FINAL EXAM: Hormone, lipid Flashcards
catabolism of fatty acids
produces acetyl-CoA
produces reducing power (NADH, FADH2)
takes place in mitochondria
Anabolism of fatty acids
requires acetyl-CoA and malonyl-CoA (chief substrate)
requires reducing power from NADPH
cytosol in animals; chloroplast in plants (where NADPH is present)
fatty acids are built in several passes
processing one acetate unit (2 carbons) at a time
where does the acetate come from in fatty acid synthesis?
activated malonate in the form of malonyl-CoA (Acetyl-CoA with another carboxyl)
malonyl-CoA synthesis
first committed step of synthesis of fatty acids
rate limiting step
energy from ATP used to add carboxyl group to acetyl-CoA
loss of carboxyl group will provide energy for condensation of acetyl group onto growing FA chain
what provides energy for condensation of acetyl group onto growing FA chain?
loss of carboxyl group
malonyl-CoA is formed from
acetyl-CoA and bicarbonate
malonyl-CoA formation from acetyl-CoA and bicarbonate
reaction carboxylates acetyl CoA
bicarbonate is the source of CO2
catalyzed by: acetyl-CoA carboxylase
acetyl-CoA carboxylase
catalyzes malonyl-CoA formation in 3 reactions
3 reactions of acetyl-CoA carboxylase
CO2 is activated by phosphorylation by ATP
biotin (cofactor) receives CO2
CO2 transferred to acetyl-CoA
**animals: all on one polypeptide chain in one enzyme)
Fatty acid synthase (FAS)
catalyzes synthesis of fatty acids
repeating 4 step sequence that elongates the fatty acyl chain by 2 carbons at each step
FAS mechanism
uses NADPH as electron donor (2 redox in reverse; reduce FA, oxidize NADPH)
uses 2 -SH groups on FAS as activating group
FAS in vertebrates and fungi
FAS I
FAS in plants and bacteria
FAS II
FAS 1
focus
single pp chain in verts
leads to single product: palmitate (16:0)
C15 and C16 are from the acetyl-CoA used to prime the reaction
FAS 2
made of separate, diffusible enzymes
makes many products (saturated, unsaturated, branched, many lengths)
plants and bacteria
goal of fatty acid synthesis
attach 2C acetate unit from malonyl-CoA to a growing chain then reduce it
Fatty acid synthesis reaction 4 enzyme catalyzed steps
condensation
reduction
dehydration
reduction
condensation in fatty acid synthesis
condensation of growing chain with activated acetate
reduction of fatty acid synthesis
reduction of carbonyl to hydroxyl
dehydration of fatty acid synthesis
alcohol to trans-alkene
reduction 2 of fatty acid synthesis
reduction of alkene to alkane
fatty acid synthesis : chain stuff
growing chain is initially attached to a cys on FA synthase via a thioester linkage
during condensation: growing chain is transferred to acyl carrier protein
after each 4 steps, elongated chain is transferred back to the cys of fatty acid synthase
Acyl carrier protein (ACP)
shuttle in fatty acid synthesis
covalently attached prosthetic group 4’-phosphopantetheine
delivers acetaldehyde (first step) or malonate (next steps) to FAS
shuttles growing chain from one active site to another during the four step reaction
part of FAS1
4’-phosphopantetheine
prosthetic group on ACP
