BioChem Ch 36
Integration of Carbohydrate and Lipid Metabolism
 Regulatory mechanisms that direct compounds through pathways of metabolism involved in storage and use of
fuels are in turn controlled by hormones, concentration of available fuels, and energy needs of body
o Changes in hormone levels, concentration of fuels, and energy requirements affect activity of key
enzymes in major pathways of metabolism
 Intracellular enzymes generally regulated by activation and deactivation, phosphorylation and
dephosphorylation, induction and repression of synthesis, or by degradation
o Activation and inhibition of enzymes cause immediate changes in metabolism
o Phosphorylation and dephosphorylation of enzymes affect metabolism slightly less rapidly
o Induction and repression of enzyme synthesis are much slower processes, usually affecting metabolic
flux over period of hours
o Degradation of enzymes decreases amount available to catalyze reactions
 Pathways of metabolism have multiple control points and multiple regulators at each control point; function of
complex mechanisms is to produce graded response to stimulus and provide sensitivity to multiple stimuli so
that an exact balance is maintained between flux through a given step (or series of steps) and need or use for
product
o Pyruvate dehydrogenase – regardless of insulin levels, it cannot become fully activated in presence of
products and absence of substrates
 Major hormones that regulate pathways of fuel metabolism are insulin and glucagon
o In liver, all effects of glucagon reversed by insulin, and some of pathways that insulin activates inhibited
by glucagon; thus pathways of carb and lipid metabolism generally regulated by changes in
insulin/glucagon ratio
 Although glycogen is critical storage form of fuel because blood glucose levels must be carefully maintained,
adipose triacylglycerols are quantitatively major fuel store
o After meal, both dietary glucose and fat stored in adipose tissue as triacylglycerol, which is released
during fasting, when it provides main source of energy for tissues of body
o Length of time we can survive without food depends mainly on size of body’s fat stores
Regulation of Carbohydrate and Lipid Metabolism in Fed State
 After meal, liver synthesizes glycogen and triacylglycerol; glycogen stored in liver
o Although liver synthesizes triacylglycerol, it doesn’t store it; packages it in VLDL and secretes it in blood
o Fatty acids of VLDL triacylglycerols secreted from liver stored as adipose triacylglycerols
o Adipose tissue has almost infinite capacity to store fat, limited mainly by ability of heart to pump blood
through capillaries of expanding adipose mass
 Body stores fat mostly where it does not interfere too much with mobility (abdomen, hips,
thighs, and buttocks)
o To convert glucose to either glycogen or triacylglycerol, it is first converted to glucose-6-phosphate by
glucokinase (liver enzyme with high Km for glucose)
 Because of its low affinity for glucose, it is most active in fed state when concentration of
glucose is particularly high because hepatic portal vein carries digestive products directly from
intestine to liver
 Synthesis of glucokinase induced by insulin (which is elevated after meal) and repressed by
glucagon (which is elevated during fasting)
 Liver can metabolize glucose only whne sugar levels are high and not when sugar levels low
o Conversion of glucose-6-phosphate to glycogen requires glycogen synthase
 Glycogen synthase activated by dephosphorylation that occurs when insulin elevated and
glucagon decreased (and by increased level of glucose)
o For lipogenesis, glucose-6-phosphate converted through glycolysis to pyruvate – key enzymes are
phosphofructokinase-1 (PFK-1) and pyruvate kinase
 PFK-1 allosterically activated in fed state by F2,6P2 (activator for next bullet) and AMP
 PFK-2 (enzyme that produces F2,6P2) dephosphorylated and kinase activity active after meal
 Pyruvate kinase activated by dephosphorylation, stimulated by increase of insulin/glucagon
ratio in fed state
o
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
