Glycogen Metabolism Purpose: Glycogen is a branched polymer of glucose; it is the stored form of G. Purpose: Glycogen is a branched polymer of glucose;

Glycogen MetabolismGlycogen Metabolism • Purpose: Glycogen is Purpose: Glycogen is

a a branchedbranched polymer polymer of glucose; it is the of glucose; it is the stored form of G.stored form of G.

• The The manymany branches branches each have a C#4 end each have a C#4 end at which GP and GS at which GP and GS can act for can act for rapidrapid response. response.

• Glycogen is stored Glycogen is stored after a meal for after a meal for release: release:

• FromFrom liver when liver when blood [G] is low to blood [G] is low to supply brain; OR supply brain; OR

• InIn muscle for rapid muscle for rapid activity.activity.

Main Enzymes of Glycogen Main Enzymes of Glycogen MetabolismMetabolism• 1. Glycogen 1. Glycogen PhosphorylasePhosphorylase (GP) (GP): releases : releases

G as G as G1PG1P: :

• GGnn + Pi + Pi G Gn-1n-1 + G1P (no ATP cost) + G1P (no ATP cost)

• GP removes G GP removes G onlyonly from C#4 ends of chains from C#4 ends of chains that are at least five G’s from a branchthat are at least five G’s from a branch

• G1PG1P equilibrates with G6P; this is not regulated equilibrates with G6P; this is not regulated

• G1P G1P G6P G6P

• 2. Glycogen Synthase (GS)2. Glycogen Synthase (GS): adds G (as UDP : adds G (as UDP – G) – G) onlyonly to C#4 ends of chains. to C#4 ends of chains.

• a) Preliminary: G a) Preliminary: G G6P G6P G1P ; then: G1P G1P ; then: G1P + UTP + UTP PPi + UDP – G PPi + UDP – G

• b)b) GSGS rxn: G rxn: Gnn + UDP – G + UDP – G UDP + G UDP + Gn +1n +1

Other Enzymes of Other Enzymes of Glycogen Glycogen

MetabolismMetabolism

• 1. Debranching 1. Debranching enzyme: after GP enzyme: after GP has removed all but has removed all but the last 4 G the last 4 G residues from a residues from a branch, this branch, this enzyme: enzyme:

• 1) catalyses 1) catalyses transfer of 3 G transfer of 3 G residues to the C#4 residues to the C#4 end of a nearby end of a nearby branch branch andand

• 2) catalyses 2) catalyses hydrolysishydrolysis of the of the 1 1 6 linkage, 6 linkage, producing Gproducing G

Other Enzymes of Glycogen Other Enzymes of Glycogen MetabolismMetabolism• 2. Branching Enzyme: transfers C#1 of a 7G 2. Branching Enzyme: transfers C#1 of a 7G

residue segment (from a branch at least 11 G residue segment (from a branch at least 11 G long) to the C#6 of a residue at least 4 G away.long) to the C#6 of a residue at least 4 G away.

Regulation of Regulation of GPGP, , GSGS

• 1. GP is designated by 2 systems a/b and m/o, 1. GP is designated by 2 systems a/b and m/o, which we will not use. Instead, we will refer to which we will not use. Instead, we will refer to the enzymes as: phosporylated (P) or the enzymes as: phosporylated (P) or dephosphorylated (DP) ( dephosphorylated (DP) ( GSGS is also P, DP). is also P, DP).

• 2. GP and GS are phosphorylated in response to 2. GP and GS are phosphorylated in response to glucagon (in the liver) (low blood [G]) and glucagon (in the liver) (low blood [G]) and adrenalin (muscle) (fight/flight), activating GP adrenalin (muscle) (fight/flight), activating GP for release of G and inactivating GS. for release of G and inactivating GS.

• 3. GP kinase (GPK): GP + ATP 3. GP kinase (GPK): GP + ATP ADP + GP – P. ADP + GP – P.

• 4. They are dephosphorylated in response to 4. They are dephosphorylated in response to insulininsulin inactivating GP, activating GS to store inactivating GP, activating GS to store G.G.

