Fatty acid metabolism Flashcards
Acetyl-CoA is used to produce _____
FA metabolism
Palmitate
What form are lipids exported from the liver?
Very low-density lipoproteins (VLDL) particles
Citrate shuttle
FA synthesis occurs in the cytosol, requiring acetyl-CoA to be transferred out of the mitochondria via the citrate shuttle
Once in the cytosol, citrate is converted into oxaloacetate and acetyl-CoA by citrate lyase
This process occurs when the citrate concentration in mitochondria is high due to inhibition of isocitrate dehydrogenase by high levels of ATP.
What enzymes produce NADPH
Inducible malic enzyme of the citrate shuttle
glucose-6-phosphate DH and 6-phosphogluconate DH of the PPP
First comitted step in FA synthesis
Formation of malonyl-CoA from acetyl-CoA and CO2
Also the rate-limiting step for FA synthesis
ACC requires biotin (B7)
What enzyme recycles biotin
Biotinidase liberates biotin covalently bound to endogenous enzymes and dietary proteins
What is a biotin a cofactor for?
Carboxylases of gluconeogenesis, FA synthesis, and branched-chain AA catbolism
Short-term regulation of ACC
Activated by citrate and inacctivated by palmitoyl CoA (- feedback) or other long-chain FA’s. (Involve changes in the polymerization/depolymerization state of the enzyme)
Reversible phosphorylation/dephosphorylation - mediated by hormone-sensitive
ACC response to phosphorylation
Inhibits ACC, due to rise in AMP levels leading ot the activation of AMP-activated protein kinase (AMPK)
ACC response to increased insulin
Activates a phosphatase, which dephosphorylates and activates ACC
Long-term regulation of ACC
Gene transcription - induction of ACC synthesis by insulin
Insulin stimulates sterol regulatory element binding protein (SREBP1) gene transcription - SREBP1 stimulates ACC transcription
FA synthase
Catalyzes the conversion of acetyl-CoA and malonyl-CoA to palmitate (16:0)
FA synthase is a dimer with seven active sites in a linear array
Role of acetyl-CoA and malonyl-CoA in FA synthesis
Acetyl-CoA - primer
successive 2-carbon units are added from malonyl-CoA from the ACC reaction
Acyl carrier protein (ACP)
A domain of FA synthase, bears a vitamin B5-related compound, phosphopantetheine cofactor, which carries the growing FA chains during synthesis
B5 - pantothenic acid
Main source of NADPH
Is generated from the cytosolic conversion of malate to pyruvate by malic enzyme.
Reaction series performed by FA synthase
Condensation, reduction, dehydration, and reduction - repeated until a 16-C saturated FA is formed (palmitate)
After, a thioesterase domain of FAS releases palmitate from the complex.
Upon release from FAS, palmitate is immediately converted into _____
Palmitoyl-CoA
FAS regulation
Activated by insulin (transcriptional)
Inhibited by glucagon
Triglyceride Synthesis
What tissue contains glycerol kinase?
Liver tissue
Glycerol kinase converts glycerol to glycerol-3-phosphate
Glycerol phosphate acyltransferase (GPAT)
In the liver and adipose tissue, glycerol-3-phosphate is combined with two fatty acyl-CoAs to form phosphatidic acid by GPAT
RLS
Phosphatidate Phosphohydrolase (PAP)
Cleaves the phosphate group of phosphatidic acid to form dacylglycerol
Diacylglycerol Acyltransferase (DGAT)
Acylated diacylglycerol to triacylglycerol
Rate-limiting step of triacylglycerol synthesis
Glycerol phosphate acyltransferase (GPAT) is the rate-limiting step
Activation of phosphatidate phosphohydrolase (PAP)
PAP is activated by oleate
Regulation of DGAT
Transcriptionally regulated by SREBP1
Most abundant phospholipid in eukaryotes
Phosphatidyl choline - choline obtained from diet or turnover of phospholipids
What enzyme controls the mobilization of fatty acids from adipose tissue when ATP is needed?
Hormone-sensitive lipase (HSL)
HSL and other lipases break down trigs from adipose tissue into free FAs and glycerol
FAs bind to what in the blood?
Albumin acts as a carrier protein, bringing FAs to the liver, muscle, etc. for oxidation
Fate of glycerol liberated from adipose by HSL
Transported to the liver and kidneys, where it can be converted to intermediates of gluconeogenesis
How does insulin affect HSL
Insulin is anti-lipolytic and inhibits HSL activity - insulin/glucagon ratio is main regulator
Although adipose tissue does not respond directly to glucagon, the fall in insulin activates HSL
Positive regulators of HSL
Epi, cortisol, and ACTH
An individual contains a mutation in a particular muscle enzyme, which leads to weight loss due to unregulated muscle fatty acid oxidation. Which of the following could be such an protein?
