glucogenesis and glycogen metabolism

Question 1

what tissues require a continuous supply of glucose as a metabolic fuel

Answer 1

Brain
- Uses 120g of glucose per day

Red blood cells

Kidney Medulla

The lens and the cornea of the eye

Testes

Exercising muscle

Question 2

how much glucose is in the body - in glycogen stores and in blood

Answer 2

~190g from Glycogen stores

~20g circulating in the blood

Question 3

In the absence of dietary input, liver glycogen can only meet demand for how many hours

Answer 3

10 - 18hrs

Question 4

once body stores of glucose are depleted glucose is formed from what precursors

Answer 4

Lactate

Pyruvate

Glycerol (from triacylglycerol breakdown - mammals cannot synthesise Glucose directly from fatty acids)

α-ketoacids (from amino acid breakdown) = some of the amino acids that make up proteins

Question 5

is pyruvate to glucose called reverse glycolysis and why

Answer 5

Sadly not…

Due to the 3 irreversible reactions in Glycolysis this cannot occur, thus glucose is synthesised in a unique pathway

So Gluconeogenesis is done

Question 6

what is gluconeogenesis

Answer 6

the formation of Glucose from non-carbohydrate sources

(pyruvate to glucose)

Question 7

how much of gluconeogenesis occurs in the liver

Answer 7

90%

Question 8

Kidneys provide what percentage of newly synthesised glucose molecules and whats their role in Gluconeogenesis

Answer 8

10%

they play a minor role except in prolonged starvation, when they become major glucose producing organs

Question 9

what reactions are unique to gluconeogenesis

Answer 9

3 irreversible reactions:

• Glucose –> Glucose-6-phosphate (Hexokinase)
• Fructose-6-phosphate –> Fructose-1,6-bisphosphate (phosphofructokinase)
• Phosphoenolpyruvate —-> Pyruvate (Pyruvate kinase)

In gluconeogenesis 4 alternate enzyme catalysed reactions are used to circumvent the 3 irreversible steps

• Bypass 1 – Pyruvate kinase = 3 steps
• Bypass 2 – Phosphofructokinase = 1 step
• Bypass 3 – Hexokinase = 1 step

Other reactions are a reversal of glycolysis

Question 10

explain bypass 1 - pyruvate to phosphoenolpyruvate step 1

Answer 10

Step 1: Carboxylation of Pyruvate:

Pyruvate —> Oxaloacetate by the enzyme Pyruvate carboxylase

• Pyruvate carboxylase is only found in the mitochondria of LIVER and KIDNEY cells, so why it only occurs in these cells

Question 11

explain Bypass 1: Pyruvate to Phosphoenolpyruvate step 2

Answer 11

Oxaloacetate, formed in the mitochondria, must enter the cytosol where the other enzymes of gluconeogenesis are located

However, oxaloacetate is unable to cross the inner mitochondrial membrane directly

• It is first converted into malate by the enzyme mitochondrial malate dehydrogenase
• Malate can cross the mitochondrial membrane, where is it then re-oxidised back into oxaloacetate by cytosolic malate dehydrogenase

Basically were going from oxaloacetate to oxaloacetate in another area by converting it to malate and back to oxaloacetate

Decarboxylation of cytosolic oxaloacetate:

Oxaloacetate isdecarboxylatedand phosphorylated in thecytosol byphosphoenolpyruvate. carboxykinase(PEP-carboxykinase).
- Reaction is driven by the hydrolysis of GTP.​

• PEP then enters the reversible reactions of glycolysis until itreaches fructose 1,6-bisphosphate.

Question 12

explain bypass 2: Fructose-1,6-bisphosphate to Fructose-6-phosphate

Answer 12

Dephosphorylation by hydrolysis of fructose1,6-bisphosphate bypasses the irreversiblephosphofructokinase reaction of glycolysis and is catalysed by the enzyme Fructose bisphosphatase​ - this takes one of the phosphates away

The reaction is an important regulatory site ofgluconeogenesis.

Question 13

explain Bypass 3: Glucose-6-phosphate to Glucose

Answer 13

Hydrolysis of glucose-6-phosphate by glucose-6-phosphatase bypasses the irreversiblehexokinase reaction of glycolysis.​

Glucose 6-phosphatase is present in the LIVER andKIDNEY, but not in muscle.

Question 14

what is the Tissue location of glucose 6-phosphatase and pyruvate carboxylase

Answer 14

Glucose-6-phosphatase and Pyruvatecarboxylase occur in cells of the the liver and kidney, but notin muscle.​

• Thus, muscle cannot contribute to blood glucoseby gluconeogenesis, nor can it produce glucosefrom glucose 6-phosphate derived from muscleglycogen.​
• Only liver (and kidney) can contribute to the bloodglucose pool.

Question 15

what are Gluconeogenic precursors

Answer 15

molecules that can give rise to anet synthesis of glucose.​

Question 16

what do substrates for Gluconeogenesis include

Answer 16

all the intermediates of glycolysis and the citricacid cycle.​

Question 17

what are the most importantgluconeogenic precursors obtained from thedeamination of glucogenic amino acids

Answer 17

Glycerol,
lactate
and the⍺-ketoacids

Question 18

explain the substrate - glycerol, for Gluconeogenesis

Answer 18

Is released during hydrolysis oftriacylglycerolsin adiposetissue and is delivered in the blood to the liver.​

Glycerol is phosphorylated to glycerol phosphate, which isoxidised to the glycolytic intermediate,dihydroxyacetonephosphate.