Regulatory effectors of GP, GPKRegulatory effectors of GP, GPK• GP-DP is: GP-DP is: • 1) activated by AMP. MR: GP provides GlP 1) activated by AMP. MR: GP provides GlP G6P G6P

for ATP production in for ATP production in glycolysisglycolysis, and in , and in OPOP via via PDH, TCA, ET, OP. ML: [AMP] is high when ATP PDH, TCA, ET, OP. ML: [AMP] is high when ATP use is rapid and ATP production is needed. use is rapid and ATP production is needed. 2) inhibited by ATP. MR in 1). ML: when 2) inhibited by ATP. MR in 1). ML: when [ATP] is high, GP doesn’t need to release G to [ATP] is high, GP doesn’t need to release G to produce more 3) inhibited by produce more 3) inhibited by G6P. MR: G6P is an indirect product of GP. ML: G6P. MR: G6P is an indirect product of GP. ML: when [G6P] is high, GP doesn’t need to make when [G6P] is high, GP doesn’t need to make more.more.

• GP-P is inhibited by glucose. MR: G is indirect GP-P is inhibited by glucose. MR: G is indirect product: GlP product: GlP G6P G6P G. ML: no need for more G. ML: no need for more fuel when plenty is availablefuel when plenty is available

• GPK is activated by CaGPK is activated by Ca2+2+. MR: GPK activates GP, . MR: GPK activates GP, which provides fuel for ATP production. ML: Cawhich provides fuel for ATP production. ML: Ca2+2+ triggers muscle contraction, ATP production is triggers muscle contraction, ATP production is needed. needed.

Regulatory effectors of GSRegulatory effectors of GS

• GS-DP is activated by G6P: MR: G6P GS-DP is activated by G6P: MR: G6P is indirect substrate: G6P is indirect substrate: G6P GlP GlP UPD-G (feed forward) ML: when G6P UPD-G (feed forward) ML: when G6P is plentiful, it’s time to store G. is plentiful, it’s time to store G.

Hormonal Regulation of Glycogen Hormonal Regulation of Glycogen MetabolismMetabolism• 1. The “hunger hormone”, glucagon is a signal to 1. The “hunger hormone”, glucagon is a signal to

release G to blood from the release G to blood from the liverliver via via glycogenolysis and gluconeogenesis. Liver GP is glycogenolysis and gluconeogenesis. Liver GP is activated, GS inhibited.activated, GS inhibited.

• 2. Adrenalin (epinephrine) is a signal to “break 2. Adrenalin (epinephrine) is a signal to “break down” down” musclemuscle glycogen to produce G6P for ATP glycogen to produce G6P for ATP production (in G’lys and in OP via PDH, TCA, ET production (in G’lys and in OP via PDH, TCA, ET and OP) for fight-or-flight. Muscle GP is activated, and OP) for fight-or-flight. Muscle GP is activated, GS inhibited.GS inhibited.

• When either one binds its cell-membrane When either one binds its cell-membrane receptor: receptor:

• a. the hormone-receptor complex binds to a. the hormone-receptor complex binds to adenylate cyclase and activates it to catalyze: adenylate cyclase and activates it to catalyze: ATP ATP PPi + cAMP PPi + cAMP

• b. cAMP is the internal or “2nd” messenger. It b. cAMP is the internal or “2nd” messenger. It binds to protein kinase (PrK) regulatory (r) binds to protein kinase (PrK) regulatory (r) subunits, dissociating them from the catalytic (c) subunits, dissociating them from the catalytic (c) subunits, which are then active.subunits, which are then active.