Conversion of pyruvate to fatty acids requires source of acetyl-CoA in cytosol; pyruvate can only be
converted to acetyl-CoA in mitochondria, so it enters mitochondria and forms acetyl-CoA through
pyruvate dehydrogenase (PDH) reaction
 PDH dephosphorylated and most active when its supply of substrates and ADP high (products
used and insulin is present)
 Pyruvate also converted to oxaloacetate (catalyzed by pyruvate carboxylase, which is activated
by acetyl-CoA) – because acetyl-CoA can’t cross mitochondrial membrane directly to form fatty
acids in cytosol, it condenses with oxaloacetate, producing citrate, and when not required for
TCA cycle activity, it crosses membrane and enters cytosol
 As fatty acids produced under conditions of high energy, high NADH/NAD+ ratio in mitochondria
inhibits isocitrate dehydrogenase, which leads to citrate accumulation within mitochondrial
matrix; as citrate accumulates, it’s transported out into cytosol to donate carbons for fatty acid
synthesis
o In cytosol, citrate cleaved by citrate lyase (inducible enzyme) to form oxaloacetate and acetyl-CoA
 Acetyl-CoA used for fatty acid biosynthesis and cholesterol synthesis (pathways activated by
insulin)
 Oxaloacetate is recycled to pyruvate via cytosolic malate dehydrogenase and malic enzyme
(which is inducible)
 Malic enzyme generates NADPH for reactions of fatty acid synthase complex
 NADPH produced by 2 enzymes of pentose phosphate pathway (glucose-6-phosphate
dehydrogenase and 6-phosphogluconate dehydrogenase)
 Glucose-6-phosphate dehydrogenase induced by insulin
o Acetyl-CoA converted to malonyl-CoA, which provides 2-carbon units for elongation of growing fatty acyl
chain on fatty acid synthase complex
 Acetyl-CoA carboxylase (enzyme that catalyzes conversion to malonyl-CoA) controlled by
 Activation by citrate (causes enzyme to polymerize) and inhibited by long-chain fatty
acyl-CoA
 Phosphatase stimulated by insulin activates enzyme by dephosphorylation
 Induction – quantity of enzyme increases in fed state
 Malonyl-CoA provides carbons for synthesis of palmitate on fatty acid synthase complex; also
inhibits carnitine palmitoyl transferase I (CPTI, also called carnitine acyltransferase I)
 CPTI prepares long-chain fatty acyl-CoA for transport into mitochondria
 In fed state, when acetyl-CoA carboxylase active and malonyl-CoA levels elevated, newly
synthesized fatty acids converted to triacylglycerols for storage rather than being
transported into mitochondria for oxidation and formation of ketone bodies
o In well-fed individual, quantity of fatty acid synthase complex increased; genes that produce enzyme
complex induced by increases in insulin/glucagon ratio
 Amount of complex increases slowly after few days of high-carb diet
 Glucose-6-phosphate dehydrogenase (generates NADPH in pentose phosphate pathway) and
malic enzyme (produces NADPH) also induced by increase of insulin
 Palmitate produced by synthase complex converted to palmitoyl-CoA and elongated and
desaturated to form other fatty acyl-CoA molecules, which are converted to triacylglycerols,
which are packaged and secreted into blood as VLDL
Lipoprotein triacylglycerols in chylomicrons and VLDL hydrolyzed to fatty acids and glycerol by lipoprotein lipase
(LPL), an enzyme attached to endothelial cells of capillaries in muscle and adipose tissue
o Enzyme found in muscle, particularly heart muscle, and has low Km for blood lipoproteins (acts even
when lipoproteins present at very low concentrations in blood)
o Fatty acids enter muscle cells and are oxidized for energy
o Enzyme found in adipose tissue has higher Km and is most active after a meal when blood lipoprotein
levels elevated
Measurement of triglycerides in blood samples performed using coupled assay; sample incubated with lipase
that converts triglyceride to glycerol and 3 fatty acids
o
Glycerol converted to glycerol-3-phosphate by glycerol kinase and ATP, and glycerol-3-phosphate then
oxidized by bacterial glycerol-3-phosphate dehydrogenase to produce DHAP and H2O2
o H2O2, in presence of peroxidase, will oxidize to colorless substrate, which turns color when oxidized
