose and muscle cells and stored as fat

(Glucose -> pyruvate -> acetyl-CoA -> FAs -> TAGs)

The TAGs can be packaged into VLDLs in the liver and exocytosed for use

Question 3

What is triacylglycerol synthesis in adipocytes driven by?

Answer 3

Insulin

Question 4

What does insulin stimulate?

Answer 4

GLUT4, LPL, glycolysis via hexokinase and DNL (de novo lipogenesis)

Question 5

What does glucose provide for DNL?

Answer 5

Glycerol-phosphate backbone and acetyl-coA

Question 6

What is dietary fat packaged into?

Answer 6

Chylomicrons to be delivered to tissues

Question 7

How is glucose synthesised into TAGs in the liver?

Answer 7

Acetyl-CoA from the CAC (after glycolysis) is synthesised to FAs via the FA synthesis complex and then into TAGs and packaged into VLDL particles

Question 8

How do FAs reach the adipose tissue from the chylomicrons and VLDL partiles?

Answer 8

Insulin activates LPL which hydrolyses TAGs in the VLDLs and chylomicrons to release FAs into the tissue where they are synthesised into TAGs

Question 9

What is glycogen?

Answer 9

A branched polysaccharide with and alpha1,4 and alpha1,6 glycosidic bond that is stored in the liver and muscle and as granules in the cytoplasm

Question 10

What is the process of glycogen synthesis?

Answer 10

Glucose enters the cell via GLUT4 (after simulation from insulin), it is converted to G6P via hexokinase, using ATP and using mutase it changes orientation to G1P. UDP-glucose is generated from G1P using UTP and then glycogen synthase produces glycogen

Question 11

What does stimulation of the insulin signalling pathway cause?

Answer 11

Activation of PKB (AKT) and activation of PP1

Question 12

What does PKB do?

Answer 12

Phosphorylates GSK3 which inactivates it. GSK3 normally phosphorylates glycogen synthase also inactivating it and allowing glycogen synthesis to occur

(Also phosphorylates AS160 to facilitate GSV vesicle release containing GLUT4 in muscle and adipose cells, L31)

Question 13

What does PP1 do?

Answer 13

Dephosphorylates the already phosphorylated glycogen synthase in order to activate it for glycogen synthesis to occur

Question 14

What does glycogen synthesis require?

Answer 14

Inputs of ATP and UTP and an activated high-energy precursor called UDP-glucose

Question 15

What is liver glycogen important for?

Answer 15

Maintaining blood glucose levels

Question 16

What is the size of glycogen granules in the liver cells?

Answer 16

~0.1um

Question 17

What is the size of mitochondria in liver cells?

Answer 17

~10um

Question 18

Why can muscle glycogen only be used within muscle cells?

Answer 18

As muscle cells lack glucose-6-phosphatase, which is required to pass glucose into the blood

Question 19

Name the key enzymes for glycogen synthesis

Answer 19

Glycogen synthase and branching enzyme

Question 20

How do glycogen synthase and branching enzyme work?

Answer 20

Glycogen synthase: Creates a chain of glucose monomers with α(1→4) glycosidic linkages.
Branching enzyme: Transfers glucose residues from the linear chain to create branches with α(1→6) glycosidic linkages