Integración Metabólica y Vías Unidireccionales

CICLOS FUTILES

Los ciclos fútiles, que se podrían producir entre las reacciones irreversibles situadas al mismo nivel de la glucolisis y la gluconeogénesis, consumirían ATP. En general no se producen fisiológicamente gracias a la regulación conjunta de ambas vías: glucolítica y gluconeogénica. Si los consideramos como ciclos de sustrato, sirven para amplificar las señales metabólicas y para producir calor.

allowed to proceed simultaneously at a high rate in thesame cell, a large amount of chemical energy would bedissipated as heat. This uneconomical process has beencalled a futile cycle. However, as we shall see later,such cycles may provide advantages for controllingpathways, and the term substrate cycle is a better de-scription. Similar substrate cycles also occur with theother two sets of bypass reactions of gluconeogenesis(Fig. 15–15).

We begin our examination of the coordinated regu-lation of glycolysis and gluconeogenesis by consideringthe regulatory patterns seen at the three main control

points of glycolysis. We then look at the regulation ofthe enzymes of gluconeogenesis, leading to a consider-ation of how the regulation of both pathways is tightly,reciprocally coordinated.

Hexokinase Isozymes of Muscle and Liver AreAffected Differently by Their Product, Glucose 6-Phosphate

Hexokinase, which catalyzes the entry of free glucoseinto the glycolytic pathway, is a regulatory enzyme.There are four isozymes (designated I to IV), encoded

Chapter 15 Principles of Metabolic Regulation: Glucose and Glycogen576

hexokinase

Glucose

Glucose 6-phosphate

glucose 6-phosphatase

phosphohexose isomerase

phospho-fructokinase-1

Fructose 6-phosphate

Fructose 1,6-bisphosphate

(2) Glyceraldehyde 3-phosphate

fructose1,6-bisphosphatase

(2) 1,3-Bisphosphoglycerate

(2) 3-Phosphoglycerate

(2) 2-Phosphoglycerate

(2) Phosphoenolpyruvate

(2) Pyruvate

(2) Oxaloacetate

pyruvate carboxylase

PEP carboxykinase

pyruvate kinase

Dihydroxyacetonephosphate

Dihydroxyacetonephosphate

aldolase

triose phosphateisomerase

triose phosphateisomerase

glyceraldehyde phosphatedehydrogenase

phosphoglycerate kinase

phosphoglycerate mutase

enolase

Glycolysis Gluconeogenesis

FIGURE 15–15 Glycolysis and gluconeogenesis. Opposing path-ways of glycolysis (pink) and gluconeogenesis (blue) in rat liver.Three steps are catalyzed by different enzymes in gluconeogenesis

(the “bypass reactions”) and glycolysis; seven steps are catalyzedby the same enzymes in the two pathways. Cofactors have beenomitted for simplicity.

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in the two pathways are regulated in a coordinated andreciprocal manner. In general, when sufficient concen-trations of acetyl-CoA or citrate (the product of acetyl-CoA condensation with oxaloacetate) are present, orwhen a high proportion of the cell’s adenylate is in theform of ATP, gluconeogenesis is favored. AMP promotesglycogen degradation and glycolysis by activating glyco-gen phosphorylase (via activation of phosphorylase ki-nase) and stimulating the activity of PFK-1.

All the regulatory actions discussed here are trig-gered by changes inside the cell and are mediated byvery rapid, instantly reversible, allosteric mechanisms.Another set of regulatory processes is triggered fromoutside the cell by the hormones insulin and glucagon,which signal too much or too little glucose in the blood,respectively, or by epinephrine, which signals the im-pending need for fuel for a fight-or-flight response. Thesehormonal signals bring about covalent modification(phosphorylation or dephosphorylation) of target pro-teins inside the cell; this takes place on a somewhat longertime scale than the internally driven allosteric mecha-nisms—seconds or minutes, rather than milliseconds.

