MS-1 FUND 1: 09:10-10:00
Thursday, Sept. 4, 2014
Dr. Young
Integration of Metabolism
Transcriber: Sushma Boppana
Editor: Ramya Singireddy
Page 1 of 3
Abbreviations: Fatty acids (FA), glucose-6-phosphate (G6P), protein phosphatase 1 (PP1), lipoprotein lipase (LPL), fatty acid synthase (FAS), acetylCoA carboxylase 1 (ACC1), hormone-sensitive lipase (HSL), red blood cell (RBC)
Introductory Comments: Second hour of Integration of Metabolism lecture by Dr. Young.
I.
II.
III.
IV.
V.
Clarification from last hour (slide 21) 48:10
a. when blood glucose levels are high (fed state), liver wants to store excess calories as glycogen or make fatty
acids (FA)
b. FA exported into circulation by VLDL and then go to adipose tissues
c. All enzymes set up to put glucose into glycogen or FAs - a long route to get from glucose to fatty acids
Fed state – skeletal muscle (slide 22) 51:45
a. Insulin levels go up, binds to receptor
i. Activates GLUT 4, increases glucose levels in cell, which increases glucose-6-phosphate (G6P) levels via
hexokinase, this activates glycogen synthase
1. Promotes glycogen synthesis
ii. Leads to activation of protein phosphatase 1 (PP1) which dephosphorylates and activates glycogen
synthase, and more (inhibit inhibitor 1, activate glycogen binding subunit, inhibit phosphorylase kinase)
1. Switch off glycogen breakdown, promote glycogen synthesis
iii. Insulin: promotes uptake of amino acids, is a very potent stimulator of protein synthesis, and is a strong
anabolic signal
1. Greatly increases protein synthesis
2. Also shuts off proteolysis (reciprocal pathways)
Fed state – adipose (slide 23) 52:58
a. Pathway for storage of excess triglycerides
b. For example, if we eat a carbohydrate-rich meal, the liver makes FA which is packaged in VLDL and enters into
circulation. Then lipoprotein lipase (LPL) will be activated by insulin.
c. LPL will hydroglyze FA off of triglycerides and FA can be taken up into cell
d. Insulin activates GLUT4 to take up more glucose
i. Glucose converted to FA via fatty acid synthase (FAS) and acetyl-CoA carboxylase 1 (ACC1) which are
also activated by insulin
e. Big issue with dieting – want to keep insulin down because insulin does everything it can to store fat and it
switches off lipolysis (inhibits hormone sensitive lipase – HSL)
Maintenance of blood glucose levels – fasting (slide 24) 55:00
a. Not taking in glucose from diet, have to increase production of glucose by liver
b. Decrease use of glucose by skeletal muscle, adipose, and liver
c. Leave glucose for cells that really need it – red blood cells (RBC) and brain
d. Glucose sources during fasting/starvation (graph on slide 25)
i. Data from humans, fasted against their will for 40 days
ii. Initially have enough glucose in circulation to survive 4 hours of fasting
iii. Increase breakdown of glycogen within store to maintain blood glucose levels (in skeletal muscle and liver)
1. Only so much glycogen, after 1.5 days liver glycogen store is depleted
iv. After 2 days of fasting relying 100% on gluconeogenesis to keep you alive
1. Resting blood glucose levels resets to 4 mM (instead of regular level of 5.5mM)
2. You can still survive because brain begins to use other molecules for fuel instead of glucose
e. Post-feeding fast (16-20 hours after a meal)
i. You have high rates of lipolysis: insulin levels go down, glucagon levels go up, HSL is activated,
triglycerides are broken down, and glycerol is released into circulation.
1. Glycerol in liver is used as gluconeogenic precursor
2. FA used by skeletal muscle and liver
a. Drop in insulin levels prevents glucose utilization by the muscle and the muscle seeks to use
alternative fuels (such as FAs).
b. In liver, FA provides ATP to drive gluconeogenesis. Also, the liver has lots of acetyl-coA from
β-oxidation, so this is converted to ketone-bodies. Ketone bodies are an alternative fuel
during prolonged starvation for tissues including the skeletal muscle, kidneys, and the brain.
