Exam 5

Question 1

What are triacylglycerols and what are the stages of their catabolism

Answer 1

Triacylglycerols are lipids used for energy storage. They have a glycerol backbone and fatty acid chains that are attached via ester bond

Stage 1: Lypolysis
Stage 2: Transport/Activation

Question 1

What is lipolysis?

Answer 1

Process that separates glycerol backbone from fatty acids and uses 3 different types of lypases

Question 2

What controls lypases?

Answer 2

low energy state indicators: glucagon and epinephrine

Perilipin - restructures lipid droplet, releases coactivator (CA) ATGL

Question 3

Describe Stage 1: Lipolysis

Answer 3

Hormones, epinephrine & glucagon, stimulate lipolysis - insulin inhibits it by promoting lipid storage

Triacylglyceride —-> Glycerol + Fatty Acids via lipase

When glycerol is absorbed by the liver:
Glycerol attacked by glycerol kinase, using ATP –> ADP

Glycerol —> Glycerol 3-phosphate

Glycerol 3-phosphate is attacked by Glycerol Phosphate Dehydrogenase, using NAD+ —> NADH + H+

Glycerol 3-phosphate —> Dihydroxyacetone phosphate <—-> Glyceraldehyde 3-phosphate

The last two are glycolytic/gluconeogenic intermediates

Question 4

Where are lipolysis, fatty acid activation, and B-Oxidation occurring?

Answer 4

Lipolysis: takes place in adipose tissue, where triglycerides are broken down into glycerol and free fatty acids

Fatty acid activation: occurs in cytosol, where fatty acids are activated by conversion into acyl-CoA before being transported into mitochondria

B-Oxidation: occurs in mitochondrial matrix, where activated fatty acids undergo sequential breakdown to generate acetyl-CoA, NADH, and FADH2 for energy production

Question 5

Name the enzymes and lipases involved in lipolysis

Answer 5

1. Adipose Triglyceride Lipase (ATGL): enzyme catalyzes the first step in lipolysis by hydrolyzing triglycerides into diglycerides and free fatty acids
2. Hormone-Sensitive Lipase (HSL): Once ATGL is done, HSL steps in to further break down diglycerides into monoglycerides and additional free fatty acids
3. Monoacylglycerol Lipase (MAGL): enzyme completes the process by hydrolyzing monoglycerides, releasing the final glycerol molecule and last fatty acid

• Perilipins: These regulate lipase activity by controlling access of lipases to lipid droplets in adipocytes
• Protein Kinase A (PKA): enzyme that activates HSL in response to hormones like epinephrine and glucagon

Question 6

Name enzymes that cleave fatty acids from glycerol backbone

Answer 6

ATGL - converts triglycerides into diglycerides
HSL - converts diglycerides into monoglycerides
MAGL - hydrolyzes monoglycerides, releasing glycerol

Question 7

How is the glycerol backbone metabolized?

Answer 7

Glycerol is transported to the liver, where it undergoes phosphorylation by glycerol kinase, converting it to glycerol-3-phosphate

It’s then oxidized by glycerol-3-phosphate dehydrogenase, forming DHAP - an intermediate that enters glycolysis/gluconeogenesis

Question 8

Describe Stage 2: Fatty acid degradation (B-oxidation)

Answer 8

Fatty Acid Activation: occurs in cytosol. Fatty acyl-CoA synthetase (acyl-CoA ligase) activates free fatty acids by attaching CoA, forming fatty acyl-CoA. This requires ATP, which gets converted into AMP+ PPi

Transport into mitochondrial matrix:
1. Carnitine Shuttle: fatty acyl-CoA cannot cross the mitochondrial membrane directly, so it uses the carnitine shuttle:
- CPT-I converts fatty acyl-CoA –> acyl-carnitine in the outer mitochondrial membrane
- Acyl-carnitine is transported into the mitochondrial matrix via carnitine-acylcarnitine translocase
- CPT-II regenerates fatty acyl-CoA inside the mitchondrial matrix

B-Oxidation
All steps (1-4) occur on B carbon:
1. Oxidation
2. Hydration
3. Oxidation
4. Cleavage

Question 9

What are the enzymes used in the transportation/activation stage and where are they located?

