TAG metabolism

Question 1

old glycerol-phosphate/ kennedy pathway

Answer 1

glycerol-3-phosphate + FA-CoA catalyzed by GPAT–> lysophosphatidate +FA CoA catalyzed by AGPAT –> phosphatidate catalyzed by PAP –> diacylglycerol + FA CoA catalyzed by DGAT –> TAG

Question 2

diseases involving GPATs

Answer 2

none known currently

Question 3

how many AGPATs are there, and where are they expressed?

Answer 3

5 AGPATs, expressed in all tissues (AGPAT2 highest in adipose), and all localized to the ER

Question 4

AGPAT diseases

Answer 4

lack of AGPAT2 gives rise to Berardinelli-Seip syndrome

Question 5

what is Berardinelli-Seip syndrome?

Answer 5

disease of abnormal fat accumulation; mutations in AGPAT2 gene–>almost no AGPAT activity; occurence: 1 in 10 million (very rare); various ethnic origins;

Question 6

symptoms of BS syndrome?

Answer 6

TG accumulates in liver and muscle; diabetes, hypertriglyceridemia–> risk factor for heart disease

Question 7

BS therapy?

Answer 7

restriction of dietary fat; minimize need to store TAG in adipose tissue so it doesn’t go to liver and muscle

Question 8

stages of liver damage in BS/abnormal fat accumulation in liver?

Answer 8

steatosis (fat deposits= liver enlargement); fibrosis (scar tissue forms)–>cirrhosis (growth of connective tissues destroys liver cells)

Question 9

fld mouse

Answer 9

fatty liver dystrophy; adipose tissue deficieny, small is size; enlarged pale liver (due to accumulation of TAGs); hypertriglyceridemia (prone to athersclerosis;

Question 10

what causes fld mouse?

Answer 10

mutation in lipin (lipin=PAP)

Question 11

lipin mutations in humans?

Answer 11

have been observed; however, no fat dystrophy
however, humans that have mutations in lipin gene–>have excessive accumulation of myoglobin in the circulation–very odd

Question 12

how was DGAT knocked out in mice?

Answer 12

replacement of specific region of a gene by hom recomb to make a nonfunctional protein–> replace gene with neo (a marker); targeting vector introduced into mouse mebryonic stem cells via electroporation (rare event)–> incorporate–>select cell with KO–>grow on plates

Question 13

what happens in DGAT1 KO mice?

Answer 13

do not become obese on high fat diet; still synthesize TAGs (to a lesser degree)

Question 14

how can DGAT KO mice still have TAGs?

Answer 14

there is another DGAT which was inhibited at high Mg conc in the assay–> not present that high in actual mice

Question 15

DGAT1 is an integral membrane protein in the ER membrane, but mainly found in ER lumen

Answer 15

ya

Question 16

DGAT2 is also an IM protein in the ER, but mainly found on the cytosolic side

Answer 16

ya

Question 17

what happens to DGAT2 KO mice?

Answer 17

die within hours of birth

–>DGAT2 maintains the skin permeability barrier–>DGAT2 -/- mice become dehydrated and cannot contain the water

Question 18

link between DGAT2 and skin barrier function?

Answer 18

lamellar membranes are not ordered–>allows lots of water to pass through; proper lamellar function is based on 3 lipids: cholesterol, linoleic acid, and ceramide
–linoleic acid is the most important in maintaining the integrity of the skin barrier–>TGs need to be incorporated into linoleic acid by DGAT2, but without no incorporation happens–> faulty membrane

Question 19

DGAT1 knockout experiment in humans

Answer 19

dGAT1 inhibitor given orally, TAG levels in blood measured after a meal, with and without inhibitor–> drug will inhibit TAG absorption

Question 20

how does DGAT1 inhibitor function?

Answer 20

inhibits TAG reabsorption by inhibiting resynthesis in the small intestine

Question 21

where is DGAT1 found?

Answer 21

in small instestine and adipose tissue

Question 22

DGAT1 -/- in humans symptoms

Answer 22

days after birth, children has frequent vomiting, diarrhea, low body weight, loss of protein, dehydration

Question 23

mutation of DGAT1–> a T turned to a C; having one good copy is sufficient, but being homozygous leads to problems
–> T to C mutation (in intron) causes skipping of exon 8; in frame deletion, 25 amino acids, but the active site is still present–>removing the exon 8 causes its rapid degradation

Answer 23

ya

Question 24

still possible to reduce DGAT2 activity in mice–>keep one good copy, happy healthy mice

Answer 24

ya