CPT I
Old-time physicians sometimes diagnosed illness on the basis of the odor of patients. Untreated diabetics sometimes have breath with a ‘fruity’ odor due to the presence of a volatile ketone body. Which of the following might be the cause?
Acetone
An individual suffers from a defect in one of the enzymes required for the synthesis of carnitine. If this individual does not have adequate intake of carnitine in the diet and is fasting, which of the following would be the most likely observation in this person compared to conditions of normal carnitine intake?
Elevated long-chain FA levels in the blood
Parents of a 3-month-old infant arrive at the ER agitated and frightened by the extreme lethargy and near comatose state of their child. Examination shows the infant to be severely hypoglycemic accompanied by low measurable ketones in the urine and blood. Blood analysis also indicates an elevation in Suberic acid (C8) as well as C8-acylcarnitines. A deficiency in which of the following enzymes is most likely responsible for these observations?
Medium-chain acy-CoA dehydrgenase (MCAD)
Avidin, a protein present in raw egg whites, binds to Biotin (Vitamin H or B7) with one of the highest affinities known in nature (Kd= 10-15M). In your research project at Quillen, you are studying the effect of avidin on various purified enzymes that participate in intermediary lipid metabolism . Which of the following reactions would you expect to be inhibited?
Acteyl-CoA carboxylase (ACC)
Which of the following statements about acetyl-CoA carboxylase is TRUE:
It catalyzes the rate-limiting step of FA synthesis
Which of the following enzymes would you block if you wanted to inhibit the synthesis of triglycerides specifically, without inhibiting the synthesis of phosphatidic acid or other phospholipids.
Diacylglycerol acyltransferase (DGAT)
One of the ways by which Insulin activates hepatic lipogenesis is:
by increasing SREBP1 transcription
What allosterically inhibits the rate-limiting enzyme in FA synthesis?
Fatty acyl-CoAs allosterically inhibit ACC
Only physiological inhibitor of carnitine:palmitoyl transferase-1 (CPT1)
Malonyl-CoA
Because FAs inhibit FA synthesis, they inhibit the production of intermediate malonyl-CoA…therefore, reduction of malonyl-CoA increases FA oxidation
CPT-1 is the rate-limiting enzyme of beta-oxidation
Difference between primary and secondary carnitine deficiency
I - you dont have it/cant make it
II - all the carnitine is bound and can’t be used
Carnitine shuttle
Pathway
Shuttles long-chain FA’s into the mitochondria to undergo β-oxidation
The major energy (ATP) producing pathway in the body
Fatty acid catabolism
Tissues that utilize FA’s as an energy source
β-oxidation of long-chain FA’s (C16-22)
ATP yielded by β-oxidation of one palmitate molecule
129 ATP (C16)
Complete oxidation of one molecule of glucose yields ______ ATP
34-36
What type of oxidation yields di-carboxylic acids?
When fatty acids and intermediates of βoxidations accumulate due to disease or blockage of the β-oxidation pathway, they may undergo ω-oxidation in the endoplasmic reticulum instead, which generates di-carboxylic acids (carboxyl groups at both ends) that appear in the urine.
Where are FA’s that are too large to undergo β-oxidation in the mitochondria initially oxidized?
The perioxisome performs initial oxidation of very large FA’s until octanoyl-CoA is formed, at which point they undergo mitochondrial oxidation.
Where are branched FA’s oxidized?
Branched fatty acids (like Phytanic acid from plants) are oxidized by a different pathway (α-oxidation) in the peroxisomes.
What FA’s can produce new glucose molecules?
Odd chain FA’s
In the last round of the β oxidation of odd-chain fatty acids, one acetyl-CoA and one propionyl-CoA are produced, which can enter the TCA cycle (as succinyl-CoA) and undergo gluconeogenesis, serving as a glucose source.
FA’s activated form
CoA thioesters; happens at different locations according to their chain length
Carnitine:palmitoyltransferase I (CPT I)
Located on the outer mitochondrial membrane and transfers long-chain fatty acyl groups from CoA to carnitine.
CPT I is the rate-limiting enzyme for β-oxidation
What helps fatty acylcarnitine cross the inner mitochondrial membrane
Translocase
What transfers the fatty acyl group back to CoA?
