1 - Lipid Metabolism

Question 1

Fat Metabolism

Answer 1

Fat metabolism is closely associated with carbohydrate metabolism

Question 2

Fatty acids’ roles

Answer 2

1. Fatty acids’ roles include the following:
   
   1. Storage/transport as triacylglycerols for fuels
   2. Components of membrane sphingolipids and glycerophospholipids
   3. Precursors of eicosanoids
   4. Precursors of regulatory molecules

Question 3

Saturated fatty acids

Answer 3

1. Saturated fatty acids can be packed together very tightly on account of their geometry (linear and zigzagged), while unsaturated fatty acids do NOT pack well together because of their geometry (double bonds introduce bends in molecule). This explains the fact that saturated fatty acids have higher melting points than unsaturated fatty acids.
   
   1. Saturated ones can yield a lot of NADPH (a reducing agent)?
2. Saturated fatty acids have no double bonds, while unsaturated have one or more double bonds. They can be mono-unsaturated or polyunsaturated fatty acids (P.U.F.A)
3. Double bonds are almost always in the cis configuration
4. Double bonds are spaced at intervals of three carbons
5. Trans fats are the product of partial hydrogenation of cis-unsaturated fats. They have been strongly implicated in coronary heart disease and might be possibly associated with other diseases.
   
   1. Trans fats are bad in general; not found in nature

Question 4

Classification of fatty acids

Answer 4

1. In physiological systems, fatty acids are generally ionized at the ambient pH (RCOO-) and technically should be written with the suffix “ate,” not “oic acid”
2. The most common fatty acid in the human are palmitic (16:0) or palmatate and stearic acid (18:0)
   
   1. Structural lipids and triacylglycerols contain primarily fatty acids of at least sixteen carbons
3. Essential fatty acids:
   
   1. Linoleic acid - 18:2 (9,12)
      
      1. Omega-6 since 18-12=6
      2. liNoleic (N located 6 from the end)
   2. α-Linolenic acid - 18:3 (9,12,15)
      
      1. Omega-3 since 18-15=3
      2. linoleNic (N located 3 from the end)
   3. Essential fatty acids are polyunsaturated (PUFA)
   4. These 2 essential fatty acids are linoleic acid and α-linolenic acid
4. Precursor of postaglandins:
   
   1. Arachidonic acid - 20:4 (5,8,11,14)
5. Here, the second number designate number of double bonds there are; numbers in paranphases designate the location of the double bonds

Question 5

Lipid acid numbering

Answer 5

1. Carbon 2 is known as the alpha carbon; carbon 3 is the beta carbon, and so forth
2. The carbon of the terminal methyl is known as the omega (last) carbon regardless of the total length
3. The asterisk (in the book) designates one of the double bonds found in omega-6 (no need to know this point)
4. Dr. Marvit said we don’t have to know how to actually number any fatty acids (confirm it for me)

Question 6

Ovierview of the lecture

Answer 6

1. There are minimally 4 processes that will be discussed at length in this unit:
   
   1. Synthesis of fatty acids
   2. Synthesis of TAG’s
   3. Release of fatty acids from TAG’s
   4. Oxidation of fatty acids
2. Unless stated otherwise, all processes discussed in this lecture are in reference to palmitate (a long-chain saturated fatty acids containing 16 carbons)
3. Generally, fatty acid synthesis “de novo” (“de novo” = “from scratch”) occurs during times of excess energy. This is because fatty acids are supplied dietarily
4. Major site for fatty acid synthesis is liver. Other sites include: adipose tissue, mammary gland, brain, and kidney

Question 7

Carboxylation of Acetyl-CoA

Answer 7

1. The first committed step of fatty acid synthesis is the carboxylation of acetyl-CoA to malonyl-CoA
2. The carboxylation of acetyl CoA yields malonyl CoA
3. Acetyl CoA carboxylase (ACC):
   
   1. Utilizes biotin
      
      1. Biotin serves as a coenzyme in other carboxylations (pyruvate carboxylase)
   2. Carboxylation rxn requires ATP
4. The dimer of ACC is the inactive enzyme
5. The polymer of ACC is the the active enzyme
6. Citrate is involved in the fat metabolism (since it involves ATP for building things up)
7. CO₂ is added to Acetyl CoA and thus becomes biotin (blue-lettered part of Malonyl CoA in the figure)

Question 8

AMP-activated protein kinase

Answer 8

1. AMPK (AMP-activated protein kinase) phosphorylates ACC, thusly inactivating the enzyme
   
   1. AMP-activated means High level of AMP (or low level of ATP) activates AMPK
2. Glucagon and epinephrine both inactivate ACC
3. Insulin activates ACC
4. What effect would elevated levels of palmitoyl-CoA have on ACC?
   
