Lipogenesis and PPP

Question 1

Recall the overall scheme for the synthesis of fatty acids from glucose

Answer 1

acetyl CoA makes FA
- reductive pathway that uses NADPH
occurs in liver and adipose tissue
process happens in cytoplasm

glucose -> G6P –> Pyruvate –> acetyl CoA –> citrate –> acetyl CoA –> malonyl CoA –> FA

Question 2

PPP

Answer 2

Pentose phosphate pathway
–> makes the reductant : NADPH
uses the enzyme G6PDH (G6Pdehydrogenase)
converts G6P and produces 1 NADPH molecule
and another NADPH molecule after other steps and intermediates

1) takes G6P out of glycolysis
2) generates 2 NADPH molecules
3) left with a 5C (hence the name pentose??) that cannot go back into glycolysis/gluconeogenesis
4) lots of enzymes and steps to get back to 3C/6C

Question 3

G6PDH

Answer 3

G6P dehydrogenase
produces NADPH
can also act as antioxidant
–> absence of this enzyme can cause anemia and RBC become bad at carrying oxygen
=> also more likely for oxidative damage to occur (since no NADPH means no antioxidant)

Question 4

Understand the reaction catalysed byACC

Answer 4

ACC = acetyl CoA Carboxylase
forms malonyl CoA (3C)
* takes a bicarbonate ion in cell to add CO2 onto end of acetyl CoA
* involves use of ATP and biotin
* malonyl CoA : activated and primed for lipogenesis

ATP is only used in this step of FA synthesis

Question 5

Understand the regulation of ACC

Answer 5

ACC = acetyl CoA Carboxylase
like PDH (pyruvate dehydrogenase) and GS (glycogen synthase)

[ACC is inactive when phosphorylated by AMP-activated protein kinase
Active when dephosphorylated by protein phosphotase 2A]

==> active when dephosphorylated
1. stimulated by insulin
2. stimulated by citrate (can switch on phosphotase = activate the enzyme)
3. inhibited by the product (fatty acyl CoA)

Question 6

Describe the reaction sequence of FAS

Answer 6

FAS : fatty acyl synthase
* malonyl CoA synthesised by ACC binds to FAS
* decarboxylated; CO2 that was added previously by ACC is now lost
* negative charge leaves it activated, attacks chain next to it to join the 2 chains together
* NADPH –> NADP to reduce the C=O bond into C-OH
* dehydrate C=C-OH into C=CH
* NADPH –> NADP reduction again to remove the double bonds into single bonds CH3CH2CH2

each round uses 2 NADPH
no ATP used
usually 14-16 carbons long
not all FAs are saturated though! (unsaturated = double bonds)

Question 7

Describe the process of esterification

Answer 7

glycerol-3-phosphate + 3 FA-CoA = makes triacylglycerol (triglycerides)
produces 3 H2O molecules and 3 ester bonds

FAs are joined to CoAs after they dissocaite from FAS

esterification enzyme is also upregulated by insulin (includingn GLUT-4, GS, PDH, ACC and FAS)

glycerol-3-phosphate can be made and phosphorylated from glycerol using the enzyme glyercolkinase (GyK) but this only occurs in the liver
–> WAT steals G3P from glycolysis

Question 8

G3P generation

Answer 8

glucose –> G6P –> dihydroxyacetone phosphate (DHAP)
DHAP forms G3P
glycerol can also produce G3P using GyK in the liver
G3P combines with FA-CoAs to form triacylglyercides.

Question 9

The effect of insulin on the activity of pyruvate dehydrogenase

Answer 9

PDH needs to be swtiched on to oxidise pyruvate into acetylCoA
insulin acitvates PDH by stimulating the PDH phosphotase
PDH phosphotase dephosphorylates the enzyme into its active form (to allow it to convert Pyruvate –> acetyl CoA)

Question 10

Evaluate the probability of the different fates of acetyl CoA in lipogenic tissue

Answer 10

For lipogenesis :
acetyl Coa enters krebs cycle for just one step to convert into citrate (citrate can be transported from mitochondria, out into cytoplasm and then back into acetyl CoA)

however:
acetyl CoA –> malonyl CoA requires ATP
need to go through a couple rounds of Krebs to produce ATP and provide the energy

[PDH requires CoA so converting acetyl CoA–> citrate gives back CoA for PDH to continue its function]

Question 11

Explain the role of citrate in controlling the major anabolic and catabolic pathways

Answer 11

lipogenesis:
citrate (also a regulator of ACC) in cytoplasm can stimulate ACC for FA synthesis

Gluconeogenesis:
Citrate can be converted back into oxaloacetate in the cytoplasm, providing a substrate for gluconeogenesis.

Feedback Inhibition:
Citrate regulates its own synthesis by feedback inhibition of the enzyme phosphofructokinase-1 (PFK-1) in glycolysis

Question 12

Outline the mechanisms for the return of oxaloacetate to the mitochondria after citrate cleavage

Answer 12

citrate in cytoplasm uses ATP citrate lyase (ACL) to go citrate in cytoplasm = acetyl CoA + oxaloacetate
1. acetyl CoA + oxalo = citrate (mitochondria)
2. citrate transported into cytoplasm
3. ACL breaks of acetyl CoA from citrate and leaves oxalo behind
4. oxalo reduce into malate (using NADH –> NAD)
5. malate oxidised into pyruvate (produces NADPH) and returns to mitochondria
6. NADPH is the reductant needed for FA synthesis

Question 13

**

malonyl CoA inhibition

Answer 13

malonyl CoA can inhibit transport of FA back into mitochondria –> swithces off beta oxidation
no FA oxidation when we want to make it!

how it works:
inhibits CAT-1 (carnitine acyltransferase 1)
–> while we’re synthesising FA, we don’t want to be transporting FA into mitochondria to oxidise them

Question 14

SUMMARY

Answer 14

insulin regulates GLUT-4, GS, PDH, ACC, FS, and the esterification enzyme

1. glucose enters the cell through GLUT-4
2. glucose –> G6P
   1) G6P used to make glycogen (GS) in liver
   2) G6P taken for PPP
   3) G6P –> pyruvate
3. pyruvate (PDH)–> acetyl CoA
4. acetyl CoA –> citrate
5. citrate into Krebs or transported out of mitochondria into cytoplasm
6. citrate –> acetyl CoA + oxaloacetate
7. acetyl CoA (ACC) –> malonyl CoA (FAS) –> FA
8. FA-CoAs + G3P (esterification) = triacylglyceride