Fructose 2,6-Bisphosphate Is a Potent Regulator of Glycolysis and Gluconeogenesis

The special role of liver in maintaining a constant bloodglucose level requires additional regulatory mechanismsto coordinate glucose production and consumption.When the blood glucose level decreases, the hormoneglucagon signals the liver to produce and release moreglucose and to stop consuming it for its own needs. Onesource of glucose is glycogen stored in the liver; anothersource is gluconeogenesis.

The hormonal regulation of glycolysis and gluco-neogenesis is mediated by fructose 2,6-bisphosphate,an allosteric effector for the enzymes PFK-1 andFBPase-1 (Fig. 15–22):

When fructose 2,6-bisphosphate binds to its allostericsite on PFK-1, it increases that enzyme’s affinity for itssubstrate, fructose 6-phosphate, and reduces its affin-ity for the allosteric inhibitors ATP and citrate. At thephysiological concentrations of its substrates ATP andfructose 6-phosphate and of its other positive and neg-ative effectors (ATP, AMP, citrate), PFK-1 is virtuallyinactive in the absence of fructose 2,6-bisphosphate.Fructose 2,6-bisphosphate activates PFK-1 and stimu-lates glycolysis in liver and, at the same time, inhibitsFBPase-1, thereby slowing gluconeogenesis.

Although structurally related to fructose 1,6-bisphosphate, fructose 2,6-bisphosphate is not an in-termediate in gluconeogenesis or glycolysis; it is a reg-ulator whose cellular level reflects the level of glucagonin the blood, which rises when blood glucose falls. Thecellular concentration of fructose 2,6-bisphosphate isset by the relative rates of its formation and breakdown(Fig. 15–23a). It is formed by phosphorylation offructose 6-phosphate, catalyzed by phosphofructoki-nase-2 (PFK-2), and is broken down by fructose 2,6-bisphosphatase (FBPase-2). (Note that these en-zymes are distinct from PFK-1 and FBPase-1, whichcatalyze the formation and breakdown, respectively, offructose 1,6-bisphosphate.) PFK-2 and FBPase-2 aretwo distinct enzymatic activities of a single, bifunctionalprotein. The balance of these two activities in the liver,which determines the cellular level of fructose 2,6-bisphosphate, is regulated by glucagon and insulin (Fig.15–23b). As we saw in Chapter 12 (p. 441), glucagonstimulates the adenylyl cyclase of liver to synthesize3!,5!-cyclic AMP (cAMP) from ATP. Then cAMP acti-vates cAMP-dependent protein kinase, which transfersa phosphoryl group from ATP to the bifunctional pro-tein PFK-2/FBPase-2. Phosphorylation of this proteinenhances its FBPase-2 activity and inhibits its PFK-2 activity. Glucagon thereby lowers the cellular level offructose 2,6-bisphosphate, inhibiting glycolysis andstimulating gluconeogenesis. The resulting productionof more glucose enables the liver to replenish blood glu-cose in response to glucagon. Insulin has the oppositeeffect, stimulating the activity of a phosphoprotein phos-phatase that catalyzes removal of the phosphoryl groupfrom the bifunctional protein PFK-2/FBPase-2, activat-ing its PFK-2 activity, increasing the level of fructose2,6-bisphosphate, stimulating glycolysis, and inhibitinggluconeogenesis.

HCH2

POAO

HO

"O

OB

H

H

OH

CH2

OH

POA

O O

OB

O O


OO" O"

O"

15.3 Coordinated Regulation of Glycolysis and Gluconeogenesis 581


ATP

ADP

AMP

citrate


ATP

ADP

Gluconeogenesis

Glycolysis

PFK-1 FBPase-1

Pi

H2O

FIGURE 15–21 Regulation of fructose 1,6-bisphosphatase-1 (FBPase-1)and phosphofructokinase-1 (PFK-1). The important role of fructose2,6-bisphosphate in the regulation of this substrate cycle is detailedin subsequent figures.