   1. Allosteric control occurs as feedback inhibition by palmitoyl-CoA and activation by citrate.
   2. When there are high levels of palmitoyl-CoA, the final product of saturated fatty acid synthesis, it allosterically inactivates acetyl-CoA carboxylase to prevent a build-up of fatty acids in cells.
   3. Citrate acts to activate acetyl-CoA carboxylase under high levels, because high levels indicate that there is enough acetyl-CoA to feed into the Krebs cycle and produce energy

Question 9

Fatty acid synthase

Answer 9

1. In bacteria, fatty acid synthesis is catalyzed by six separate enzymes, plus acyl carrier protein
2. In mammals, fatty acid synthesis is catalyzed by separate domains of multi-catalytic, homo-dimeric, polypeptide, which includes acyl carrier protein
3. ACP contains phosphopantetheine, a derivative of pantothenic acid
4. 5-chloro-2-(2,4-dichlorophenoxy)phenol inhibts one of the bacterial enzymes associated with fatty acid synthesis
5. Questions associated with the usage of this product have arisen.
6. This product can be found in some:
   
   1. Soaps
   2. Cosmetics
   3. Deodorants
   4. Toothpastes
7. Triclosan is another example where its use is being questioned; used to prevent bacterial contamination

Question 10

Elongation cycle of fatty acid synthesis

Answer 10

1. Elongation cycle of fatty acid synthesis consits of 4 stages:
   
   1. Condensation of acetyl-ACP and malonyl-ACP, forming acetoacetyl-ACP
   2. Reduction of acetoacetyl-ACP, with the utilization of NADPH i.e. reductive =synthesis
   3. Dehydration
   4. Reduction with the utilization of NADPH

Question 11

Interrelationship between glucose metabolism and palmitate metabolism

Answer 11

(Dr. Marvit said details are not important; get the bigger picture)

1. The glycolytic pathway produces pyruvate, which is the primary source of the mitochondrail acetyl CoA to be used for fatty acid synthesis. It also produces cytosolic reducing equivalents of NADH. Pyruvate enters the mitochondria
2. Mitochondrial oxaloacetate (OAA) is produced by the first step in the gluconeogenic pathway
3. Acetyl CoA is produced in the mitochondria and condenses with OAA to form citrate, the first step in the tricarboxylic acid cycle
4. Citrate leaves the mitochondria and is cleaved in the cytosol to produce cytosolic acetyl CoA
5. Cytosolic reducing equivalents (NADH) produced during glycolysis contribute to the reduction of NADP+ to NADPH needed for palmitoyl CoA synthesis
6. The carbons of cytosolic acetyl CoA are used to synthesize palmitate, with NADPH as the source of reducing equivalents for the pathway

Question 12

Generation of NADPH from NADH

Answer 12

Understand the bigger picture first

Question 13

Palmitate

Answer 13

1. Palmatate (Palmitic acid)
   
   1. Excess carbohydrates in the body are converted to palmitic acid.
   2. Palmitic acid is the first fatty acid produced during fatty acid synthesis and the precursor to longer fatty acids.
   3. As a consequence, palmitic acid is a major body component of animals
   4. Palmitate negatively feeds back on acetyl-CoA carboxylase (ACC), which is responsible for converting acetyl-CoA to malonyl-CoA, which in turn is used to add to the growing acyl chain, thus preventing further palmitate generation.
2. Palmitate (palmitic acid) can undergo:
   
   1. Elongation in the smooth endoplasmic reticulum
      
      1. 16, 18, 20, etc… can be elongated
   2. Desaturation in the smooth endoplasmic reticulum i.e. double bond formation
   3. Incorporation into TAG

Question 14

Triacylglyceride (TAG)

Answer 14

1. Typically TAG consist of the following:
   
   1. Fatty acid at carbon 1 is saturated
   2. Fatty acid at carbon 2 is unsaturated
   3. Fatty acid at carbon 3 can be either saturated or unsaturated
2. Synthesis of TAG’s can occur in both liver and adipose tissue
   
   1. The glycerol released during TAG degradation can NOT be metabolized by adipocytes because they apparently lack glycerol kinase.
   2. Rather, glycerol is transported through the blood to the liver, where it can be phosphorylated. The resulting glycerol phosphate can be used to form TAG in the liver, or can be converted to DHAP by reversal of the glycerol phosphate dehydrogenase reaction; DHAP can
      participate in glycolysis or gluconeogenesis.
3. Most of the TAG in the body is stored in adipocytes. The liver packages the TAG into VLDL (very low density lipoprotein), which it secretes into the bloodstream and delivers to peripheral tissues
   
