11: FA/Keto Flashcards
Understand the roles of insulin, lipoprotein lipase, adipocyte lipid binding protein, thiokinase, acyl transferases, and glycerol-3-phosphate in TG storage.
insulin: Fat storage is promoted by insulin. Fat storage occurs when energy from the diet is abundant (i.e., in the fed state) and is favored
by high levels of insulin relative to counter-regulatory hormones. Insulin also promotes glucose uptake by adipocytes, by promoting translocation of GLUT4 glucose transporters to the plasma membrane. This glucose is the source of glycerol-3-phosphate (see below). Finally, insulin in the fed state leads to the inhibition of at least one enzyme (hormone-sensitive lipase) that hydrolyzes stored TG
lipoprotein lipase: Adipocytes release lipoprotein lipase in response to insulin. FA are unloaded from chylomicrons and VLDL and taken up by adipocytes. Insulin promotes glucose uptake into adipocytes. So fat cells have everything they need to store TG. Insulin promotes the secretion of lipoprotein lipase from adipocytes. The released lipase becomes bound to nearby capillary walls. The increased presence of lipoprotein lipase results in greater hydrolysis of TG of large lipoprotein particles (chylomicrons and VLDL).
adipocyte lipid binding protein: NEFA released through the action of lipoprotein lipase are transported into adipocytes by FA transporters (FATP1). Insulin promotes translocation of these FA transporters to the plasma membrane (much like GLUT4). Once inside the adipocyte, long-chain FA are bound to ALBP (= adipocyte lipid binding protein). In the fat cell, the FA may be used as fuel or stored as TG.
thiokinase: The next step is conversion of the NEFA to fatty acyl-CoA through the action of thiokinase (aka acyl-CoA synthetase).
acyl transferases & glycerol-3-phosphate: For storage, acyl groups are transferred from fatty acyl-CoA to glycerol-3-phosphate in reactions catalyzed by acyl transferases. The glycerol-3-P is derived from glycolysis. The first two acyl transfers yield phosphatidic acid (PA), which is dephosphorylated to diacylglycerol before the third acyl group is added to produce TG. By the combined action of thiokinase and acyl transferases, non-esterified fatty acids (NEFA) are esterified to a glycerol backbone.
Understand the roles of the following in fat mobilization: insulin; counterregulatory hormones; cAMP; phosphorylation of hormone-sensitive lipase and of perilipin; adipocyte lipid binding protein; serum albumin.
Fat mobilization involves de-esterification: the hydrolytic release of FA from TG. This process of mobilization of FA from TG (by far the major form of stored lipid) is also commonly called lipolysis. FA are successively released from TG, diacylglycerol (DG), and monoacylglycerol (MG) by esterases.
insulin: Opposes fat mobolization
counterregulatory hormones:
cAMP: HSL is phosphorylated by cAMP-dependent protein kinase, typically in response to catecholamines; phosphorylated HSL is more active.
phosphorylation of hormone-sensitive lipase and of perilipin: The best-studied of the fat cleaving esterases is called hormone-sensitive lipase (HS-lipase, HSL). Itreleases FA preferentially from DG and MG — i.e., mainly responsible for catalyzing the second and third hydrolysis steps, yielding NEFA and glycerol). It’s activated by catecholamines and glucagon (see below), hence the name HSL. It is NOT the rate limiting step of lypoysis: adipose triglyceride lipase catalyzes the rate-limiting step in lipolysis. The key regulatory event in lipolysis (more important than phosphorylation of HSL) appears to be the parallel phosphorylation (also by cAMP-dependent protein kinase) of another protein, perilipin. Mature adipocytes contain at least two types of lipid storage droplets: the prominent, centrally located, large droplet and the much smaller, peripheral droplets interposed between the central droplet and the plasma membrane. The peripheral droplets are the ones that are metabolically active in lipolysis. Prior to hormone stimulation, access of HSL and other lipases to peripheral fat droplets is prevented by a layer of perilipins, which are preferentially located to the smaller droplets. Phosphorylation of perilipins in response to hormone disrupts the perilipin sheet, allowing HSL and other lipases to get at TG in fat droplets.
adipocyte lipid binding protein: ALBP rapidly shuttles mobilized FA to the cell surface, where they are off-loaded onto serum albumin.
serum albumin: NEFA are transported in the bloodstream bound to albumin, which comprises about half the total protein in the plasma.
Know the Delta (Δ) and omega (ω) nomenclatures for unsaturated fatty acids.
What is it for: Palmitic, Oleic, Linoleic.
Mnemonic: My penis penetrated sarah’s oily lumpy landfill again
Palmitic = 16:0
Oleic = 18:2Δ9
Linoleic = 18:2Δ9,12
Note how a DB makes suffix oleic
Δ System: The first number represents the number of carbon atoms while the number following the colon is the number of double bonds. When there are double bonds, a Δ is followed by the position of the first carbon of each double bond, counting from the carboxyl carbon as number 1.
The ω system is similar to the Δ system except that the position of the double bonds is numbered from the methyl (CH3-) carbon, the farthest carbon from the carboxyl group—the ω end (omega is the last letter in the Greek alphabet). Usually only the position of the first double bond is given since double bonds are almost always three carbons apart (still must put # of double bonds though).