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Chapter 15 Principles of Metabolic Regulation: Glucose and Glycogen582P

FK

-1 a

ctiv

ity

(% o

f Vm

ax)

0

100

[Fructose 6-phosphate] (mM)(a)

(c)

80

60

40

20

0.05 0.1 0.2 0.4 0.7 1.0 2.0

FB

Pas

e-1

acti

vity

(% o

f Vm

ax)

0

100

[Fructose 1,6-bisphosphate] ( M)(b)

80

60

40

20

50 1000 0

4.0

!F26BP

"F26BP

"F26BP

!F26BP

#


F26BP


ATP

ADP

Gluconeogenesis

Glycolysis

PFK-1 FBPase-1

Pi

H2O

FIGURE 15–22 Role of fructose 2,6-bisphosphate in regulation ofglycolysis and gluconeogenesis. Fructose 2,6-bisphosphate (F26BP)has opposite effects on the enzymatic activities of phosphofructoki-nase-1 (PFK-1, a glycolytic enzyme) and fructose 1,6-bisphosphatase(FBPase-1, a gluconeogenic enzyme). (a) PFK-1 activity in the absenceof F26BP (blue curve) is half-maximal when the concentration offructose 6-phosphate is 2 mM (that is, K0.5 $ 2 mM). When 0.13 µM

F26BP is present (red curve), the K0.5 for fructose 6-phosphate is only

0.08 mM. Thus F26BP activates PFK-1 by increasing its apparent affin-ity (Fig. 15–18) for fructose 6-phosphate. (b) FBPase-1 activity is in-hibited by as little as 1 !M F26BP and is strongly inhibited by 25 !M.In the absence of this inhibitor (blue curve) the K0.5 for fructose 1,6-bisphosphate is 5 !M, but in the presence of 25 !M F26BP (red curve)the K0.5 is % 70!M. Fructose 2,6-bisphosphate also makes FBPase-1more sensitive to inhibition by another allosteric regulator, AMP. (c)Summary of regulation by F26BP.



PFK-2 FBPase-2

(a)

ATP

ADP

Pi

ADP

ATP

H2O

Pi

glucagon[cAMP])(

[F26BP]

Stimulates glycolysis,inhibits gluconeogenesis

Inhibits glycolysis,stimulates gluconeogenesis

cAMP-dependentprotein kinase

phospho-proteinphosphatase

insulin

FBPase-2(inactive)

PFK-2(active)

OH

PFK-2(inactive)

FBPase-2(active)

O P O"

O"

O[F26BP]

(b)

FIGURE 15–23 Regulation of fructose 2,6-bisphosphate level. (a) Thecellular concentration of the regulator fructose 2,6-bisphosphate(F26BP) is determined by the rates of its synthesis by phosphofructo-kinase-2 (PFK-2) and breakdown by fructose 2,6-bisphosphatase

(FBPase-2). (b) Both enzymes are part of the same polypeptide chain,and both are regulated, in a reciprocal fashion, by insulin andglucagon. Here and elsewhere, arrows are used to indicate increas-ing (h) and decreasing (g) levels of metabolites.



PF

K-1

act

ivit

y (%

of V

max

)

0

100

[Fructose 6-phosphate] (mM)(a)

(c)

80

60

40

20

0.05 0.1 0.2 0.4 0.7 1.0 2.0

FB

Pas

e-1

acti

vity

(% o

f Vm

ax)

0

100

[Fructose 1,6-bisphosphate] ( M)(b)