What are ketone bodes? (slide 27) 59:24
a. FA released during starvation are broken down in liver to acetyl-CoA then into ketone bodies
b. Ketone bodies can be used as an alternative fuel source
c. Three ketone bodies
i. Acetoacetate and D-3-hydroxybutyrate are the two main ketone bodies (can be utilized by the body)
ii. Acetone byproduct of acetoacetate (unstable) but cannot be metabolized by humans, so we breath it out
1. Used as indicator of ketoacidosis (can smell acetone on breath)
MS-1 FUND 1: 09:10-10:00
Thursday, Sept. 4, 2014
Dr. Young
Integration of Metabolism
Transcriber: Sushma Boppana
Editor: Ramya Singireddy
Page 2 of 3
Abbreviations: Fatty acids (FA), glucose-6-phosphate (G6P), protein phosphatase 1 (PP1), lipoprotein lipase (LPL), fatty acid synthase (FAS), acetylCoA carboxylase 1 (ACC1), hormone-sensitive lipase (HSL), red blood cell (RBC)
2. Ketoacidosis typically seen when on Atkins diet, prolonged starvation, uncontrolled Type I diabetes
VI. Synthesis of Ketone bodies
a. Occurs in fasting/starvation state. Occurs in mitochondrial matrix of liver cells. Has accompanying high rates of
lipolysis and β oxidation
b. Synthesis of acetoacetate
i. 3 enzymes involved
ii. Step 1: Start with two acetyl-CoAs (from β oxidation) and convert to acetoacetyl CoA by β-ketothiolase
1. β-ketothiolase is the last enzyme from β oxidation and works in reverse in this reaction compared to
β oxidation
iii. Step 2: Acetoacetyl CoA is converted to HMGCoA intermediate by adding another acetyl-CoA (catalyzed
by HMGCoA synthase)
iv. Step 3: Take off acetyl-CoA from HMGCoA to make the final product, acetoacetate (catalyzed by HMGCoA
cleavage enzyme)
1. HMGCoA synthase and HMGCoA cleavage enzyme – 2 unique enzymes for ketone body synthesis
c. Synthesis of β-hydroxybutyrate and acetone
i. Acetoacetate used to make the other two ketone bodies
ii. Use β-hydroxybutyrate dehydrogenase to make β-hydroxybutyrate – reduction reaction
iii. Slow, spontaneous decarboxylation makes acetone
d. Relative proportions of ketone bodies synthesized
i. Quantitatively less formed than the other two ketone bodies. Have to excrete acetone, can smell on breath
of ketotic people
ii. There is more βhydroxybutyrate naturally than acetoacetate (2-5 times greater). During fasting you have
high rates of β oxidation and increased NADH/NAD+ ratio.
1. NADH pushes reaction towards β-hydroxybutyrate
e. Utilization of ketone bodies by peripheral tissues
i. Go into circulation (leave mitochondria, liver) to be utilized by peripheral tissues
ii. One more unique enzyme – β ketoacyl-CoA transferase
1. This pathway also uses β hydroxybutyrate dehydrogenase and β ketothiolase (from ketone
synthesis)
iii. Step 1: β hydroxybutyrate is converted to Acetoacetate by β hydroxybutyrate dehydrogenase
iv. Step 2: Acetoacetate accepts CoA from Succinyl CoA to make acetoacetylCoA (catalyzed by β ketoacylCoA transferase)
v. Step 3: AcetoacetylCoA is then cleaved to make two acetyl-CoA by β ketothiolase
vi. Ketone bodies are transportable forms of acetylCoA because AcetylCoA by itself is too big to be
transported across membranes.
vii. β ketoacyl-CoA transferase very important in utilizing ketone bodies.
f. How are synthesis and degradation regulated?
i. Substrate availability:
1. Acetyl-CoA abundant during starvation because high rates of β oxidation
2. When fasting, ketone body level also rises
ii. Futile cycle prevented by gene expression level regulation (tissue specific gene expression)
1. Liver has HMGCoA synthase and cleavage enzyme but does not have β-ketoacyl-CoA transferase.
So, it can only produce ketone bodies but cannot utilize them.