Hormonal Regulation of Glycogen Hormonal Regulation of Glycogen MetabolismMetabolism

• c. PrK catalyses the phosphorylation of a variety of c. PrK catalyses the phosphorylation of a variety of proteins, including:proteins, including:

• the tandem E (G’lys, G’neo)the tandem E (G’lys, G’neo)• pyruvate kinase (G’lys)pyruvate kinase (G’lys)• ACoA carboxylase (FA synthesis)ACoA carboxylase (FA synthesis)• Glycogen synthase, inactivating itGlycogen synthase, inactivating it• Glycogen phophorylase kinase (GPK), activating it.Glycogen phophorylase kinase (GPK), activating it.• d. GPK-P catalyses phosphorylation of glycogen d. GPK-P catalyses phosphorylation of glycogen

phosphorylase (GP), activating it.phosphorylase (GP), activating it.• e. Net hormonal effect: GP activated: Gn e. Net hormonal effect: GP activated: Gn G; G;

andand GS inactive, GS inactive, preventing oppositionpreventing opposition to GP. to GP. • PrK also phosphorylates phosphoprotein PrK also phosphorylates phosphoprotein

phosphatase inhibitor 1 (PPI-1) causing it to bind to phosphatase inhibitor 1 (PPI-1) causing it to bind to and inactivate PP1, the enzyme that and inactivate PP1, the enzyme that dephosphorylates GP, GS, etc .dephosphorylates GP, GS, etc .

Dephosphorylation of (GPK, GP, Dephosphorylation of (GPK, GP, GS)GS)• Dephosphorylation of these enzymes (GPK, GP, Dephosphorylation of these enzymes (GPK, GP,

GS) is catalyzed by phosphoprotein GS) is catalyzed by phosphoprotein phosphatase 1 (PP1).phosphatase 1 (PP1).

• In muscle,In muscle, phosphorylation phosphorylation of a regulatory of a regulatory glycogen binding protein, Gglycogen binding protein, GMM in response to in response to insulin (which causes insulin (which causes dedephosphorylation of phosphorylation of other Es) at site 1 activates PP1. This results in other Es) at site 1 activates PP1. This results in the opposite activities to the above (GS active the opposite activities to the above (GS active to store the plentiful G, GP to store the plentiful G, GP notnot, to prevent , to prevent opposing GS).opposing GS).

• Phosphorylation of GPhosphorylation of GMM at site 2 ( alone or in at site 2 ( alone or in addition to site 1) by PrK inactivates PP1, addition to site 1) by PrK inactivates PP1, preventing it from opposing PrK.preventing it from opposing PrK.

• In liver, the switch from the phospho- to the In liver, the switch from the phospho- to the dephospho- state of GP, GPK, and GS cannot dephospho- state of GP, GPK, and GS cannot occur without the accumulation of glucose. It is occur without the accumulation of glucose. It is said that said that “GP is the glucose sensor”:“GP is the glucose sensor”:

• a) In the phospho (active) form, the P’s on GP a) In the phospho (active) form, the P’s on GP are “buried” where PP1 can’t get at them. are “buried” where PP1 can’t get at them.

• b) When G binds to active GP-P, its b) When G binds to active GP-P, its conformation changes, “exposing” the P’s so conformation changes, “exposing” the P’s so PP1 can “clip them” off. PP1 can “clip them” off.

• c) PP1 binds strongly to GP-P in the R form; and c) PP1 binds strongly to GP-P in the R form; and is not active toward other phosphoproteins in is not active toward other phosphoproteins in this state. Only after GP-P binds G and PP1 this state. Only after GP-P binds G and PP1 dephosphorylates GP is PP1 released and dephosphorylates GP is PP1 released and active. active.

• d) PP1 has a much higher affinity for GP-P than d) PP1 has a much higher affinity for GP-P than for GS, GPK, etc, so it must first “work its way for GS, GPK, etc, so it must first “work its way through” nearly all the GP-P, dephosphorylating through” nearly all the GP-P, dephosphorylating it, before it has much effect on GS.it, before it has much effect on GS.

Amplification cascadeAmplification cascade• GP and GS are regulated by effectors like AMP, GP and GS are regulated by effectors like AMP,

ATP, G, and G6P. So why has regulation by ATP, G, and G6P. So why has regulation by enzyme phosphorylation evolved? The big enzyme phosphorylation evolved? The big advantage is speed and advantage is speed and magnitudemagnitude of response: of response: each enzymatic step each enzymatic step amplifiesamplifies the signal, and the signal, and subsequent ones multiply previous ones:subsequent ones multiply previous ones:

• 1) each hormone-receptor complex activates one 1) each hormone-receptor complex activates one adenylate cyclase, which can produce, say, 1000 adenylate cyclase, which can produce, say, 1000 cAMP/sec.cAMP/sec.