80

60

40

20

50 1000 0

4.0

!F26BP

"F26BP

"F26BP

!F26BP

#


F26BP


ATP

ADP

Gluconeogenesis

Glycolysis

PFK-1 FBPase-1

Pi

H2O

FIGURE 15–22 Role of fructose 2,6-bisphosphate in regulation ofglycolysis and gluconeogenesis. Fructose 2,6-bisphosphate (F26BP)has opposite effects on the enzymatic activities of phosphofructoki-nase-1 (PFK-1, a glycolytic enzyme) and fructose 1,6-bisphosphatase(FBPase-1, a gluconeogenic enzyme). (a) PFK-1 activity in the absenceof F26BP (blue curve) is half-maximal when the concentration offructose 6-phosphate is 2 mM (that is, K0.5 $ 2 mM). When 0.13 µM

F26BP is present (red curve), the K0.5 for fructose 6-phosphate is only

0.08 mM. Thus F26BP activates PFK-1 by increasing its apparent affin-ity (Fig. 15–18) for fructose 6-phosphate. (b) FBPase-1 activity is in-hibited by as little as 1 !M F26BP and is strongly inhibited by 25 !M.In the absence of this inhibitor (blue curve) the K0.5 for fructose 1,6-bisphosphate is 5 !M, but in the presence of 25 !M F26BP (red curve)the K0.5 is % 70!M. Fructose 2,6-bisphosphate also makes FBPase-1more sensitive to inhibition by another allosteric regulator, AMP. (c)Summary of regulation by F26BP.



PFK-2 FBPase-2

(a)

ATP

ADP

Pi

ADP

ATP

H2O

Pi

glucagon[cAMP])(

[F26BP]

Stimulates glycolysis,inhibits gluconeogenesis

Inhibits glycolysis,stimulates gluconeogenesis

cAMP-dependentprotein kinase

phospho-proteinphosphatase

insulin

FBPase-2(inactive)

PFK-2(active)

OH

PFK-2(inactive)

FBPase-2(active)

O P O"

O"

O[F26BP]

(b)

FIGURE 15–23 Regulation of fructose 2,6-bisphosphate level. (a) Thecellular concentration of the regulator fructose 2,6-bisphosphate(F26BP) is determined by the rates of its synthesis by phosphofructo-kinase-2 (PFK-2) and breakdown by fructose 2,6-bisphosphatase

(FBPase-2). (b) Both enzymes are part of the same polypeptide chain,and both are regulated, in a reciprocal fashion, by insulin andglucagon. Here and elsewhere, arrows are used to indicate increas-ing (h) and decreasing (g) levels of metabolites.


Glycogen Synthase Is Also Regulated by Phosphorylation and Dephosphorylation

Like glycogen phosphorylase, glycogen synthase can ex-ist in phosphorylated and dephosphorylated forms (Fig.15–27). Its active form, glycogen synthase a, is un-phosphorylated. Phosphorylation of the hydroxyl sidechains of several Ser residues of both subunits convertsglycogen synthase a to glycogen synthase b, which isinactive unless its allosteric activator, glucose 6-phosphate, is present. Glycogen synthase is remarkablefor its ability to be phosphorylated on various residuesby at least 11 different protein kinases. The most im-portant regulatory kinase is glycogen synthase kinase3 (GSK3), which adds phosphoryl groups to three Serresidues near the carboxyl terminus of glycogen syn-thase, strongly inactivating it. The action of GSK3 is hi-erarchical; it cannot phosphorylate glycogen synthaseuntil another protein kinase, casein kinase II (CKII),has first phosphorylated the glycogen synthase on anearby residue, an event called priming (Fig. 15–28a).

In liver, conversion of glycogen synthase b to theactive form is promoted by PP1, which is bound to theglycogen particle. PP1 removes the phosphoryl groupsfrom the three Ser residues phosphorylated by GSK3.Glucose 6-phosphate binds to an allosteric site on glyco-gen synthase b, making the enzyme a better substratefor dephosphorylation by PP1 and causing its activation.By analogy with glycogen phosphorylase, which acts asa glucose sensor, glycogen synthase can be regarded as

a glucose 6-phosphate sensor. In muscle, a differentphosphatase may have the role played by PP1 in liver,activating glycogen synthase by dephosphorylating it.