• Dihydroxyacetone phosphate —> Glyceraldehyde-3-phosphate
• Triose phosphate isomerase

Question 19

explain the substrate - lactate, for glucogenesis

Answer 19

Is released into the blood by cells that lack mitochondria, suchas red blood cells and by exercising skeletal muscle.​

In theCori cycle, blood-borne glucose is converted byexercising muscle to lactate, which diffuses into the blood.​

This lactate is taken up by the liver and converted to glucose,which is released back into the circulation.

In exercising muscles, a lot of lactate is produced

Question 20

explain substrate - alpha-ketoacid, for gluconeogenesis

Answer 20

Amino acids whose catabolism yields pyruvate or one of theintermediates of the citrate cycle are called glucogenic (orglycogenic).​

These intermediates are substrates for gluconeogenesis andtherefore can be used to contribute to liver glycogen in liverand muscle.

Question 21

The body has developed mechanisms for storinga reserve pool of glucose in the form of what

Answer 21

glycogen

Question 22

what happens When glycogen stores are depleted

Answer 22

the body thensynthesises glucose by gluconeogenesis in the first instance.

Question 23

Dietary intake of glucose and the precursors of glucose iswhat

Answer 23

sporadic,and a constant source of blood glucose is of absolute metabolicimportance.

(Our bodies need glucose)

Question 24

explain the amounts of liver and muscle glycogen

Answer 24

About 400g of glycogen makes up 1-2% of the fresh weight ofresting muscle and about 100g glycogen makes up the freshweight of a well-fed adult liver.​

What limits the production of glycogen at these levels is notknown.

Question 25

explain glycogens structure

Answer 25

Glycogen is a branched chain homopolysaccharide madeexclusively from⍺-D-glucose.​

They have reducing and non reducing ends

The primary glycosidic bonds are⍺1-4 and between every 8-10 residues there is a branch consisting of an⍺1-6 glycosidicbond.​

Glycogen molecules can have a molecular weight ≤ 108. Theyexist as discrete cytoplasmic granules containing most of theenzymes for synthesis and breakdown

Question 26

what is glycogen granule

Answer 26

The core is made up of the protein Glycogenin, and is surrounded by the branches of Glucose polymers

Glycogen molecules are big

Due to the alpha 1 – 6 bonds, its very dense so more glucose can be in the same space

Question 27

what is glycogen synthesis

Answer 27

The process occurs in the cytosol, and requires energy suppliedby ATP (for the initial phosphorylation of glucose) anduridinetriphosphate (UTP).

Question 28

glycogen in glycogen synthesis is synthesised from what

Answer 28

molecules of⍺-D-glucose that areinitially attached touridinediphosphate(UDP).

(UDP is like ADP etc)

Question 29

explain glycogen synthesis step 1: Synthesis of UDP-Glucose

Answer 29

Glucose attached touridinediphosphate(UDP-glucose) isthe source of all glucosyl residues that are added to thegrowing glycogen molecule.​

First, glucose 6-phosphate is converted to glucose 1-phosphate byphosphoglucomutase

Cant have glycogen without glycogenin

Question 30

explain glycogen synthesis step 2: Synthesis of a Glycogen primer to initiate Glycogen synthesis

Answer 30

The main enzyme involved in glycogenpolymerisation,glycogensynthaseis responsible for making the⍺1-4linkages inglycogen.​

However, this enzyme can only add to an existing chain of between5 to 8 glucoseresidues.​

Thus, glycogen synthesis requires a primer, a role that is carried outbyglycogenin

Question 31

what is glycogenin structure

Answer 31

Dimer of 2 identical 37kDsubunits​

Question 32

what does glycogenin catalyse

Answer 32

catalyses the addition of glucose to itself bycovalently attaching glucose from UDP-glucose to the hydroxylgroup of tyrosine-194 in eachsubunit.

This is an autocatalytic reaction facilitated byglycogeninitiator synthaseactivity

Question 33

Glycogeninalso has______activity which allows what

Answer 33

glucosyltransferaseactivity

which allowsthe addition of up to 8 glucose residues from UDP-glucose toits partner in theglycogenindimer in this sequence ofautoglycosylations.

Question 34

explain glycogen synthesis step 3: Elongation of Glycogen chains by Glycogen synthase

Answer 34

Elongation of a glycogen chain involves the transfer of glucosefrom UDP-glucose to the non-reducing end of the growingchain.​

The enzyme responsible for making these⍺-1,4 linkages inglycogen isglycogensynthase​

UDP released when the new⍺-1,4 linkage is formed can beconverted back to UTP by thenucleosidediphosphokinase​
UDP + ATP UTP + ADP

The Glycogen branching enzyme (amylo-⍺(1:4)→⍺(1:6)transglycosylase) transfers the end of one chain to an earlier part of the chain by a 1,6-glycosidic bond