flexible arm to tether acyl chain while carrying intermediates from one enzyme subunit to the next
sulfhydryl group binds to form thioester
general 4 step FAS1 reaction in mammals: PREP
ACP binds acetyl group from acetyl CoA (CoA released)
ACP transfers acetyl group to cys on FAS1 (or fatty acyl chain later rounds)
ACP binds malonyl CoA and CoA leaves
general 4 step FAS1 reaction in mammals: Step 1
condensation reaction attaches the attached acetyl group (or longer fatty acyl chain) to 2C from malonyl group
- released CO2 from malonyl group
- released acetyl group from cys
**decarboxylation (loss of CO2 facilitates the reaction)
general 4 step FAS1 reaction in mammals: Step 2
1st reduction
NADPH reduces the beta-keto intermediate to an alcohol
general 4 step FAS1 reaction in mammals: step 3
dehydration
OH group from beta carbon and H from alpha carbon are eliminated, creating trans-alkene double bond (and releases water)
general 4 step FAS1 reaction in mammals: step 4
2nd reduction
NADPH reduces double bond to yield saturated alkane
end product of FAS1 reaction
saturated acyl group lengthened by 2 carbons
product of first round of FAS1
butyryl-ACP (bound to sulfur of ACP)
butyryl group transferred to cys of FAS1
new malonyl group from malonyl-CoA binds to ACP
after new round of 4 steps: 6C product is made and bound to ACP
stoichiometry of synthesis of palmitate (16:0)
7 AcCoA + 7 CO2 + 7 ATP
**7 acetyl-CoAs are carboxylated to make 7 malonyl-CoA using ATP
7 Malonyl-CoA + 7ADP + 7Pi
** 7 cycles of condensation, reduction, dehydration, reduction using NADPH to reduce the beta-keto group and trans-double bond
Palmitate + 7 CO2 (off Malonyl) + 8 CoA + 14 NADP+ + 6 H2O
why are only 6 H2O made in palmitate synthesis not 7?
1 H2O lost to hydrolyzing palmitate off the enzyme
in nonphotosynthetic eukaryotes:
acetyl-CoA made in mitochondria but
fatty acids made in cytosol
acetyl-CoA is transported indirectly into cytosol with cost of 2 ATP per Acetyl-CoA
** cost of FA synthesis is 3 ATPs per 2C unit
(1 for malonyl CoA, 2 for transport)
how is Acetyl-CoA which is generated in the mitochondria shuttled to the cytosol?
Acetyl-CoA is converted to citrate
Acetyl-CoA + oxaloacetate = citrate
catalyzed by citrate synthase
passes through citrate transporter in inner membrane
what happens to the citrate now in cytosol?
cleaved by citrate lyase
regenerates Acetyl-CoA and oxaloacetate
requires ATP
Acetyl-CoA can now be used for lipid synthesis
what happens to the oxaloacetate now in the cytosol after citrate is cleaved?
malate dehydrogenase reduces oxaloacetate to malate
2 fates for malate in cytosol
1 - converted in cytosol to pyruvate via malic enzyme (produces NADPH)
NADPH used for lipid synthesis
Pyruvate sent back to mitochondria via pyruvate transporter
converted back to oxaloacetate by pyruvate carboxylase, requires ATP
2 - transported back into mitochondria via malate/alpha-ketoglutarate transporter where it is oxidized to oxaloacetate
acetyl-CoA carboxylase regulation of fatty acid synthesis
Acetyl-CoA carboxylase: catalyzes the rate-limiting step (acetyl-CoA to malonyl-CoA)
inhibited by: palmitoyl-CoA
activated by: citrate
what does citrate signal?