Glycogen Synthase Kinase 3 Mediates the Actions of Insulin

As we saw in Chapter 12, one way in which insulin trig-gers intracellular changes is by activating a protein ki-nase (protein kinase B, or PKB) that in turn phosphor-ylates and inactivates GSK3 (Fig. 15–29; see also Fig.12–8). Phosphorylation of a Ser residue near the aminoterminus of GSK3 converts that region of the protein toa pseudosubstrate, which folds into the site at which thepriming phosphorylated Ser residue normally binds(Fig. 15–28b). This prevents GSK3 from binding thepriming site of a real substrate, thereby inactivating theenzyme and tipping the balance in favor of dephosphor-ylation of glycogen synthase by PP1. Glycogen phos-phorylase can also affect the phosphorylation of glyco-gen synthase: active glycogen phosphorylase directlyinhibits PP1, preventing it from activating glycogen syn-thase (Fig. 15–27).

Although first discovered in its role in glycogen me-tabolism (hence the name glycogen synthase kinase),GSK3 clearly has a much broader role than the regula-tion of glycogen synthase. It mediates signaling by in-sulin and other growth factors and nutrients, and it actsin the specification of cell fates during embryonic de-velopment. Among its targets are cytoskeletal proteins


Insulin

ADP

ATP

3ADP 3ATPGSK3

CKII

HOHO

HO

Glycogensynthase

a

Glycogensynthase

bInactive

PP1 3Pi

Active

GlucoseInsulin Glucose6-phosphate

Glucagon,epinephrine

Phosphoserinesnear carboxylterminus

P

PP

FIGURE 15–27 Effects of GSK3 on glycogen synthase activity.Glycogen synthase a, the active form, has three Ser residues near itscarboxyl terminus, which are phosphorylated by glycogen synthasekinase 3 (GSK3). This converts glycogen synthase to the inactive (b)form (GSb). GSK3 action requires prior phosphorylation (priming) bycasein kinase (CKII). Insulin triggers activation of glycogen synthase bby blocking the activity of GSK3 (see the pathway for this action inFig. 12–8) and activating a phosphoprotein phosphatase (PP1 inmuscle, another phosphatase in liver). In muscle, epinephrine acti-vates PKA, which phosphorylates the glycogen-targeting protein GM

(see Fig. 15–30) on a site that causes dissociation of PP1 from glycogen.Glucose 6-phosphate favors dephosphorylation of glycogen synthaseby binding to it and promoting a conformation that is a good substratefor PP1. Glucose also promotes dephosphorylation; the binding ofglucose to glycogen phosphorylase a forces a conformational changethat favors dephosphorylation to glycogen phosphorylase b, thus re-lieving its inhibition of PP1 (see Fig. 15–29).

8885d_c15_586 2/26/04 2:02 PM 86 mac76 mac76:385_reb:

Consiste en un acoplamiento de dos rutas metabólicas (glucolisis y gluconeogénesis) en dos órganos distintos (músculo e hígado), que permite a las células musculares poder disponer de la energía necesaria en todo momento. El músculo obtiene ATP a partir de la degradación de glucosa en la glucolisis. Cuando las condiciones del ejercicio son anaeróbicas la glucosa se degrada a piruvato y éste se reduce a lactato. El lactato es exportado a la circulación y es captado por el hígado. El hígado sintetiza glucosa de nuevo a partir de lactato por la ruta gluconeogénica.

Ciclo de CORI

1

Ciclo de Cori

Ciclo Glucosa/Alanina

Glucosa

Urea

(2) Piruvato

(2) Alanina

(2)-NH2

6 ATP

4 ATP Enzima?

Glucosa

(2) Piruvato (2) Lactato

(2) Alanina

2 ATP

α-Ceto ácido

α aminoácido

Sangre

Hígado Músculo

En general involucra AA transferasas y GPT moviendo nitrógeno desde el músculo al hígado. GPT: Glutamato piruvato transaminasa. Note la similitud con el ciclo de Cori!!