• 2) 4 cAMP can --> 2 active PrK which can 2) 4 cAMP can --> 2 active PrK which can produce, say, 100 GPK-P/sec.produce, say, 100 GPK-P/sec.

• 3) Each GPK-P can phosphorylate, say, 100 GP/s.3) Each GPK-P can phosphorylate, say, 100 GP/s.• So, one H-R complex results in: 1000 cAMP x So, one H-R complex results in: 1000 cAMP x

2PrK/4 cAMP x 100 GPK-P/PrK x 100GP-P/GPK-2PrK/4 cAMP x 100 GPK-P/PrK x 100GP-P/GPK-P=5,000,000 activated GP/sec, rather than 1 P=5,000,000 activated GP/sec, rather than 1 Enzyme/1 effector. That's why it’s called the" Enzyme/1 effector. That's why it’s called the" amplification cascade".amplification cascade".

Glucokinase (GK)Glucokinase (GK)• GK catalyzes G + ATP GK catalyzes G + ATP G6P + ADP, same as G6P + ADP, same as

HXK.HXK.• GK is a liver enzyme; muscle doesn’t have it (has GK is a liver enzyme; muscle doesn’t have it (has

HXK).HXK).• The properties of GK are more suited to The properties of GK are more suited to

maximum glycogen synthesis when [G] is high. maximum glycogen synthesis when [G] is high. HXK can also support rapid glycogen synthesis, HXK can also support rapid glycogen synthesis, but not as well as GK.but not as well as GK.

• HXK is “designed” to keep up with extremely HXK is “designed” to keep up with extremely rapid glycolysis (if that’s the pace PFK sets): rapid glycolysis (if that’s the pace PFK sets): when PFK consumes F6P rapidly, G6P is also when PFK consumes F6P rapidly, G6P is also consumed rapidly, (G6P consumed rapidly, (G6P F6P) so that HXK is F6P) so that HXK is not inhibited by G6P.not inhibited by G6P.

• HXK has a very low Km (high affinity) for G, so it HXK has a very low Km (high affinity) for G, so it can go at almost Vmax rate even if [G] is low, but can go at almost Vmax rate even if [G] is low, but high [G] doesn’t increase rate. high [G] doesn’t increase rate.

GK, HXKGK, HXK• But GK is But GK is not not inhibited by inhibited by G6PG6P, so when , so when [G] is high it can [G] is high it can produce a much produce a much higher [G6P] higher [G6P] which which kinetically kinetically pushespushes glycogen glycogen synthase: via synthase: via G6P G6P GlP GlP UDPGUDPG

• GK has a much GK has a much higher Km higher Km (lower affinity) (lower affinity) for G so its rate for G so its rate is nearly is nearly proportional to proportional to [G] across the [G] across the physiological [G] physiological [G] range.range.

FAs, Fatty AcidsFAs, Fatty Acids• FAs are a much more efficient form of stored FAs are a much more efficient form of stored

fuel: 9kcal/g (9 Cal/g) vs. (4 Cal/gG); also fuel: 9kcal/g (9 Cal/g) vs. (4 Cal/gG); also glycogen binds two times its weight of Hglycogen binds two times its weight of H22O. O. A typical man would have to store ~ 90 kg of A typical man would have to store ~ 90 kg of glycogen (~200lbs) if he was to have the glycogen (~200lbs) if he was to have the same energy as in the ~15 kg fat he stores.same energy as in the ~15 kg fat he stores.

• Although glycolysis is a major fuel consuming Although glycolysis is a major fuel consuming pathway, FAs are pathway, FAs are thethe main fuelmain fuel (except in (except in brain, RBCs, rapid muscle activity).brain, RBCs, rapid muscle activity).