excess energy to be converted to fat
in CAC: citrate is made from acetyl-CoA
When [acetyl-CoA] increases in mitochondria, citrate is synthesized and exported to cytosol when ATP is high
regulation of FA synthesis in plants and bacteria
regulation in plants and bacteria does not rely on citrate
plant acetyl-CoA carboxylase is activated by changes during light reaction of photosynthesis
bacteria use lipids for membranes, not for energy storage
- have complex regulation with guanine nucleotides — coordinates with cell growth/ division
transport or attachment of FA requires conversion to
Fatty acyl-CoA
palmitate can be lengthened to longer chain FA
elongation systems in the endoplasmic reticulum and mitochondria create longer FA
each step adds 2C
Stearate (18:0) is the most common product
- one more 2C unit
palmitate and stearate can be desaturated
Palmitate (16:0) —> palmitoleate (16:1 d9)
stearate (18:0) —> oleate (18:1 d9)
what catalyzes the desaturation of palmitate and stearate
fatty acyl-CoA desaturase
fatty acyl-CoA desaturase
O2 reduced to make 2H2O and FA oxidized to produce cis double bond — needs 4 electrons
2 e- and 2H+ come from saturated FA for O2 reduction
2 e- come from oxidation of 2 cytochrome b5
cyt b5 re-reduced by cyt b5 reductase using FADH2
FAD re-reduced by NADPH - NADP+ is formed
- *oxygen is reduced to water and FA and NADPH are oxidized
- Bond between C9 and C10 is oxidized
plants can desaturate at positions beyond C9
humans have: d4, d5, d6, d9 desaturases but not beyond d9
plants:
linoleate 18:2 d9,12
alpha-linoleate 18:3 d9,12,15
these are essential to humans to help control membrane fluidity (polyunsaturated FA = more fluid)
precursor for backbone of fat and phospholipids
glycerol 3-P
most glycerol -3P comes from
reducing dihydroxyacetone phosphate from glycolysis via glycerol 3P dehydrogenase
some DHAP made through start of gluconeogenesis
some G3P made from glycerol via glycerol kinase with ATP (liver, kidney)
phosphatidic acid
glycerol 3P bound by 2 FA on C1 and C2
precursor to triacylglycerols and phospholipids
FA are attached to glycerol-3P by acyl transferases
releases CoA
acyl transferases
attaches FA to glycerol-3P and releases CoA
advantages of making phosphatidic acid
can be made into triacylglycerol or glycerophospholipid
triacyl: remove Pi, add FA
glycero: add head group on Pi
phosphatidic acid phosphatase
MAKES TRIACYLGLYCEROL
removes phosphate from phosphatidic acid
yields 1,2-diacylglycerol
hydroxyl on 3C is acylated with 3rd FA by acyl transferase
yields triacylglycerol
peptide hormones
insulin and glucagon
bind to receptors that span the membrane and induce conformational change that produces a second messenger
results in signal amplification and changes at many targets
insulin signaling pathway
RTK, phosphorylation
inc. cell proliferation/growth
lipid synthesis
glucagon synthesis
protein synthesis
glucose uptake
glucagon signaling pathway
GPCR activates
insulin
synthesized by beta cells of pancreas as preproinsulin
processed in 2 steps into active form (irreversible covalent regulation)
secreted in response to high glucose after a meal; gets glucose out of blood
glucagon
synthesized by alpha cells of pancreas as proglucagon
cleaved into active form
synthesized when insulin levels drop in response to lower glucose; increases blood glucose
peptide hormone insulin
insulin is produced to lower blood sugar
take up glucose into cells from blood
utilize glucose-glycolysis, glycogen synthesis, fatty acid synthesis
prevent intracellular production of glucose
prevent utilization of other molecules for energy
affects of insulin
binds to receptors in muscle, brain, liver, adipose
muscle and liver: promotes glucose uptake, glycogen synthesis
adipocytes: promotes triacylglycerol synthesis and inhibits breakdown of ^
effects of insulin on blood glucose:
inc glucose uptake (muscle, adipose)
target enzyme:
inc. glucose transporter GLUT4
effects of insulin on blood glucose:
inc glucose uptake (liver)
target enzyme:
inc. glucokinase expression
effects of insulin on blood glucose:
inc glycogen synthesis (liver, muscle)
inc. glycogen synthase
effects of insulin on blood glucose:
dec. glycogen breakdown (liver, muscle)
dec glycogen phosphorylase
effects of insulin on blood glucose:
inc. glycolysis, acetyl-CoA production (liver, muscle)
inc PFK-1 by PFK2 (allosteric)
inc pyruvate dehydrogenase complex
effects of insulin on blood glucose:
inc. fatty acid synthesis (liver)
inc. acetyl-CoA carboxylase
effects of insulin on blood glucose:
inc triacylglycerol synthesis (adipose)
inc lipoprotein lipase
carb metabolism in liver
hepatocytes:
- GLUT2 transporter for diffusion of glucose in/out
- glucokinase (hexokinase IV)
glucokinase
in hepatocytes
produces glucose-6P from glucose transported into hepatocyte by GLUT2
higher Km than other hexokinases (10mM vs 4) — glucose-6P isn’t made when glucose is low
NOT INHIBITED BY GLUCOSE-6P so glucose-6P can be made continually
fates for glucose-6P in liver
dephosphorylate to yield free glucose to go to other tissues
make into liver glycogen
enter glycolysis, make acetyl-CoA and then ATP for hepatocytes themselves
enter PPP to yield NADPH and ribose-5P
metabolism of FA in liver
make lipids that contain fatty acids
break down FA into acetyl-CoA to make ATP
make acetyl-CoA into ketone bodies to be secreted for use in other organs
use acetyl-CoA to make sterols
secrete FA to be used in other organs
in the liver: insulin stimulates ________ and inactivates
glycogen synthase and inactivates glycogen phosphorylase
UDP glucose —> glycogen
in the liver glycolysis is stimulated
phosphofructokinase activated by inc. in fructose-2,6-bisphosphate (its allosteric regulator)
- through dephosphorylation and activation of enzyme that makes it
pyruvate kinase activated by reversible covalent modification (phosphorylation)
insulin stimulates
conversion of excess glucose to glycogen and/or triacylglycerol
in muscle and adipose, insulin stimulates
glucose uptake (GLUT4) increases within plasma membrane
muscles can store excess glucose as
glycogen
in adipose, insulin stimulates
triacylglycerol synthesis and decreases triacylglycerol breakdown
insulin changes transcription of more than 150 genes
inc: enzymes in glycolysis, PPP, lipid synthesis
dec: enzymes in gluconeogenesis
glucagon role
acts in opposite to insulin
glucagon goals
break down glycogen stores
increase gluconeogenesis in liver
release glucose into bloodstream
mobilize FA from fat for alt. energy source
produce ketone bodies for alt. energy source
glucagon raises blood glucose and ketone bodies by
changes in liver metabolism
glucagon: activates glycogen phosphorylase
inactivates glycogen synthase
glycogen —> glucose-1P —> glucose-6P —> glucose
glucagon: promotes gluconeogenesis
stimualtes Fructose 1,6-bisphosphatase (inhibits glycolysis at phosphofructokinase-1) through allosteric regulation
inhibits pyruvate kinase by covalent modification
increases PEP carboxykinase — produces PEP from oxaloacetate
effect of inhibiting pyruvate kinase by covalent modification
prevents PEP from being converted to acetyl-CoA
accumulation of phosphoenolpyruvate favors gluconeogenesis
glucagon: inhibits acetyl-CoA carboxylase by covalent modification
decreases [malonyl-CoA] leading to increased ketone body formation
glucagon affects adipose tissue to spare glucose for the brain
at adipose: activates triacylglycerol hydrolysis
activates hormone-sensitive lipase
results in FA transport to other tissues so that glucose is spared for the brain
fuel use over 4 hours of human metabolism
immediately after a meal: glucose increases; insulin stimulates glycolysis, triacylglycerol synthesis, glycogen synthesis
2 or more hours: blood glucose drops; glucagon secreted, liver glycogen is broken down to glucose for other tissues
after 4 hours: more glucagon produced, triacylglycerol hydrolysis occurs, FA become fuel for muscle and liver
effects of prolonged fasting
muscle used for fuel
liver deaminates or transaminates AA
FA oxidized to acetyl-CoA, but oxaloacetate is depleted to make glucose so ketone bodies formed and exported to other tissues
liver deamination or transamination of AA
converts amino groups to urea
carbon skeletons of glucogenic amino acids converted to pyruvate, then glucose via gluconeogenesis
provides glucose for brain