Alanine Transports Ammonia from Skeletal Muscles to the Liver

Alanine also plays a special role in transporting aminogroups to the liver in a nontoxic form, via a pathwaycalled the glucose-alanine cycle (Fig. 18–9). In mus-cle and certain other tissues that degrade amino acidsfor fuel, amino groups are collected in the form ofglutamate by transamination (Fig. 18–2a). Glutamatecan be converted to glutamine for transport to the liver,as described above, or it can transfer its !-amino groupto pyruvate, a readily available product of muscleglycolysis, by the action of alanine aminotransferase(Fig. 18–9). The alanine so formed passes into the bloodand travels to the liver. In the cytosol of hepatocytes,alanine aminotransferase transfers the amino groupfrom alanine to !-ketoglutarate, forming pyruvate andglutamate. Glutamate can then enter mitochondria,where the glutamate dehydrogenase reaction releasesNH4

! (Fig. 18–7), or can undergo transamination withoxaloacetate to form aspartate, another nitrogen donorin urea synthesis, as we shall see.

The use of alanine to transport ammonia fromskeletal muscles to the liver is another example of theintrinsic economy of living organisms. Vigorously con-tracting skeletal muscles operate anaerobically, produc-ing pyruvate and lactate from glycolysis as well as

Chapter 18 Amino Acid Oxidation and the Production of Urea664

BOX 18–1 BIOCHEMISTRY IN MEDICINE

Assays for Tissue DamageAnalyses of certain enzyme activities in blood serumgive valuable diagnostic information for a number ofdisease conditions.

Alanine aminotransferase (ALT; also called glutamate-pyruvate transaminase, GPT) and aspar-tate aminotransferase (AST; also called glutamate-oxaloacetate transaminase, GOT) are important in thediagnosis of heart and liver damage caused by heartattack, drug toxicity, or infection. After a heart attack,a variety of enzymes, including these aminotrans-ferases, leak from the injured heart cells into thebloodstream. Measurements of the blood serum con-centrations of the two aminotransferases by the SGPTand SGOT tests (S for serum)—and of another en-zyme, creatine kinase, by the SCK test—can pro-

vide information about the severity of the damage.Creatine kinase is the first heart enzyme to appear inthe blood after a heart attack; it also disappearsquickly from the blood. GOT is the next to appear, andGPT follows later. Lactate dehydrogenase also leaksfrom injured or anaerobic heart muscle.

The SGOT and SGPT tests are also important inoccupational medicine, to determine whether peopleexposed to carbon tetrachloride, chloroform, or otherindustrial solvents have suffered liver damage. Liverdegeneration caused by these solvents is accompaniedby leakage of various enzymes from injured hepato-cytes into the blood. Aminotransferases are most use-ful in the monitoring of people exposed to these chem-icals, because these enzyme activities are high in liverand can be detected in very small amounts.

Bloodglucose

Glucose

Glucose

Pyruvate

Pyruvate

Bloodalanine

Alanine

Alanine

Glutamate

Glutamate

Amino acids

Muscleprotein

Urea

NH4!

NH4!

urea cycle

alanineaminotransferase

alanineaminotransferase

gluconeo-genesis

glycolysis

-Ketoglutarate"

-Ketoglutarate"

LiverFIGURE 18–9 Glucose-alanine cycle. Alanine serves as a carrier ofammonia and of the carbon skeleton of pyruvate from skeletal mus-cle to liver. The ammonia is excreted and the pyruvate is used to pro-duce glucose, which is returned to the muscle.