• Because of the above, glycogen storage is Because of the above, glycogen storage is limited and “xs G” is converted to fat via limited and “xs G” is converted to fat via glycolysis, PDH, CS and FA synthesis.glycolysis, PDH, CS and FA synthesis.

FA Use as FuelFA Use as Fuel• FAs are released from storage (as triacylglycerol) FAs are released from storage (as triacylglycerol)

by the hydrolytic action of hormone-sensitive by the hydrolytic action of hormone-sensitive lipase, which is activated by phosphorylation by PrK lipase, which is activated by phosphorylation by PrK in response to adrenalin or glucagon, deactivated in response to adrenalin or glucagon, deactivated by dephosphorylation by PP1 in response to insulin.by dephosphorylation by PP1 in response to insulin.

• FA’s then travel from FA’s then travel from adipose cellsadipose cells (“cytosol” is (“cytosol” is mainly a fat globule) via blood to cells that use mainly a fat globule) via blood to cells that use them. them.

• FA’s are prepared in the cytosol (cytoplasm) for FA’s are prepared in the cytosol (cytoplasm) for transport to the mitochondrial matrix, where they transport to the mitochondrial matrix, where they are converted to ACoA in are converted to ACoA in oxidation oxidation..

• FA activationFA activation: (costs : (costs 22 ATP) ATP)

• a) FA + ATP a) FA + ATP PPi + FA – AMP; PPi + FA – AMP;

• b) FA – AMP + CoASH b) FA – AMP + CoASH AMP + FA – SCoA AMP + FA – SCoA

• CoA from cytosol doesn’t enter matrix (or vice-CoA from cytosol doesn’t enter matrix (or vice-versa).versa).

• Instead, on the outer surface of the inner Instead, on the outer surface of the inner membrane, FA is transferred to carnitine membrane, FA is transferred to carnitine (releasing CoASH to the cytosol) in a reaction (releasing CoASH to the cytosol) in a reaction catalyzed by carnitine acyltransferase I (CATI) catalyzed by carnitine acyltransferase I (CATI) (aka Carnitine Palmitoyl TransferaseI, or CPTI). (aka Carnitine Palmitoyl TransferaseI, or CPTI).

• A transport protein in the inner membrane brings A transport protein in the inner membrane brings fatty-acyl carnitine into the matrix (in exchange fatty-acyl carnitine into the matrix (in exchange for carnitine delivered outside). for carnitine delivered outside).

• CATII (CPTII) on inner surface transfers FAcyl CATII (CPTII) on inner surface transfers FAcyl group from carnitine to CoASH. group from carnitine to CoASH. ((PalmitatePalmitate is the 16C saturated FA) is the 16C saturated FA)

oxidationoxidation + TCA + ET + + TCA + ET + OPOP ATPATP

oxidationoxidation converts the fatty acyl group converts the fatty acyl group to ACoA. to ACoA. Net reactionNet reaction for for completecomplete oxidationoxidation of palmitate: of palmitate:

• CC1515HH3131COSCoA + 7CoASH + 7FAD + COSCoA + 7CoASH + 7FAD + 7NAD7NAD++ 8ACoA + 7FADH 8ACoA + 7FADH22 + 7NADH + 7NADH

• ATP production from palmitateATP production from palmitate• 8XTCA: +8GTP + 24NADH + 8FADH28XTCA: +8GTP + 24NADH + 8FADH2• ET: -31NADH – 15FADH2ET: -31NADH – 15FADH2• OP: [+3(31) + 2(15)] ATP = + 123 ATPOP: [+3(31) + 2(15)] ATP = + 123 ATP• 123ATP + 8GTP – 2 “ATP” (ATP 123ATP + 8GTP – 2 “ATP” (ATP AMP in AMP in

FA acivation ) = 129ATPFA acivation ) = 129ATP

Ketone BodyKetone Body Production Production

• The moderate rate of production of The moderate rate of production of acetoacetate, acetoacetate, hydroxybutyrate and acetone hydroxybutyrate and acetone that that occurs normallyoccurs normally in the liver in the liver mitochondrial matrix delivers “water soluble FA mitochondrial matrix delivers “water soluble FA fragments” to cells via blood for use as fuel. fragments” to cells via blood for use as fuel.