o Measurement of color change directly proportional to amounts of glycerol generated in sample
 Type 1 DM associated with severe deficiency or absence of insulin production by β-cells of pancreas
o One of effects of insulin is to stimulate production of LPL
o Because of low insulin levels, patients tend to have low levels of LPL
o Hydrolysis of triacylglycerols in chylomicrons and VLDL decreased and hypertriglyceridemia results
 Insulin stimulates adipose cells to synthesize and secrete LPL, which hydrolyzes chylomicron and VLDL
triacylglycerols
o Apoprotein CII (apoCII) donated to chylomicrons and VLDL by HDL; activates LPL
o Fatty acids released from chylomicrons and VLDL by LPL are stored as triacylglycerols in adipose cells
 Glycerol released by LPL not used by adipose cells because they lack glycerol kinase
 Glycerol can be used by liver cells because they have glycerol kinase
o In fed state, liver cells convert glycerol to glycerol moiety of triacylglycerols of VLDL, which is secreted
from liver to distribute newly synthesized triglycerides to tissues
o Insulin causes number of glucose transporters in adipose PMs to increase; glucose enters cells and is
oxidized, producing energy and providing glycerol-3-phosphate moiety for triacylglycerol synthesis (via
dihydroxyacetone phosphate intermediate of glycolysis)
o 20-30% of patients with insulinoma gain weight as part of their syndrome; although they are primed by
high insulin levels both to store and use fuel more efficiently, they will gain weight unless they don’t
consume more calories than is required for daily expenditure during illness (many times they will have
extra carbs to avoid symptoms of hypoglycemia)
Regulation of Carbohydrate and Lipid Metabolism During Fasting
 During fasting, insulin/glucagon ratio decreases; liver glycogen degraded to produce blood glucose because
enzymes of glycogen degradation activated by cAMP-directed phosphorylation
o Glucagon stimulates adenylate cyclase to produce cAMP, which activates PKA
o Protein kinase A phosphorylates phosphorylase kinase, which then phosphorylates and activates
glycogen phosphorylase; PKA also phosphorylates (and inactivates) glycogen synthase
 Gluconeogenesis stimulated because synthesis of phosphoenolpyruvate carboxykinase, fructose-1,6bisphosphatase, and glucose-6-phosphatase induced and because there is increased availability of precursors
o Fructose-1,6-bisphosphatase activated because levels of its inhibitor (F2,6P2) low
o During fasting, activities of corresponding enzymes of glycolysis decreased
 Induction of enzyme synthesis requires activation of transcription factors
o CREB (cAMP response element-binding protein) – phosphorylated and activated by cAMP-dependent
protein kinase, which is activated on glucagon or epinephrine stimulation
o C/EPBs (CCAAT enhancer-binding proteins) – activated for enzyme synthesis
 Type 2 DM – produce insulin, but adipose tissue partially resistant to its actions, so it doesn’t produce as much
LPL as normal person, which is one reason why LDL and chylomicrons remain elevated in blood
 Those with type 1 DM can have hyperglycemia because insulin levels tend to be low and glucagon levels tend to
be high; patient’s muscle and adipose cells don’t take up glucose at a normal rate, and they produce glucose by
glycogenolysis and gluconeogenesis; as a result, their blood glucose levels elevated
o Similar for type 2 DM, but in their case, they produce insulin but tissues are resistant to its actions
 During fasting, as blood insulin levels fall and glucagon levels rise, level of cAMP rises in adipose cells, so PKA
activated and causes phosphorylation of hormone-sensitive lipase (HSL)
o Phosphorylated HSL is active and cleaves fatty acids from triacylglycerols
o HSL also activated by epinephrine, ACTH, and GH
o Glyceroneogenesis and resynthesis of triglyceride by adipocyte regulate rate of release of fatty acids
during fasting
 Insulin normally inhibits lipolysis by decreasing lypolytic activity of HSL in adipocyte; those with type 1 DM have
increased lipolysis and a subsequent increase in concentration of free fatty acids in blood