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Integración Metabólica y Vías Unidireccionales

Temas

•  Perfiles Metabólicos •  Análisis del Metabolismo •  Unidireccionalidad •  Metabolismo en un Organismo Multicelular

Perfiles en el metabolismo de combustible

Tejido Combustible Combustible Combustible almacenado preferido exportados

Cerebro Ninguno Glucosa (cuerpos Ninguno

cetónicos) Músculo esq Glucógeno Acidos grasos Ninguno (reposo) Músculo esq Ninguno Glucosa Lactato, Alanina (ejercicio) Músculo Ninguno Acidos grasos Ninguno Cardiaco Tejido adiposo Triacilgliceroles Acidos grasos Acidos grasos, glicerol Hígado Glucógeno Aminoácidos Acidos grasos, glucosa

triacilgliceroles glucosa, ácidos grasos cuerpos cetónicos

Principales hormonas que controlan metabolismo de combustible

Hormona Acciones bioquímicas Acciones fisiológicas Insulina permeabilidad celular a glucosa (musc. y Tej. Adip.) Señala el estado de ingesta

Glucólisis Concentración de glucosa en sangre Síntesis de glucógeno Almacenamiento de combustible Síntesis de triacilgliceroles Crecimiento y diferenciación celular Gluconeogénesis Lipólisis Degradación de proteínas Síntesis de proteínas, DNA y RNA

Glucagón Concentración de cAMP (hígado y tej. adiposo) Liberación de glucosa del hígado

Glucogenólisis Concentración de glucosa en sangre Síntesis de glucógeno Hidrólisis de triacilgliceroles Gluconeogénesis Glucólisis

Adrenalina Concentración de cAMP en músculo Liberación de glucosa del hígado

Movilización de triacilgliceroles Uso de glucosa por el músculo Glucogenólisis Concentración de glucosa en sangre Síntesis de glucógeno

Análisis del Metabolismo

Vías Catabólica y anabólica, ocurren simultáneamente, deben actuar de manera regulada, ordenadamente, respuesta total

•  Solo uno pocos intermediarios conectan sistemas

principales - azúcar-Ps, alfa-ceto ácidos, derivados CoA, y PEP

•  ATP & NADPH relacionan catabolismo & anabolismo

Mecanismos frecuentes de regulación metabólica

Anabolismo y catabolismo deben coordinarse con precisión

•  Interacciones alostéricas •  Modificación covalente •  Niveles enzimáticos •  Compartimentalización •  Especialización metabólica en los órganos

Interacciones alostéricas Los posibles puntos de control son enzimas

que catalizan reacciones irreversibles

•  Fosfofrutoquinasa (glicólisis): Activadores AMP, ADP, fructosa 2,6 bifosfato. Inhibidores ATP y citrato

•  Acetil CoA carboxilasa (síntesis de ácidos grasos): Activador citrato. Inhibidor palmitoil CoA

Modificación Covalente

Algunas enzimas son controladas por modificación covalente, además de

interacción alostérica

•  Glucógeno fosforilasa: dos formas (fosforilasa a [activa] y fosforilasa b [relativamente inactiva]. Fosforilación de Ser14, aumenta actividad de la enzima.

•  Glucógeno sintasa: dos formas (sintasa I [activa] y sintasa

D [menos activa]. La enzima disminuye su actividad cuando se fosforila.

Estequiometría Metabólica Tres tipos de estequiometría en sistemas

biológicos

• Reacción estequiométrica - el número de cada tipo de átomo en una reacción

• Estequiometría de apareamiento obligada - requiere el acoplamiento de transportadores de electrones

• Estequiometría acoplada – el número de moléculas de ATP en la vías involucra comsumirlo o producirlo -

Naturaleza de equivalente de ATP Una perspectiva diferente

•  ΔG para hidrólisis de ATP dice que a concentraciones

de equilibrio de ADP y Pi deberían ser muy superiores a ATP

•  Sin embargo, una célula donde esto sea cierto, es porque está muerta

•  Control cinético sobre las vías catabólicas asegura que la razón [ATP]/[ADP][Pi] permanezca muy alto

•  Esto permite que hidrólisis de ATP puede servir como la fuerza conductora para la mayoría de los procesos bioquímicos

El ATP equivalente Cuál es el “coeficiente de acoplamiento" para producir o

consumir ATP?