2. Some peripheral tissues have high levels of β-ketoacyl-CoA transferase – can use ketone bodies
instantly
a. Muscle, heart, renal cortex
3. Brain can adapt to ketone body utilization, can turn gene for the transferase on during prolonged
starvation
VII. Interdependence between ketone body formation, FA oxidation, and gluconeogenesis in liver (slide 37)
1:10:40
a. During starvation, the body needs to maintain blood glucose levels
i. Decrease use of glucose by peripheral tissues
ii. Increase use of alternative fuels
iii. Increase gluconeogenesis in liver
b. Signal: insulin drops, glucagon goes up, adrenaline and lipolytic hormones will go up
c. Integration – Increase in lipolysis provides a lot of fatty acids into circulation. FAs are used as alternative fuel
(muscle will leave glucose alone) and the liver will use FAs to make ketone bodies, which are another
alternative source  peripheral tissues leave blood glucose alone
MS-1 FUND 1: 09:10-10:00
Thursday, Sept. 4, 2014
Dr. Young
Integration of Metabolism
Transcriber: Sushma Boppana
Editor: Ramya Singireddy
Page 3 of 3
Abbreviations: Fatty acids (FA), glucose-6-phosphate (G6P), protein phosphatase 1 (PP1), lipoprotein lipase (LPL), fatty acid synthase (FAS), acetylCoA carboxylase 1 (ACC1), hormone-sensitive lipase (HSL), red blood cell (RBC)
i. Burning fat through β oxidation will provide energy (not carbon) to drive gluconeogenesis
d. Why would a defect in β oxidation lead to hypoglycemia during fasting? If you cannot perform β oxidation during
fasting, the liver will not have energy for gluconeogenesis and also have problem with ketogenesis – it cannot
make acetylCoA from FAs. The muscle will use FA and glucose, but you are not producing glucose, so you
deplete fuel sources which leads to hypoglycemia.
VIII. Circulating glucose, FA, and ketone bodies during prolonged starvation (slide 39) 1:14:10
a. Glucose relatively maintained, drops and stays at 3.5 mM and stay for a while
b. FA go up, 3x as up in circulation
c. After 2 days of fasting, levels of ketone bodies shoot up, helps maintain blood glucose levels
i. Used by lots of tissues, including brain
d. After about 1 day, start breaking down proteins – amino acids provide gluconeogenic and ketonic precursors
i. Have to get rid of nitrogen via urea cycle
IX. End of Lecture Take-Away Points (slides 41-on) 1:16:30
a. α-ketoglutarate is the common nitrogen acceptor or all reactions involving transaminase reactions
i. See this a lot in urea cycle
b. Tryptophan is an important precursor to nicotinamide (important for NAD), serotonin (mood), and melatonin
(sleep)
c. Quantitatively chylomicrons and VLDL are the major carriers of triacylglycerols.
i. Physiologically, HDL and LDL are the more important ones for cardiovascular health.
d. Triacylglycerols are packaged into chylomicrons in the intestinal epithelial cell.
i. Liver makes VLDL
e. Arachidonic acid is the parent compound for prostaglandins.
i. Coming from linoleate (an essential FA)
f. Phosphoribosyl pyrophosphate (PRPP) is important for purine and pyrimidine synthesis as well as salvage
pathways
i. IMP is a purine precursor, can’t make pyrimidines from it
g. Folic acid directly affects methylation (1 carbon metabolism by adding a methyl group or a formyl group)
h. Carbon monoxide inhibits the cytochrome oxidase (complex IV) in the electron transport chain
i.
Slides 49-59 were not discussed.
Student Question 1: Can acetyl-CoA go directly from glycolysis to fatty acid synthesis?
Answer: No, there is a separation between cytosol and mitochondria in these processes. Don’t have an enzyme
in cytosol to convert pyruvate to acetyl-CoA, have to do that in mitochondria; then don’t have a transporter for
acetyl CoA to go back into cytosol so has to be converted to citrate with 1 st enzyme of TCA cycle. Citrate, once
transported to cytosol, can be lysed by citrate lyase to remake acetyl-CoA. Symbiosis – back when mitochondria
were bacteria
<END OF LECTURE 1:37:10>