• Since this process involves unregulated Since this process involves unregulated enzymes, the enzymes, the buildup of ACoAbuildup of ACoA in diabetes in diabetes overproducesoverproduces these compounds to toxic these compounds to toxic levels.levels.

FA SynthesisFA Synthesis

• FA synthesisFA synthesis is is a liver pathwaya liver pathway

• The net effect is to build up the CHThe net effect is to build up the CH22 chain chain by joining ACoAs’ acetyl gps. and reducing by joining ACoAs’ acetyl gps. and reducing (and hydrogenating) the C=O of ACoA.(and hydrogenating) the C=O of ACoA.

• The ACoAs for FA synth The ACoAs for FA synth don’tdon’t come from come from oxidation. Rather it’s the “xs G” that oxidation. Rather it’s the “xs G” that enters liver cells after a meal and goes enters liver cells after a meal and goes through through insulin stimulated glycolysis insulin stimulated glycolysis and PDHand PDH..

• But PDH is in matrix, FA sythase is in cytosol: But PDH is in matrix, FA sythase is in cytosol: (ACoA doesn’t cross inner membrane)(ACoA doesn’t cross inner membrane)

• 1. high ACoA from PDH stimulates PC 1. high ACoA from PDH stimulates PC high high oxac.oxac.

• 2. (ACoA + oxac 2. (ACoA + oxac citrate) in matrix; then citrate) in matrix; then transport citrate to cytosol.transport citrate to cytosol.

• 3. in cyto: citrate + ATP 3. in cyto: citrate + ATP ACoA + oxac + ADP ACoA + oxac + ADP + Pi (catalyzed by citrate lyase)+ Pi (catalyzed by citrate lyase)

• 4. oxac + NADH 4. oxac + NADH malate + NAD+ then, malate + NAD+ then, malate can enter matrix, ORmalate can enter matrix, OR

• 5. in cyto: mal + NADP+ 5. in cyto: mal + NADP+ NADPH + pyr + NADPH + pyr + COCO22 ; (pyr goes to matrix). This rxn is catalyzed ; (pyr goes to matrix). This rxn is catalyzed by the malic enzyme.by the malic enzyme.

• 6. The NADPH is needed for FA synthesis (below)6. The NADPH is needed for FA synthesis (below)

• 7. The cyto 7. The cyto ACoA is ACoA is activated for activated for joining by joining by conversion to conversion to malonyl CoA malonyl CoA (carboxylated) (carboxylated) by ACoA by ACoA carboxylase carboxylase (ACoAC): (ACoAC): ACoA + COACoA + CO22 +ATP ---> ADP +ATP ---> ADP + Pi + malCoA+ Pi + malCoA

FA SynthaseFA Synthase• In E Coli, this consists of a number of separate In E Coli, this consists of a number of separate

enzymes, but in animals 2 identical subunits each enzymes, but in animals 2 identical subunits each contain the enzymatic activities for contain the enzymatic activities for all the rxnsall the rxns ( ( oxidation has a different enzyme for each step).oxidation has a different enzyme for each step).

• The substrate remains bound to the long The substrate remains bound to the long phosphopantethein prosthetic group (Fig. 25-29, phosphopantethein prosthetic group (Fig. 25-29, p931), which “carries” it to each of the various p931), which “carries” it to each of the various active sites. This is on ACP (acyl-carrier protein)active sites. This is on ACP (acyl-carrier protein)

Phases of FA Synthase Reaction Phases of FA Synthase Reaction “Cycle”“Cycle”• Loading: the acetyl group of ACoA is Loading: the acetyl group of ACoA is

transferred to a cys-S (viaACP) and the transferred to a cys-S (viaACP) and the malonyl group of mal-CoA to ACP-S.malonyl group of mal-CoA to ACP-S.

• Condensation, Reduction: CCondensation, Reduction: C22 chains ( chains (ofof malCoA malCoA fromfrom ACoA) are linked, releasing ACoA) are linked, releasing COCO22, then reduced to –CH, then reduced to –CH22-CH-CH22. .