Carnitine:palmitoyltransferase II (CPT II)
Short and medium - chain FAs
Do not need the carnitine shuttle to enter mitochondria and undergo β-oxidation
Short = 4-6C
Medium = 6-12C
Regulation of β-oxidation
Conditions that favor FA syn, inhibit Regulation of β-oxidation
- Rate of FA ox is mainly controlled by FA availability in the blood.
- CPT-1 is inhibited by malonyl-CoA, which is synthesized in the cytosol of many tissues by acetyl-CoA carboxylase.
In skeletal muscles and the liver, it is inhibited when it is phosphorylated by the AMPactivated protein kinase (AMP-PK, perhaps the main energy master switch in the body). Thus, during exercise—when AMP levels increase—AMP-PK is activated, thereby inactivating acetyl-CoA carboxylase.
When do ketone bodies form as a by-product of β-oxidation?
Ketone bodies are formed in liver mitochondria when the rate of fatty acid ß-oxidation exceeds
utilization of the end product, acetyl-CoA
Ketone bodies include acetoacetate, β-hydroxybutyrate, and acetone
Precursor of bile acids
Cholesterol
Cholesterol is also precursor for steroid hormones and vitamin D
Sources of cholesterol
Diet (via chylomicrons), local synthesis, and what comes from peripheral tissues by reverse cholesterol transport.
25% - diet
75% - synthesised de novo
How is cholesterol conc. reduced in the body?
It is exreted in bile and can be converted to bile acids for fat digestion
Mammals cannot degrade cholesterol
Enterohepatic circulation
95% of the bile acids excreted by the liver are re-absorbed in the ileum and come back to the liver
The body needs to synthesize ~1.5g cholesterol/day to replace what is lost by fecal excretion, used as a precursor to other molecules, or by tissue sheeding
All C atoms of cholesterol come from _____
Acetyl-CoA
Synthesis occurs in the cytosol and later on in the ER membranes of cells
Summarized cholesterol and isoprenoid pathway
Farnesyl pyrophosphate
Hydroxy-methyl-glutaryl-CoA reductase
HMG-CoA reductase = one of the most regulated enzymes in the cell
Stimulated by insulin, thyroxine, and estrogen
Inhibited via glucagon
Can be inhibited or activated by sterol-regulatory-element-binding-proteins (SREBP) family of transcription factors
Sterol Regulatory Elements (SRE)
Specific nucleotide sequences found within SREBPs that are recognized and bound by the SREBP protein to activate transcription.
SREBP Regulation
SREBP proteins reside in ER membrane as an “immature” form. In this form, they are bound to SREBP cleavage activating protein (SCAP - a cholesterol sensor)
When cholesterol levels fall in the ER, INSIG is cleaved, and SCAP-SREBP complex heads to the golgi, where SREBP undergoes proteolytic cleavages.
The now mature SREBP moves to the nucleus, where it alters the expression of any gene with an SRE sequence.
SREBP-1
Gives basal expression of genes for both cholesterol and FA metabolism
SREBP-2
Stimulates genes primarily involved in cholesterol metabolism
Additionally, SREBP-2 is a major regulator for the expression of the LDL receptor (LDL-R)
Cholesterol ester
Stored in lipid droplets
Much more hydrophobic than cholesterol and become trapped in lipid droplets to their hydrobphobicity
Two enzymes produces CE’s = acyl-CoA-cholesterol acytransferase (ACAT) and lecithin-cholesterol acyltransferase (LCAT)
Common precursor of steroid synthesis
Pregenolone
21-α-Hydroxylase
Catalyzes the hydroxylation of the carbon atom 21 of progenolone converting it to progesterone.
Activity required for steroid hormone synthesis, including cortisol and aldosterone
What converts cholesterol to pregenolone?
Cytochrome P450 monooxygenases
Congenital Adrenal Hyperplasias
CAHs are a group of inheritable disorders associated with an inability or deficiency in the ability to produce mineralcorticoids.
More than 90% of cases are due to 21-α-Hydroxylase dysfunction
Sxs: Mineralcorticoids and glucocorticoids are virtually absent or deficient - overproduction of androgens, which leads to masculinization of external genitalia in females and early virilization in males
Dx: made by biochemical screening for elevated plasma levels of 17-hydroxyprogesterone, followed by confirmatory genetic and provacative biochemical testing (load test).
Tx: Glucocorticoid and mineralcorticoid replacements, as well as surgical correction of genital anomalies in females.
What enzyme is at the position #2?