Answer 9

1. Activation of fatty acids (cytosol)
   Enzyme: fatty acyl-CoA synthetase (AKA acyl-CoA ligase)
   Location: cytosol (outer mitochondrial membrane)
   Function: catalyzes the attachment of CoA to fatty acids, forming fatty acyl-CoA (an activated form)
2. Transport of Fatty Acids via Carnitine Shuttle
   Step 1: Conversion to Acyl-Carnitine
   Enzyme: CPT-I
   Location: Outer mitochondrial membrane
   Function: transfers acyl-CoA to carnitine, forming acyl-carnitine, which is then able to cross into mitochondriaStep 2: Transport into Matrix
   Enzyme: Carnitine-acylcarnitine translocase
   Location: inner mitochondrial membrane
   Function: exchanges acyl-carnitine for free carnitine, allowing acyl-carnitine to enter matrixStep 3: Regeneration of Fatty Acyl-CoA
   Enzyme: CPT-II
   Location: inner mitochondrial membrane
   Function: converts acyl-carnitine back into fatty acyl-CoA for B-Oxidation

Question 10

What does each round of B-Oxidation do?

Answer 10

shortens fatty acid by 2 carbons until fully degraded

Question 11

What additional enzymes are used for unsaturated fatty acid degradation?

Answer 11

Isomerase: converts cis double bonds into trans for B-oxidation

Reductase: helps metabolize polyunsaturated fatty acids

Question 12

Describe Unsaturated fatty acids

Answer 12

Even # of double bonds: require BOTH isomerase and reductase - produce SAME # of ATP as saturated fatty acid
- After oxidation, FADH2 and a diene are formed but dienes cannot be used as a substrate
- Reductase reduces diene
- Isomerase moves double bond

Odd # of double bonds: require ONLY isomerase - produce FEWER # of ATP as saturated fatty acid
- Isomerase moves double bond from C3=C4 to C2=C3, giving us the product that’d usually occur after 1st step of oxidation
- the rest of B-Oxidation yields: acyl CoA, Acetyl CoA, and NADH
- NO FADH2 generated b/e first step of oxidation was skipped

Question 13

How to find the stoichiometry for the oxidation of a fatty acid of a given length

Answer 13

(n/2 - 1) = rounds of oxidation are needed, NADH are produced, and FADH2 produced

(n/2) = acetyl CoA are produced

For C16:
Rounds of B-oxidation: 7 cycles
NADH produced: 7 molecules
FADH2 produced: 7 molecules
Acetyl-CoA produced: 8 molecules

Question 14

How to find the stoichiometry for the synthesis of a fatty acid of a given length

Answer 14

(n/2 - 1) = rounds of synthesis needed

(n/2) = acetyl CoA consumed

(n-1) ATP consumed
(1 ATP per malonyl CoA formed)
( 1 ATP per ATP-citrate lyase rxn)

2 NADPH consumed per round of synthesis

To make Palmitate (C16):
Acetyl-CoA required: 8 molecules
ATP required: 7 molecules (for carboxylation to malonyl-CoA)
NADPH required: 14 molecules (for reductions)

Question 15

Oxidation of sugars vs. oxidation of fats

Answer 15

Fats provide MORE ATP per carbon than sugars
- more reduced a carbon atom is, the more free energy is released upon oxidation
Most energy: methane (CH4)
Least energy: carbon dioxide (CO2)

Fats are more reduced —-> more H, less Oxygens

Sugars are more oxidized —> more O, less Hydrogens

Question 16

How can an inability to make insulin lead to excess production of ketone bodies and be life-threatening?

Answer 16

**Summary: **
1. OAA levels drop in liver
2. CAC slows in liver
3. Free fatty acids are released
4. Ketone bodies form (high acetyl CoA)
5. Blood pH drops (strong acid = ketone bodies)
6. Coma & death
Positive feedback loop

Type I diabetes - absolute deficiency of insulin. When insulin is absent, diabetic ketoacidosis (DKA) can occur (a condition where excess ketone body production leads to life-threatening acidosis

1. Insulin deficiency and its consequences
   - insulin normally promotes glucose uptake
   - Without insulin, glucose cannot enter cells, leading to hyperglycemia
   - The body perceives this lack of intracellular glucose as “starvation”, even though blood glucose levels are high
   - In response, glucagon secretion is increased, signaling the liver to produce more glucose via gluconeogenesis, worsening hyperglycemia
2. Shift to fat metabolism and excess ketone production
   - adipose tissue undergoes uncontrolled liploysis
   - liver converts the free fatty acids into acetyl-CoA, which would normally enter the Krebs cycle. However, w/o insulin, the Krebs cycle slows down b/e of the lack of intermediates
   
   • Excess acetyl-CoA is diverted into ketogenesis, producing ketone bodies: Acetoacetate, B-hydroxybutyrate, acetone)
     - excess ketone production leads to ketoacidosis, as ketone bodies are strong acids that lower blood pH
3. Coma & death

Question 17

What are the three stages of fatty acid synthesis?

