the normal liver: basic metabolic liver function...the liver as nutrient distribution centre • the...

The Normal Liver:

Basic metabolic liver function

Part I: Carbohydrate and intermediary metabolism

Regulation by glucagon and insulin

The Liver as nutrient distribution centre

• The liver as a metabolic distribution

centre:

• Via portal vein, receives blood from

spleen and from most of the GI

tract, esp. stomach, pancreas,

duodenum and mesentery

• Responds to insulin and glucagon

• Processing of carbohydrates, lipids

and amino acids depending on

physiological condition

Glycogen Glucose-1-P Glucose-6-P Glucose

Fructose-6-P

Fructose-1,6-bis-P

Glyceraldehyde-3-P Dihydroxyacetone-P

1,3-Bisphosphoglycerate

3-Phosphoglycerate

2-Phosphoglycerate

Phosphoenolpyruvate

Pyruvate

Acetyl-CoA

Oxaloacetate Citrate

Isocitrate

α-Ketoglutarate

Succinyl-CoA Succinate

Fumarate

Malate

CO2

CO2

CO2

Acetoacetate

Hydroxybutyrate

Leu

Phe

Tyr

Trp

Lys

Lactate

Glutamine

Glutamate

Pro

His

Arg

Ile

Malonyl-CoA

Fatty acyl-CoA Fatty acids

Glycerol Glycerol-P

Triacylglycerol

Methylmalonyl-CoA Ile

Met

Val

Thr Propionyl-CoA

Ala

Cys

Gly

Ser

Thr

Phe

Tyr

Arginino-

succinate

Arginine

NH3

Ornithine

Citrulline

CO2

Carbamoyl-P

Urea

Aspartate

Asparagine

Intermediary

metabolism

Ferrier, Biochemistry 6th ed.

Fig. 8.2 (Lippincott’s)

Intermediates of Carbo-

hydrate metabolism

Intermediates of protein

metabolism

Intermediates of lipid

metabolism

Main Learning Goals

• Understand (at a basic level) the input, output and role of key

metabolic pathways: glycolysis, TCA cycle, gluconeogenesis,

synthesis and degradation of glycogen and fatty acids,

transamination and urea cycle.

• Understand the effect of insulin and glucagon on these metabolic

pathways in the fasting and absorptive state and appreciate the

range of regulatory mechanisms

• Understand the role of the liver in buffering blood glucose levels

and in distributing nutrients

TAG

sy

nth

P

DH

G

lyco

lysi

s

Major metabolic pathways in the liver

Glucose

Acetyl-CoA

Glucose-6-P Glycogen

Pyruvate

Fatty acids

TAG

PPP

NADPH

GLUT-2 Fa

tty

acid

sy

nth

Absorptive state

Glucose

Acetyl-CoA

Glycogen

Fatty acids

Blo

od

TCA

cycle

Ketone

bodies

Pyruvate

Lactate, Amino acids

Glu

con

eogen

esis

Glucose-6-P

Hydrolysis of TAGs

β-o

xidatio

n

Ketogenesis

Major metabolic pathways in the liver

Fasting state

Glucose-6-P Glucose

Fructose-6-P

Fructose-1,6-bis-P



3-Phosphoglycerate

2-Phosphoglycerate

Phosphoenolpyruvate

Pyruvate Lactate

Glycolysis

Catabolism (breaking down) of

glucose (and most other

carbohydrates via glucose) in all

tissues.

Generation of intermediates for

other pathways

Generation of energy and (in

aerobic conditions) reducing

equivalents

End product depends on O2:

Pyruvate under aerobic

conditions, Lactate under

anaerobic conditions

Glucose-6-P

Glucose

Fructose-6-P

Fructose-1,6-bis-P



3-Phosphoglycerate

2-Phosphoglycerate

Phosphoenolpyruvate

Pyruvate

Lactate

A family of glucose transporters (GLUT) facilitates

diffusion of glucose into cells.

Many are tissue-specific: GLUT-4 in adipose tissue;

GLUT-2 in liver

GLUT-2 can facilitate both glucose entry into liver

cells and exit

Glucose-6-P

Glucose

Fructose-6-P

Fructose-1,6-bis-P



3-Bisphosphoglycerate


Phosphoenolpyruvate

Pyruvate

Lactate

ATP

Glucose concentration, mM

En

zym

e a

ctivity

Hexokinase I-III

Glucokinase

Concentration of fasting

blood glucose {

5 10 15 20

Phosphorylation traps glucose in the cell. because the

ionic phosphate cannot cross the membrane

spontaneously.

The reaction is catalysed by the enzyme Hexokinase.

Enzyme variants in most tissues (Hexokinase I-III) are

relatively slow but are fully active at very low

concentrations of glucose.