   1. Implications with child development at starvation state
   2. Chylomicron is another type of lipoprotein

Question 15

Glycerol

Answer 15

1. Glycerol can serve as the starting material for gluconeogenesis
2. In both liver and adipose tissue, glycerol phosphate can be produced from dihydroxyacetone phosphate (DHAP), an intermediary of glycolysis
3. The liver utilizes glycerol kinase to produce glycerol phosphate from glycerol. Glycerol kinase is not found in adipose tissue
4. The glucose transporter in adipocytes, GLUT-4, is insulin-dependent. What does this mean?
5. (Hint: if you answered that adipose production of TAG is controlled by plasma levels of glucose, you would be CORRECT)
6. Please remember that glycerol can serve as the starting material for gluconeogenesis. Please explain.–> *refer to the very first card*
7. The fact that there is glycerol in your system may be indicative of you being in starvation state where gluconeogenesis kicks in to make glucose

Question 16

Glycerol kinase

Answer 16

1. Glycerol kinase deficiency is an X-linked disorder associated with increased plasma glycerol levels in both plasma and urine. This disease can potentially lead to facial dysmorphisms, mental retardation, seizures, and diabetes

Question 17

Synthesis of TAG

Answer 17

1. Prior to incorporation into TAG, fatty acids are activated by attachment to CoA
2. This reaction is catalyzed by a thiokinase and is accompanied by the degradation of ATP to AMP and PPi

Question 18

Synthesis of TAG (cont.)

Answer 18

1. CoA is NOT incorporated into fatty acid when synthesizing for TAG
2. Enzymes involved are acyltransferases
3. Phosphatase powers up the reaction, leaving diacylglycerol behind.
4. (Dr. Marvit said specific names on the figure are not that important)

Question 19

Hormonal activation of TAG degradation in adipocyte

Answer 19

1. Hormone sensitive lipase can be activated by such hormones as glucagon, epinephrine, etc.
2. HSL can be inhibited by insulin and other factors
3. FFA (free or unesterified) fatty acids diffuse through the adipocyte membrane and bind to plasma albumin. The FFA’s can enter cells, in which they are oxidized
4. Insulin induces TAG synthesis
5. Epinephrine induces TAG degradation
6. TAG degradation occurs in adipocytes

Question 20

Carnitine

Answer 20

1. Carnitine is a biomolecule synthesized in the liver and kidneys from methionine and lysine
2. Carnitine can be obtained from the diet i.e. meat - “carnivore”
3. Carnitine deficiencies may be primary or secondary to other metabolic disorders
4. What effect would malonyl CoA have on the carnitine shuttle, and why?
   
   1. Malonyl CoA wil inhibit carnitine shuttle
   2. Malonyl CoA inhibits CPT-I, thus preventing the entry of long-chain acyl groups into the mitochondrial matrix. Therefore, when fatty acid synthesis is occurring in the cytosol (as indicated by the presence of malonyl CoA), the newly made palmitate cannot be transferred
      into the mitochondria and degraded.
5. Fatty acids are very much used as fuel in many occasions
6. Carnitine is industralized as pills for people for weight loss and (for same mechanism) extra energy for atheletes –> considered vogus
7. CoA leaves and Carnitine is put in place, then Carnitine is again replaced for CoA for the end product; essential for fatty acid to get into mitochondrial matrix so it can be used for energy

Question 21

Beta-oxidation of palmitate

Answer 21

1. The stoichimetric representation for the beta-oxidation (i.e. oxidationof palmitate is :
   
   1. Palmitoyl-CoA + 7FAD + 7NAD⁺ +7CoA + 7H₂O →8Acetyl-CoA + 7FADH₂+ 7NADH + 7H⁺
2. Activation of palmitate to palmitoyl CoA involves the hydrolysis of one molecule of ATP to AMP and PPi (which is hydrolyzed by pyrophosphatase to 2Pi)
3. The net energy yield from the beta-oxidation of palmitate is 129 ATP

Question 22

Beta-oxidation of palmitoyl CoA (cont.)

Answer 22

• Look at this notecard on computer for the figure

1. Beta-oxidation is called that way since it ‘chops’ two Carbons at a time
2. It can only go so far as C₂; can’t further divide up the last C-C molecule for generating FADH2 or NADH (but can be used for Acetyl CoA)
3. Thus if you have C_n molecules, you will yield following number of Acetyl CoA, FADH₂, and NADH accordingly:
   
   1. n/2 = # Acetyl CoA generated
   2. n/2 - 1 = # FADH₂ and NADH generated
4. Number of ATP each molecule can generate:
   
   1. FADH2 ==> 2 ATP via CoQ of ETC
   2. NADH ==> 3 ATP via Complex of ETC
   3. Acetyl CoA ==> 12 ATP via TCA cycle
5. Beta-oxidation does NOT apply to short chain fatty acids e.g. C4 but only for long chains
6. For a C16 fatty acid, the total number of ATP generated would be 14 ATP + 21 ATP + 96 ATP = 131 ATP - 2 ATP (where is this from?) = 129 ATP
7. This explains why fatty acids (9 Kcal) yield more Kcal than carbohydrates (4 Kcal); this is about 2-3 times more ATP than glycolysis

Question 23

Beta-oxidation (cont.)