Answer 17

Stage 1: transport of acetyl-CoA to cytoplasm
Stage 2: Formation of malonyl CoA (activation) - the committed step of fatty acid synthesis
Stage 3: Fatty Acid Elongation (Fatty acid synthase - FAS)

Question 18

Fatty acid synthesis production and consumption

Answer 18

Per round:
1 acetyl CoA consumed
2 NADPH consumed

Per molecule of acetyl CoA transferred to cytoplasm:
1 ATP consumed
1 NADPH produced

Question 19

Describe Stage 1 of fatty acid synthesis

Answer 19

Acetyl-CoA is generated in mitochondrial matrix (from pyruvate oxidation or fatty acid breakdown) but fatty acid synthesis occurs in the cytoplasm

Since acetyl-CoA cannot cross the mitochondrial membrane, it’s transported via the citrate shuttle:
- citrate synthase combines acetyl-CoA with oxaloacetate to form citrate
- citrate is exported to cytoplasm via the mitochondrial citrate transporter
- ATP-citrate lyase in the cytoplasm cleaves citrate back into acetyl-CoA and oxaloacetate
- Oxaloacetate is converted into malate or pyruvate, which can return to the mitochondria

Question 20

Describe Stage 2 of fatty acid synthesis

Answer 20

Formation of malonyl CoA (activation)

Activation of acetyl-CoA is the committed step, catalyzed by acetyl-CoA carboxylase (ACC):
Acetyl-CoA + ATP + HCO3^- —-> Malonyl-CoA

Malonyl-CoA is the key intermediate that drives fatty acid synthesis forward

ACC is heavily regulated to control fatty acid synthesis

Question 21

what is the enzyme that catalyzes the committed step of fatty acid synthesis?

Answer 21

Acetyl CoA carboxylase 1 (ACC 1)

Requires biotin as coenzyme

Question 22

Describe Stage 3 of fatty acid synthesis

Answer 22

Each round of fatty acid synthesis adds 2 carbon units using malonyl-CoA

1. Condensation
2. Reduction
3. Dehydration
4. Reduction

repeats until palmitate (C16) is formed

Question 23

How does the order of fatty acid synthesis compare to its degradation?

Answer 23

Fatty acid synthesis:
1. Condensation
2. Reduction
3. Dehydration
4. Reduction

Fatty acid degradation:
1. Oxidation
2. Hydration
3. Oxidation
4. Cleavage

Question 24

how are fatty acids elongated and desaturated form palmitate (C-16)?

Answer 24

Once palmitate (C16) is synthesized, it can undergo: Elongation: takes place in the endoplasmic reticulum, adding two-carbon units from malonyl-CoA Desaturation: occurs in endoplasmic reticulum, where fatty acyl-CoA desaturases introduce double bonds (requires oxygen and NADH)

Question 25

Describe the regulation of fatty acid metabolism by acetyl CoA carboxylase I and II - phosphorylation

Answer 25

Both I and II regulated by phosphorylation - (AMP kinase & Protein Phosphatase 2A) - when carboxylase is NOT phosphorylated, both carboxylase I and II enzyme is active and producing malonyl CoA

Question 26

Describe the regulation of acetyl CoA carboxylase I

Answer 26

Cytoplasm allosteric activator: citrate dephosphorylated = active Completely inactive if ONLY phosphorylated PARTIALLY active carboxylase when citrate and phosphoryl group are attached

Question 27

Describe the regulation of acetyl CoA carboxylase II

Answer 27

- inhibits fatty acid degradation to prevent the oxidation of fatty acids that were just made - mitochondrial malonyl CoA inhibits carnitine acyltransferase I - prevents entry of acetyl CoA into the mitochondria for fatty acid oxidation Citrate activates ACC-II, signaling that there's plenty of energy and promoting malonyl-CoA production, inhibiting CPT-I Palmitoyl-CoA inhibits ACC-II, preventing excessive malonyl-CoA formation, allowing fatty acids to enter mitochondria for oxidation Regulation via phosphorylation: AMP kinase phosphorylates ACC-II --> inhibits its activity, leading to reduced malonyl-CoA levels PP2A: dephosphorylates ACC-II ---> activates ACC-II High ACC-II activity = high malonyl CoA= fatty acid storage (blocks CPT-I) Low ACC-II activity (due to AMPK inhibition) = low malonyl-CoA = fatty acid oxidation (CPT-1 active)

Question 28

Where are ACC-I and ACC-II located?