• Reloading: existing chain transferred to Reloading: existing chain transferred to cys-S; next malonyl group to ACP-S (cys-S; next malonyl group to ACP-S (eacheach mal of mal CoA goes onto mal of mal CoA goes onto ACPACP, only acetyl , only acetyl group of ACoA (and existing chain) go onto group of ACoA (and existing chain) go onto cys-S cys-S

• Release: FA Release: FA hydrolyzedhydrolyzed from ACP from ACP

Regulation of FA MetabolismRegulation of FA Metabolism• ACoA Carboxylase (ACoA ACoA Carboxylase (ACoA mal CoA mal CoA FA FA

synthesis)synthesis)• 1.1. Inhibited by palmitoyl CoA. MR: indirect Inhibited by palmitoyl CoA. MR: indirect

product (mal CoA product (mal CoA palmitate palmitate pal CoA). pal CoA). • ML: If [pal CoA] is high, it is being produced ML: If [pal CoA] is high, it is being produced

faster than it can be used, production can slow)faster than it can be used, production can slow)• 2.2. Activated by citrate. MR: indirect Activated by citrate. MR: indirect

substrate; citrate substrate; citrate oxac + oxac + AcoAAcoA, the substrate , the substrate • ML: when[citrate] cyto is high, [citrate] mito is ML: when[citrate] cyto is high, [citrate] mito is

very high, fuel is plentiful, time to store itvery high, fuel is plentiful, time to store it• 3. Inhibited by phosphorylation in response to 3. Inhibited by phosphorylation in response to

glucagon or adrenalin. These hormones glucagon or adrenalin. These hormones promote fuel mobilization to make fuel promote fuel mobilization to make fuel available, so they inhibit storage.available, so they inhibit storage.

• 4. Activated by dephosphorylation in response 4. Activated by dephosphorylation in response to insulin. Insulin “signals fed state”, when [G] to insulin. Insulin “signals fed state”, when [G] is high it’s time to store C as glycogen and FAs.is high it’s time to store C as glycogen and FAs.

Regulation of FARegulation of FA Active Active ACoACACoACMetabolismMetabolism• Phosphorylation shifts ACoAC Phosphorylation shifts ACoAC

from active polymer form to from active polymer form to inactive monomer form.inactive monomer form.

• Carnitine Acyl Transferase I Carnitine Acyl Transferase I (CATI) (transport of FAs into (CATI) (transport of FAs into matrix for matrix for oxid’n). oxid’n).

• Inhibited by mal CoA. MR: mal Inhibited by mal CoA. MR: mal CoA is the product of the CoA is the product of the committed step in the committed step in the opposing pathway, FA opposing pathway, FA synthesis. synthesis.

• ML: When [mal CoA] is high, ML: When [mal CoA] is high, FA synthesis is rapid (in FA synthesis is rapid (in liverliver), ), with the purpose of export of with the purpose of export of these FA’s for storage. these FA’s for storage. Inhibition of CATI prevents Inhibition of CATI prevents consumption from working consumption from working against synthesis.against synthesis.

Figure 25-41Figure 25-41 All of cholesterol’s All of cholesterol’s carbon atoms are derived from carbon atoms are derived from

acetate.acetate.

Pag

e 94

2

Amino Acid (AA) Amino Acid (AA) oxidationoxidation • IntroductionIntroduction

• 1. Part of the C’s of some of the AAs are 1. Part of the C’s of some of the AAs are convertible to ACoA, either directly or via convertible to ACoA, either directly or via acetoacetate or pyr. (and less directly, so are the acetoacetate or pyr. (and less directly, so are the others via TCA int others via TCA int oxac oxac PEP PEP pyr pyr ACoA.) ACoA.) (These AAs are “ketogenic) (These AAs are “ketogenic)

• So, these C’s of So, these C’s of xsxs AA intake (in relation to need AA intake (in relation to need for protein synth) are used as fuel, just like dietary for protein synth) are used as fuel, just like dietary CHCH22O’s, fats.O’s, fats.