o Liver uses some of these fatty acids to synthesize triacylglycerols, which are then used in hepatic
production of VLDL
o
o
VLDL not stored in liver but secreted into blood, raising serum concentration
Type 1 DM patients would have low levels of LPL because of decreased insulin levels, so
hypertriglyceridemia can result from both overproduction of VLDL by liver and decreased breakdown of
VLDL triacylglycerol for storage in adipose cells
o Serum begins to appear cloudy when triacylglycerol level increases
 As fatty acids released from adipose tissue during fasting, they travel in blood complexed with albumin – fatty
acids oxidized by various tissues, particularly muscle
o In liver, fatty acids transported into mitochondria because acetyl-CoA carboxylase is inactive, malonylCoA levels low, and CPTI is active
o Acetyl-CoA produced by β-oxidation is converted to ketone bodies, which are used as energy source by
many tissues to spare use of glucose and necessity of degrading muscle protein to provide precursors
for gluconeogenesis
o High levels of acetyl-CoA in liver (derived from fat oxidation) inhibit pyruvate dehydrogenase (which
prevents pyruvate from being converted to acetyl-CoA) and activate pyruvate carboxylase (which
produces oxaloacetate for gluconeogenesis
o Oxaloacetate doesn’t condense with acetyl-CoA to form citrate because
 Under high rate of fat oxidation in liver mitochondria, energy levels in mitochondrial matrix are
high (high levels of NADH and ATP present); high NADH level inhibits isocitrate dehydrogenase,
so citrate accumulates and inhibits citrate synthase from producing more citrate
 High NADH/NAD+ ratio diverts oxaloacetate into malate, so malate can exit mitochondria (via
malate/aspartate shuttle) for use in gluconeogenesis
 During exercise, fuel used initially by muscle cells is muscle glycogen; as exercise continues and blood supply to
tissue increases, glucose taken up from blood and oxidized
o Liver glycogenolysis and gluconeogenesis replenish blood glucose supply
o Because insulin levels drop, concentration of GLUT4 glucose transporters in PM reduced, thereby
reducing glucose entry from circulation into muscle
o As fatty acids become available because of increased lipolysis of adipose triacylglycerols, exercising
muscle begins to oxidize fatty acids; β-oxidation produces NADH and acety-CoA, which slow flow of
carbon from glucose through reaction catalyzed by pyruvate dehydrogenase; thus oxidation of fatty
acids provides major portion of increased demand for ATP generation and spares blood glucose
 Because those with type 1 DM produce very little insulin, they are prone to developing ketoacidosis
o When insulin levels low, HSL of adipose tissue is very active, resulting in increased lipolysis
o Fatty acids released travel to liver, where they are converted to triacylglycerols of VLDL; also undergo βoxidation and conversion to ketone bodies
o Those with low insulin levels may develop ketoacidosis
 Patients with type 2 DM do not tend to develop ketoacidosis – insulin is tissue specific, and insulin sensitivity of
adipocytes may be greater than that of muscle and liver
o Levels of insulin required to suppress lipolysis is only 10% that required to enhance glucose use by
muscle and adipocyte
o Leads to less fatty acids being released from adipocytes in type 2 than in type 1, although in both cases,
release of fatty acids would be greater than that of normal individual
o If person with type 2 diabetes has precipitating event (i.e., release of stress hormones), then
ketoacidosis more likely to be found as stress hormones counteract effects of insulin on adipocyte
Importance of AMP and Fructose-2,6-Bisphosphate
 Switch between catabolic and anabolic pathways often regulated by levels of AMP and F2,6P2 in cells,
particularly in liver
o Because cell uses ATP in energy-requiring pathways, levels of AMP accumulate more rapidly than that of
ADP because of adenylate kinase reaction (2ADP ATP+AMP)
o Rise in AMP levels signals more energy required (usually through allosteric binding sites on enzymes and
activation of AMP-activated protein kinase), and cell switches to activation of catabolic pathways