Hexokinase-IV, or Glucokinase, in the liver has a much

higher capacity to trap glucose in the liver, but only

when glucose concentrations are high- eg after a meal.

Glucose-6-P

Glucose

Fructose-6-P

Fructose-1,6-bis-P



3-Phosphoglycerate

2-Phosphoglycerate

Phosphoenolpyruvate

Pyruvate Lactate

ATP

ATP

NAD+ + Pi

NADH +H+

In the next steps,

• Glucose-6-P is isomerized to Fructose-6-P

• F-6-P is phosphorylated again to yield F-1,6-BP

in the most critical regulated step **

• the C6 molecule F-1,6-BP is cleaved into two

C3 molecules;

• In the only oxidative step of glycolysis, GA-3P

is converted to 1,3-BPG.

• NADH + H+ generated in this oxidative step can

be regenerated under anaerobic conditions by

reducing pyruvate to lactate.

• This happens in exercising skeletal muscle and

poorly vascularized and/ or mitochondria-free

tissues.

• The liver can re-oxidise lactate to pyruvate.

NAD+ NADH +H+

**

Glucose-6-P

Glucose

Fructose-6-P

Fructose-1,6-bis-P



3-Phosphoglycerate

2-Phosphoglycerate

Phosphoenolpyruvate

Pyruvate Lactate

ATP

ATP

NAD+ + Pi

ATP

NADH +H+

• 1,3-BPG and PEP are high-energy compounds

that can transfer phosphate to ADP (substrate

level phosphorylation). That way, 4 ATP are

generated from each molecule of F-1,6.

• Accounting for the “investment” of 2 ATP per

molecule glucose early on, there is a net

generation of 2ATP per glucose in

glycolysis.

• Pyruvate kinase, the last enzyme of glycolysis,

is activated by Fructose-1,6-bisphosphate

(feed forward activation!) and repressed by

glucagon.

NAD+ NADH +H+

ATP

Fructose-6-P

Fructose-1,6-bis-P



3-Phosphoglycerate

2-Phosphoglycerate

Phosphoenolpyruvate

Pyruvate

Acetyl-CoA


Isocitrate

α-Ketoglutarate


Fumarate

Malate

CO2

CO2

CO2

Acetoacetate

Hydroxybutyrate

Leu

Phe

Tyr

Trp

Lys

Glutamine

Glutamate

Pro

His

Arg

Ile

Malonyl-CoA


Glycerol Glycerol-P

Triacylglycerol


Met

Val

Thr Propionyl-CoA

Ala

Cys

Gly

Ser

Thr

Phe

Tyr

Arginino-

succinate

Arginine

NH3

Ornithine

Citrulline

CO2

Carbamoyl-P

Urea

Aspartate

Asparagine

Glycolysis

Glucose is greatly preferred as

energy source by brain and

nervous tissue, and essential

for the adrenal medulla, testes

and mature erythrocytes.

The Liver

is tasked with maintaining

stable blood glucose levels.

It is the main tissue performing

these two maintenance

mechanisms:

Glycogenesis

Glycogenolysis

Gluconeogenesis

Oxaloacetate

CO2



Glycogen

• Is a highly branched, all-glucose poly-

saccharide with an α-1,4-linked backbone and

α-1,6-linked branches

• Resembles the amylopectin component of plant

starch, but is more highly branched.

• Is the storage form of glucose, mainly in

skeletal muscle (1-2% by weight, total ~400g)

and liver (up to 10% by weight, total ~100g)

• Synthesis after a meal and degradation during

an overnight fast are key mechanisms that

maintain blood glucose levels and are

regulated by glucagon.

Fructose-6-P

Fructose-1,6-bis-P



3-Phosphoglycerate

2-Phosphoglycerate

Phosphoenolpyruvate

Pyruvate

Oxaloacetate

CO2

Glucose-6-P Glucose

Gluconeogenesis

Glycogen stores in the liver can

supply glucose-dependent

tissues with most of the fuel

during an overnight fast, but not

much longer. Gluconeogenesis

is a pathway active in the liver

(and after prolonged fasting,

the kidney) that regenerates

glucose from non-carbohydrate

precursors.

• Lactate from skeletal

muscle is re-oxidised to

Pyruvate. This liver-muscle

cycle is called Cori Cycle.

Lactate

Glycerol Glycerol-P

α-Ketoglutarate

• Glycerol is released by

the hydrolysis of fat (TAGs)

in adipocytes

• Amino acids from tissue

protein are metabolised to

α-keto acids like

oxaloacetate and α-

ketoglutarate.