Answer 28

ACC-I : cytoplasm ACC-II : mitochondria

Question 29

Regulation of acetyl CoA Carboxylase by hormones

Answer 29

High glucagon (low blood glucose), high epinephrine - stimulates lipolysis, providing free fatty acids for B-oxidation - activates AMP kinase, phosphorylating the carboxylases, inactivating them Inactive ACC I & II: fatty acid synthesis halts (carboxylase I) ; fatty acid degradation proceeds (carboxylase II) - no inhibition of carnitine acyltransferase Insulin (high blood glucose): stimulates fatty acid synthesis, activating protein phosphatase 2A - Active ACC I & II: fatty acid synthesis proceeds (carboxylase I) ; fatty acid degradation halts (carboxylase II) - inhibition of carnitine acyltransferase

Question 30

Describe phosphatidate

Answer 30

can be formed from: lipolysis and glycolysis intermediates Step 1: glycerol-3-phosphate formation - Option 1: glycerol-3-phosphate dehydrogenase reduces DHAP (from glycolysis) to form glycerol-3-phosphate - Option 2: triacylglyceride ---> glycerol via lypolysis. Then glycerol kinase can phosphorylate the glycerol to produce glycerol-3-phosphate Step 2: attachment of fatty acids: - acyltransferase enzymes add two fatty acids to glycerol-3-phosphate to form phosphatidate

Question 31

Where does lipid synthesis occur?

Answer 31

endoplasmic reticulum at same time, liver in the ER is synthesizing triacylglycerols

Question 32

Triacylglyerol synthesis

Answer 32

liver phosphatidate ---> DAG ---> triacylglycerol

Question 33

Basic structure of phsophoglyceride

Answer 33

glycerol backbone connected to two fatty acids on left, one phosphate on right connected to an alcohol

Question 34

What are the two ways phosphoglyceride can get its alcohol?

Answer 34

Scenario 1: - phosphatidate is activated - attaching phosphatidate to CTP forming CDP-diacylglycerol - CTP is activated carrier of phosphatidate or alcohols - activated phosphatidate (CDP-diacylglycerol) reacts with alcohol to form phosphoglyceride - can be modified with kinases Scenario 2: - an alcohol is activated - activated alcohol reacts with DAG to form phosphoglyceride

Question 35

What is the key regulatory enzyme in triacylglycerol and phosphoglyceride synthesis?

Answer 35

DGAT: catalyzes final step in triacylglycerol synthesis **PAP:** converts phosphatidate into diacylglycerol (DAG), regulating lipid synthesis - regulation: removal/addition of phosphoral group from phosphatidate - decreased activity of PAP = decrease in tracylglycerol synthesis

Question 36

Lipids with sphingolipid backbone and ceramide

Answer 36

requires two substrates: 1. palmitoyl CoA 2. Surine Palmitoyl CoA + Serine ---> Ceramide Ceramide = starting point for sphingolipids and glycolipids

Question 37

Sphingolipid vs Glycolipid

Answer 37

Sphingolipid: primary hydroxyl attached to a phosphorylated alcohol moiety Glycolipid: primary hydroxyl attached to a carbohydrate

Question 38

Cholesterol - three stages

Answer 38

1. formation of isopentyl pyrophosphate 2. formation of squalene from isopentyl pyrophosophate 3. cyclization of squalene primarily occurs in liver

Question 39

Committed step in formation of cholesterol

Answer 39

Occurs in Stage 1 (1.1 starts from acetyl CoA): HMG-CoA ---> mevalonate catalyzed by HMG-CoA reductase

Question 40

4 modes of Cholesterol regulation

Answer 40

High cholesterol = BAD No cholesterol at all = WORSE inability to create cholesterol affects: steroid production, regulation of membrane fluidity 4 modes: 1. transcriptional control of HMG-CoA reductase mRNA - control of how much mRNA is produced - regulates activity of sterol regulating element binding protein (SREBP) 2. Translational control of HMG-CoA reductase mRNA - high levels of mevalonate and dietary cholesterol inhibit translation 3. Degradation of HMG-CoA reductase protein - high cholesterol drives proteolytic degradation of the reductase 4. Phosphorylation of the HMG- CoA reductase protein - phosphorylation by AMP-dependent kinase inactivates the reductase

Question 41

Simplified 4 modes of cholesterol regulation

Answer 41

1. Transcriptional control: - SREBP controls HMG-CoA reductase gene expression 2. Translational control/feedback inhibition: - high cholesterol levels inhibit HMG-CoA reductase 3. Degradation 4. Phosphorylation

Question 42

Cholesterol and triacylglycerol transport

Answer 42

Lipoproteins transport lipids in the blood: VLDL (triacylglycerol transport) LDL (delivers cholesterol to tissues) HDL (removes excess cholesterol)

Question 43

Good vs Bad cholesterol

Answer 43

Bad: LDL - delivers cholesterol but can accumulate in arteries Good: HDL - removes excess cholesterol, reducing risk of heart disease

Question 44

Increasing LDL receptors

Answer 44

More LDL receptors on cell surfaces - increase cholesterol uptake - PCSK9 blocks LDL receptor cycling and expression on surface inhibit PCSK9 --> increase surface expression of LDL receptors ---> reduce LDL levels