o As AMP levels drop and ATP levels rise, anabolic pathways activated to store excess energy
 Levels of F2,6P2 critical in regulating glycolysis versus gluconeogenesis in liver; under conditions of high blood
glucose and insulin release, F2,6P2 levels high because PFK-2 is in its activated state
o
o
F2,6P2 activates PFK-1 and inhibits fructose-1,6-bisphosphatase, thereby allowing glycolysis to proceed
When blood glucose levels low and glucagon released, PFK-2 phosphorylated by cAMP-dependent
protein kinase and is inhibited, thereby lowering F2,6P2 levels and inhibiting glycolysis, whereas favoring
gluconeogenesis
AMPK
 AMP-activated protein kinase (AMPK) is pivotal regulatory molecule in metabolism of carbs and fats; hepatic
targets of AMPK include
o Acetyl-CoA carboxylase (phosphorylation reduces activity, leading to reduced fatty acid synthesis)
o eEF2 kinase (protein activated when phosphorylated and will lead to reduction in protein synthesis)
o Glycerol-3-phosphate acyltransferase (GPAT) (phosphorylation reduces activity, leading to reduced
triglyceride synthesis)
o HMG-CoA reductase (phosphorylation reduces activity, leading to reduced cholesterol synthesis)
o Malonyl-CoA decarboxylase (MCD) (active when phosphorylated, reduces malonyl-CoA levels, allowing
fatty acid oxidation to occur)
o mTOR – reduced activity when phosphorylated, leading to reduced protein synthesis
o Tuberous sclerosis complex 2 (TSC2; increased activity when phosphorylated, leading to reduced protein
synthesis)
o Target of rapamycin complex 2 (TORC2) (protein sequestered in cytoplasm when phosphorylated,
leading to decreased expression at transcriptional level of gluconeogenic enzymes)
 Overall effect of AMPK activation is reduced fatty acid and triglyceride synthesis (via effects on acetyl-CoA
carboxylase, GPAT, and MCD), reduced cholesterol synthesis (via inhibition of HMG-CoA reductase), and reduced
protein synthesis (via effects on mTOR and TSC2)
 Concomitant increase in fatty acid oxidation to raise ATP levels
 Because all above processes are high energy-dependent, it makes sense to turn them off when energy levels
low, as exemplified by increased AMP level
 mTOR (drug that is potent immunosuppressant) is protein kinase that, when active, phosphorylates key proteins
that regulate and initiate protein synthesis
o AMPK phosphorylation of mTOR blocks activation of mTOR
o mTOR can be activated by TSC2 through complex pathway involving GTP-binding protein (Rheb or ras
homolog enriched in brain); TSC complex (consisting of TSC1 and TSC2) acts as GTPase activating protein
for Rheb
o Rheb-GTP activates mTOR, whereas Rheb-GDP doesn’t
o Phosphorylation of TSC2 by AMPK activates GTPase-activating activity of TSC2, leading to Rheb-GDP
formation, and reduced mTOR activity
o Reduced mTOR activity leads to reduction of protein synthesis
 mTOR plays critical role in transmitting signal from insulin receptor to increase in protein synthesis within cell
o Insulin receptor activation leads to Akt (PKB) activation; Akt will phosphorylate TSC1/TSC2 complex and
inactivate GTPase activating component of complex
o Under above conditions, Rheb-GTP will be long lived and mTOR will be active, leading to enhancement
of protein synthesis in cell
 AMPK can be activated in several ways (all depend on increased AMP levels in cell)
o As concentration of AMP increases, AMPK activated by allosteric means, by phosphorylation by LKB1, or
by phosphorylation by calmodulin kinase kinase
 AMPK inactivated by dephosphorylation by protein phosphatases or decrease in AMP levels
 Small changes in intracellular AMP levels can have profound effects on AMPK activity because of multiple
regulatory pathways
 AMPK – heterotrimeric complex that consists of catalytic subunit (α) and 2 regulatory subunits (β and γ)
o Allosteric activation of AMPK occurs via AMP binding to α-aubunit
o Phosphorylation activation of AMPK occurs via threonine phosphorylation on α-subunit
 Different tissues express different isoforms of α, β, and γ-subunits, giving rise to wide variety of isozymes of
AMPK in different tissues