• 2. Part (or all) of the C’s of 18 of the AAs can be 2. Part (or all) of the C’s of 18 of the AAs can be converted to TCA intermediates, which can be converted to TCA intermediates, which can be converted to G (TCA int converted to G (TCA int oxac oxac PEP PEP G). G). These are referred to as the “glucogenic” AAs. These are referred to as the “glucogenic” AAs.

• AAs from digestion of muscle protein are the main AAs from digestion of muscle protein are the main source of C for gluconeogenesis in CHsource of C for gluconeogenesis in CH22O starvationO starvation

Transaminations (Transaminations (trnsamstrnsams))• Each AA can be converted to the corresponding Each AA can be converted to the corresponding

keto acid by at least one transaminase. This AA keto acid by at least one transaminase. This AA is oxidized in this rxn, but is oxidized in this rxn, but kg is reduced to kg is reduced to glutamate at the same time so there’s not a net glutamate at the same time so there’s not a net AA oxidationAA oxidation

• The amino group transferred to The amino group transferred to kg (---> glu) is kg (---> glu) is toxictoxic when released as when released as NHNH33, this ammonia is , this ammonia is detoxifieddetoxified by conversion to urea in the urea by conversion to urea in the urea cycle (cycle (NHNH33 can be can be excretedexcreted). Net oxidation ). Net oxidation occurs by occurs by couplingcoupling of of trnsamtrnsam with glutamate with glutamate dehydrogenase (GDH).dehydrogenase (GDH).

• GDH: glu + NADGDH: glu + NAD++ kg + NADH + NH kg + NADH + NH33 • This rxn running in reverse when [NHThis rxn running in reverse when [NH33] is very ] is very

high depletes TCA ints, interferes with TCA +ET + high depletes TCA ints, interferes with TCA +ET + OPOP in brain cells and causes the in brain cells and causes the delirium/dementia in liver damaged patients.delirium/dementia in liver damaged patients.

GDH RegulationGDH Regulation

• Inhibited by ATP and GTP; Activated by Inhibited by ATP and GTP; Activated by ADP and GDPADP and GDP

• MR: GDH + MR: GDH + trnsamtrnsam TCA intsTCA ints TCA TCA ET ET OPOP: ATP production: ATP production

• ML: If [ATP] or [GTP] is high, more is not ML: If [ATP] or [GTP] is high, more is not needed; if [ADP] or [GDP] is high, ATP needed; if [ADP] or [GDP] is high, ATP synthesis is needed; synthesis is needed; TCA TCA ATPATP

Carbamoyl Phosphate Synthetase I Carbamoyl Phosphate Synthetase I RegulationRegulation• CPSI is activated by N-acetylglutamate NAGCPSI is activated by N-acetylglutamate NAG

• MR: NAG is produced from ACoA and glu: MR: NAG is produced from ACoA and glu: • ACoA + glu ACoA + glu NAG. NAG. • A high [ACoA] and/or a high [glu] increases the A high [ACoA] and/or a high [glu] increases the

rate of NAG production and the [NAG], so the rate of NAG production and the [NAG], so the [NAG] indicates the levels of AcoA and glu. [NAG] indicates the levels of AcoA and glu.

• MR, ML for ACoA: when [ACoA] is high there is MR, ML for ACoA: when [ACoA] is high there is a need for oxac to react with ACoA in the CS a need for oxac to react with ACoA in the CS rxn. GDH + rxn. GDH + trnsamtrnsam can produce TCA ints from can produce TCA ints from AAs at a high rate only if CPSI consumes the AAs at a high rate only if CPSI consumes the ammonia product of GDH.ammonia product of GDH.

• MR, ML for glu: when glu is high it has been MR, ML for glu: when glu is high it has been produced by a high rate of trnsam and there is produced by a high rate of trnsam and there is a need to convert it to a need to convert it to kg kg in GDHin GDH to maintain to maintain [[ kg] for TCA and trnsam. CPSI kg] for TCA and trnsam. CPSI mustmust consume consume the ammonia product of GDH.the ammonia product of GDH.