Aromatase
Metabolic inactivation of steroid hormones
Performed by the liver - typically includes the reduction of double-bonds, hydroxylation, and conjugation with glucuoronic acid
-These modification make the steroid analogues more soluble
Modified/conjugated steroids are typically filtered through kidneys and eliminated in urine (also can be secreted into bile and eliminated in feces)
7-dehydrocholesterol
The precursor of vitamin D
7-dehydrocholesterol is photolyzed by UV rays of sunlight to previtamin D3, which spontaneously isomerizes to D3.
D3 (cholecalciferol), is converted to calcitrol, the active hormone, by hydroxylation reactions in the liver and kidneys.
Rickets
Vit D deficiency in childhood
Inadequat calcification of cartilage and bone.
7-dehydrocholesterol in the skin is not photlyzed to previtamin D3
Penultimate step of cholesterol synthesis
7-dehydrocholesterol (penultimate = step before final step)
Vitamin D is derived from 7-dehydrocholesterol
A pharmaceutical company is developing a new drug to lower cholesterol levels. They decide to target the rate-limiting step of cholesterol synthesis. Which of the following enzymes would be the most appropriate target for this drug?
HMG-CoA reductase
A researcher is studying the regulation of cholesterol homeostasis in liver cells. She observes that when cellular cholesterol levels are high, there is a decrease in the expression of LDL receptors. Which of the following best explains this observation?
Reduction of SREBP cleavage
A 6-month-old female infant is brought to the emergency department with vomiting and dehydration. Physical examination reveals ambiguous genitalia. Laboratory tests show hyponatremia, hyperkalemia, and elevated 17-hydroxyprogesterone. Which of the following is the most likely diagnosis?
Congenital adrenal hyperplasia
A researcher is studying the regulation of cholesterol synthesis in response to different hormones. Which of the following hormones would most likely stimulate cholesterol synthesis?
Insulin
A researcher is studying the effects of different drugs on cholesterol metabolism in liver cells. She observes that one drug leads to an accumulation of free cholesterol in the cells, without affecting cholesterol synthesis. Which of the following is the most likely target of this drug?
ACAT
During fasting, adipose fat deposits are “mobilized” to provide fuel to muscle and other tissues. Order the following events in order of occurrence (first to last)
- The insulin/glucagon ratio decreases
- The level of cAMP rises in adipose cells
- Protein kinase A is activated
- HSL is activated
- Trigs are cleaved into FAs and glycerol
- FAs travel in the blood and are oxidized by muscle
During prolonged fasting, the liver produces and exports ketone bodies to serve as fuel for other organs. Order the following events in order of occurrence. (First to last)
- FAs are released from adipose
- FAs travel in blood complexed with albumin
- FAs are tranported into mitochondria
- Acetyl-CoA is produced from FA oxidation
- Excess acetyl-CoA, not used in TCA cycle, is converted to ketone bodies
- The brain adapts to use ketone bodies
In the well-fed state, which of the following are activated or induced by Insulin
Acetyl-CoA carboxylase
Glucokinase
Glycogen synthase
Pyruvate DH
Activated/induced by insulin
During fasting, glucagon and epinephrine govern the overall metabolism. Which of the following are activated or induced by the rise of one of these hormones?
Phosphoenolpyruvate carboxykinase (PEPCK)
Protein kinase A
Glucose-6-phosphatase
Hormone-sensitive lipase
Glycogen phosphorylase
Ketone bodies are used as an energy source by many tissues to spare the use of glucose and the necessity of degrading muscle protein to provide the precursors for gluconeogenesis. Which of the following tissues can utilize ketone bodies as a source of fuel?
Nervous system
Skeletal muscle
Heart muscle
Enterocytes
Which of the following tissues utilize fatty acids as a source of fuel?
Skeletal muscle
Heart muscle
Liver
Hereditary abetalipoproteinemia
A**bsence of production of apolipoprotein B (apo B). Accordingly, no lipoproteins that contain apo B (e.g., chylomicrons, LDL, VLDL) are present in serum, which leads to fat malabsorption. Because dietary intake of fats exacerbates hereditary abetalipoproteinemia, treatment includes a low-fat diet with restriction of long-chain fatty acids. Patients also require supplementation with fat-soluble vitamins.
Late manifestations of hereditary abetalipoproteinemia include retinitis pigmentosa, coagulopathy, myopathy, and spinocerebellar degeneration (due to vitamin E deficiency).
autosomal recessive condition caused by a mutation in the gene that encodes microsomal triglyceride transfer protein (MTTP).