• Coeficiente de acoplamiento son los moles de ATP producidos o consumidos por mol de substrato convertido (o producto formado)

• Oxidación celular de glucosa tiene un coeficiente de acoplamiento de 30-38 (dependiendo del tipo de célula)

• Hexoquinasa tiene un coeficiente de acoplamiento de -1

• Piruvato kinasa (en glicólisis) coeficiente de acoplamiento de +1

El significado de los 38 ATPs La “estequiometría del ATP" tiene un gran efecto

sobre la Keq de una reacción

• Considere la Keq para la oxidación de glucosa • Si se producen 38 ATP, el G celular es -967

kJ/mol y Keq = 10170, un número muy grande! • Si G = 0, se pueden hacer 58 ATP, pero la

reacción estará en equilibrio con solo la mitad de la glucosa oxidada que nosotros tenemos

• De forma que el número de 38 es un compromiso!

Significado de grandes Keq

A mayor ATP generado, más baja la constante de equilibrio, y más alto el nivel de glucosa requerida

• Si [glucosa] esta bajo este valor, no será

efectivamente utilizada • Grandes Keq significa que los niveles basales de

glucosa serán muy bajos • Grandes Keq también significan que la reacción

estará lejos del equilibrio y puede ser regulada

El valor ATP de NADH vs NADPH

• El valor ATP para NADH es 2.5-3 • El valor ATP para NADPH es mayor • NADPH transporta electrones desde vías

catabólicas a procesos biosintéticos • [NADPH]>[NADP+] por ello NADPH/NADP+ es

mejor sistema donante de e- que NADH/NAD • De manera que NADPH promueve 3.5-4 ATP!

Vías unidireccionales

Un rol "secreto" de ATP en metabolismo

•  Ambas direcciones de alguna vía opuesta debe ser favorable, de forma que el efector alostérico puede controlar efectivamente la dirección

•  El coeficiente de acoplamiento de ATP para alguna de esas secuencias ha evolucionado de manera que todo el equilibrio para la conversión es altamente favorable

Combustible para el cerebro

•  Cerebro tiene un alto metabolismo pero no tiene combustible de reserva

•  Esto significa que el cerebro requiere un suplemento constante de glucosa

•  En condiciones extremas, cerebro puede usar β-hidroxibutirato (desde ácidos grasos), convirtiendolo en acetil-CoA en TCA

•  Esto permite que el cerebro use grasas como combustible!

Creatina Kinasa en Músculo

•  Músculos deben ser preparados para una rápida provisión de energía

•  Creatina kinasa y fosfocreatina actúan como sistema tampón, proveyendo ATP adicional para la contracción

•  Glicógeno provee energía adicional, liberando glucosa para la glicólisis

•  Glicólisis baja rápidamente pH, causando fatiga muscular

Fosfocreatina Creatinina

Degradación de la proteína músculo

•  Durante alta actividad, los amino ácidos se degradan a piruvato, los cuales pueden ser transaminados a alanina

•  Alanina circula al hígado, donde es convertido nuevamente a piruvato – alimento para gluconeogénesis

•  Este es el último recurso para organismos exhaustos o que presentan alta actividad

Piruvato

Alanina

Gluatamato

α-cetoglutarato

Glutamato alanina aminotransferasa

Productos intermediario Glicólisis y ciclo de Krebs

Consumo de Proteínas

Proteínas corporales

Purinas Pirimidinas Porfirinas

Neurotransmisores Fosfolípidos enzimas

AA

NH3

Urea

AA no escenciales

Glucosa + depósito glicógeno

Energía CO2 + H2O

Acetil CoA

Acidos grasos

Integración del Metabolismo

Integración Metabólica y Vías Unidireccionales

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