Answer 23

1. The reactions of beta-oxidation consist of the following:
   
   1. Dehydrogenation (yielding FADH2)
   2. Hydration
   3. Dehydrogenation (yielding NADH)
   4. Thiolysis (cleavage of C-C bond with addition of CoA [contain thiol group])

Question 24

Summary of synthesis and degradation of long-chained, even-numbered, saturated fatty acids

Answer 24

Question 25

Summary of Synthesis and degradation of long chain, saturated, fatty acid (cont.)

Answer 25

* The process recycyles itself over and over

Question 26

Decrease in glucose level

Answer 26

1. As glucose availability decreases (due to not enough insulin or diet [no carb.]), the following can occur: 1. The decrease in circulating insulin levels leads to release of fatty acids into blood stream and uptake by liver 2. High levels of free fatty acids in liver inhibit fatty acid synthesis and lead to increased oxidation of fatty acids to acetyl Co-A i.e. build-up of acetyl CoA 3. The oxidation of fatty acids to acetyl Co-A results in an increase in the level of high energy molecules, which slows down TCA cycle in liver cells; build-up of acetyl CoA and no where to go from there --\> problematic 4. Ketone body diabetes type I

Question 27

Ketone body synthesis

Answer 27

1. Ketone bodies include the following: 1. Acetoacetate 2. 3-Hydroxybutyrate 3. Acetone (NOT a ketone body?) 2. The accumulation of liver acetyl Co-A diverts the oxaloacetate towards gluconeogenesis 3. Acetyl Co-A accumulates, leading to the production of ketone bodies 4. Acetoacetate is a ketone, while 3-hydroxybutyrate is not a ketone, but an acid 5. Acetoacetate can spontaneously be converted to acetone. The presence of acetone on one’s breath can be associated with diabetes 6. Acetone is a volatile compound that is not metabolized in the body 7. Acetone has a fruity-like odor 8. In the figure, just know the three molecules but don't worry about what's in between

Question 28

Ketogenesis (cont.)

Answer 28

1. Ketogenesisis always occurring on a basal level 2. Ketogenesis occurs in liver mitochondria (liver makes it but cannot be used itself) 3. Both the liver and red blood cells are not capable of using ketone bodies as fuel 4. Most tissues (including brain and skeletal muscle) can utilize ketone bodies as fuels 5. The brain can use ketone bodies as fuel but can NOT use fatty acids 6. Ketone bodies are water soluble 7. The utilization of ketone bodies is glucose-sparing 8. Ketone bodies are synthesized in the liver and are exported to peripheral tissues for aerobic energy utilization 1. Liver is missing *Thiophorase* so it itself can NOT use the ketone body as an energy 2. Acetyl-CoA goes through irreversible process here; instead of letting Acetyl-CoA buildup, peripheral tissues take up and use it to create energy from it; only at a base level so not a lot of yield 9. Ketone body synthesis increases under certain conditions 1. This state may be produced by a fast of less than 24 hours 2. High fat, low carbohydrate diets tend to produce increase ketone body synthesis

Question 29

Ketone related complications

Answer 29

1. *Ketonemia* is associated with starvation 1. Under non-starving conditions the brain utilizes glucose as its metabolic fuel 2. Fatty acids cannot cross the blood-brain barrier 3. After 3 days of starvation, ketone body oxidation accounts for over half of the brain’s energy source 2. Diabetic ketoacidosis (DKA) 1. acidic since acetoacetate and the other molecules (butyrate) is acidic 3. Ketonemia can be associated with poorly controlled *type I diabetes* 1. Absence of insulin can lead to the inhibition of glycolysis and lipogenesis 2. Absence of insulin can lead to stimulation of glycogenolysis, lipolysis, gluconeogenesis, and ketogenesis 3. Overproduction of ketone bodies can lead to metabolic *acidosis* 4. Osmotic diuresis can develop because of increased renal excretion of glucose/ketone bodies 5. The increase in H+ concentration (thus low pH; acidic) in a decreased volume can result in ketoacidosis

Question 30

Partial metabolic map

Answer 30

Question 31

Clinical relevance

Answer 31

1. "Ketogenic diet for the treatment of refractory epilepsy in children" 2. Ketogenic diet is necessary but too much (and thus ketonuria--\>ketone bodies in urine) is not good