See pg. 210-212
Know which FA classes are essential and why.
‘Essential’ means we can’t synthesize FA of this class; must be obtained from diet.
Polyunsaturated FA of the ω-6 and ω-3 classes are not synthesized by mammals and must be obtained from the diet. They are called essential FA.
Linoleic ω-6
Linolenic ω-3
FA of one class cannot be converted to another class by humans, but FA in the same ω class like ω-3 or ω-6 CAN BE INTERCONVERTED.
Understand how fatty acids enter mitochondria (roles of carnitine, carnitine acyltransferases, and malonyl-CoA as key regulator of fatty acid oxidation).
carnitine: The transfer goes from CoA to carnitine to CoA. Carnitine allows the acyl fat to cross the IMM.
carnitine acyltransferases:
malonyl-CoA: CPT 1 catalyzes the transfer of the LCFA moiety from CoA to carnitine. CPT 1 is inhibited by malonyl-CoA, the product of the first committed step in FA bio- synthesis.
Know the steps in β-oxidation.
FA are removed from the blood by various tissues and oxidized to yield ATP (brain cannot use FA as a fuel). Modifications to the basic procedure are required to handle the oxidation of unsaturated and odd chain fatty acids.
- Activation of the fatty acids to fatty acyl-CoA: Activation of FA is an energy-dependent step catalyzed by acyl-CoA synthetase (thiokinase). Beta-oxidation occurs in mitochondria, so the next phase after activation is getting the FA into the mitochondrial matrix.
- Entry of the acyl-CoA into the mitochondria. Phase 2a: Transfer of the LCFA moiety from CoA to carnitine. Phase 2b: Acyl-carnitine is transported across the IMM and the acyl moiety is transferred back to CoA. Acyl-CoA is regenerated at the inner face of the IMM.
- β-oxidation to produce acetyl-CoA, FADH2 and NADH: The acyl-CoA then enters the beta-oxidation pathway. Entry of LCFA into mitochondria is a major control point in metabolism. CPT 1 enzyme is inhibited by malonyl-CoA, the product of the first committed step in FA bio- synthesis. So, when FA are being synthesized they aren’t being used as fuel.
- Oxidation of acetyl-CoA through the TCA cycle.
- Utilization of electron transport systems to produce ATP.
See pg. 218
Understand how complete oxidation of palmitate yields 129 net ATP.
See pg. 221
Understand the lower energy yield from oxidation of unsaturated fatty acids.
More highly reduced compounds yield more energy on their oxidation than less reduced ones because there is more to squeeze out. You get less energy out if you start with a more oxidized compound since B oxidation is just round after round of oxidation. You start out with less.
Know the steps in ketone body metabolism and the conditions that favor ketogenesis in liver cells.
Ketone bodies are produced by the liver during fasting and starvation (and overproduced in uncontrolled Type 1 diabetes). The production and export of ketone bodies from acetyl- CoA allows for continued β-oxidation of FA by the liver with only minimal oxidation in the liver itself of the acetyl-CoA thus produced.
REGULATION OF
KETONE BODY METABOLISM:
• In fasting, low concentrations of TCA cycle intermediates & OAA is being used for gluconeogenesis so it is low & Acetyl-CoA cannot condense with it. Therefore, Acetyl-CoA diverted to KB.
• ‘Ketosis’ ( = state of making KB in liver) begins after overnight fast.
The compounds referred to as ketone bodies (KB) or ketoacids are: (1) acetoacetate, the primary product; (2) β-hydroxybutyrate, formed by reduction of acetoacetate; and (3) acetone, produced by spontaneous decarboxylation of acetoacetate — a metabolic dead end! See their structures on pg. 225.
Ketogenesis occurs in liver mitochondria and KB are released into the plasma. Acetoacetate and β-hydroxybutyrate can be used as fuel by many tissues, including the brain. During starvation, up to 75% of the energy requirements of the brain are met by these KB. KB are the preferred substrate (over glucose) utilized by heart muscle and renal cortex. They can be considered water-soluble equivalents of FA.
STEPS of Ketone body metabolism which occurs in liver mitochondria:
Take 2 Acetyl CoA (from B oxidation) & condense them
make AcetoacetylCoA
make HMG CoA
HMG CoA reacts with lyase to make Acetyl CoA + Acetoacetate
Acetoacetate is converted to acetone (spontaneous) or B hydroxybuterate
Other tissues do no have enzyme to make HMG CoA, only liver can! Therefore, the liver can only make ketone bodies.
In non liver tissues the B-hydroxybutyrate is converted to Acetoacetate then this converted Acetoacetate + already present Acetoacetate become Acetoacetyl-CoA which become 2 Acetyl-CoA that can enter TCA cycle.
Note that KETONE OVERPRODUCTION is the problem in uncontrolled Type 1 diabetes due to runaway lipolysis (due to no insulin). Glucagon is high & insulin is absent! Therefore, sudden over-production of KB: more than other tissues can burn, more than kidneys can excrete. Result = ketoacidosis (acidic blood), which if not corrected promptly, can lead to coma and death.