Opposing regulation of metabolism by

insulin vs glucagon

Lippincott, Fig. 23.5

Glucose

uptake

Glycogen synthesis

Protein

synthesis

Amino acid

uptake

Fat synthesis

Ketogenesis

Lipolysis

Gluconeogenesis

Glycogenolysis

Regulatory mechanisms

• Availability of substrates: eg Glucokinase

• Allosteric control (regulatory metabolites bind outside of the active

site of enzymes and modulate activity): eg PFK1 regulation by F-2,6-

BP

• Regulatory phosphorylation: eg phosphorylation of glycogen

synthase and phosphorylase kinase by PKA

• Changes in transcription: eg increase in expression of glycolytic

enzymes triggered by insulin

Glucose-6-P

Glucose

Fructose-6-P Regulation of Phosphofructokinase-1

(PFK-1)

PFK1 acts after isomerisation of Glucose-6-P to

Fructose-6-P and catalyses the most important

regulated step of glycolysis:

• It is the rate-limiting (slowest) step in glycolysis

• PFK-1 is allosterically activated by AMP

(activation by low energy levels in the cell) and

repressed by ATP and citrate

• PFK-1 is activated by Fructose-2,6-

bisphosphate whose biosynthesis in turn is

regulated by insulin and glucagon.

Fructose-2,6-P +

- + AMP

ATP, citrate

Fructose-1,6-bis-P

ATP

ADP

PFK-1 PFK-2

-

+ insulin

glucagon

PFK-2 regulation in detail: high glucagon

PFK-2 regulation in detail: low glucagon

Fructose-6-P

Fructose-1,6-bis-P



3-Phosphoglycerate

2-Phosphoglycerate

Phosphoenolpyruvate

Pyruvate

Oxaloacetate

CO2

Glucose-6-P Glucose

Gluconeogenesis

Glucagon is the main

regulator of gluconeogenesis.

It acts by repressing ( )

pyruvate kinase, thus

increasing the availability of

PEP for gluconeogenesis.

Glucagon also increases the

expression ( ) of PEP

carboxykinase.

Lactate

ATP

GTP

- + Glucagon

-

+

F-2,6-BP - - Glucagon +

Finally, glucagon represses

the formation of F-2,6-BP,

which is a repressor of

Fructose-1,6-bisphosphatase

in gluconeogenesis (while it is

an activator of PFK-1 in

glycolysis).

Glycogenesis and

Glycogenolysis: Regulation

• Glycogen metabolism is controlled hormonally by

glucagon and insulin.

• Glucagon triggers the production of cAMP in cells,

which in turn activates protein kinase A (PKA).

• PKA phosphorylates glycogen synthase directly,

and glycogen phosphorylase via phosphorylase

kinase.

• Phosphorylation has opposite effects on the two

enzymes: glycogen synthase becomes inactive,

while glycogen phosphorylase is activated by

phosphorylation.

• As a result, glucagon promotes glycogenolysis

and inhibitis glycogenesis.

Glucose-1-P

Glucose-6-P Glucose

Debranching

enzyme

Glycogen

phosphorylase-P

Glucose-6-

phosphatase PGM

Glycogen synthase

Branching

enzyme

Glycogen

phosphorylase Glycogen

Synthase-P

Glucagon

Phosphorylase

kinase

Protein

Kinase A

+ +

+

-

Intermediary

metabolism

Ferrier, Biochemistry 6th ed.

Fig. 8.2 (Lippincott’s)


Fructose-6-P

Fructose-1,6-bis-P





Phosphoenolpyruvate

Pyruvate

Acetyl-CoA


Isocitrate

α-Ketoglutarate


Fumarate

Malate

CO2

CO2

CO2

Acetoacetate

Hydroxybutyrate

Leu

Phe

Tyr

Trp

Lys

Lactate

Glutamine

Glutamate

Pro

His

Arg

Ile

Malonyl-CoA


Glycerol Glycerol-P

Triacylglycerol


Met

Val

Thr Propionyl-CoA

Ala

Cys

Gly

Ser

Thr

Phe

Tyr

Arginino-

succinate

Arginine

NH3

Ornithine

Citrulline

CO2

Carbamoyl-P

Urea

Aspartate

Asparagine

Pyruvate

Acetyl-CoA


Isocitrate

α-Ketoglutarate


Fumarate

Malate

CO2

CO2

CO2

TCA cycle

Acetyl-CoA


Isocitrate

α-Ketoglutarate


Fumarate

Malate

CO2

CO2

The Pyruvate Dehydrogenase complex

Pyruvate

CO2

• Pyruvate is shuttled into

mitochondria with the help of a

dedicated transporter.

• The Pyruvate Dehydrogenase

complex (PDH) is a gigantic

multi-enzyme complex with

dozens of copies each of three

enzymes E1, E2 and E3.

• The complex needs no fewer

than five cofactors, some

derived from vitamins:

Thiamine-PP (B1), lipoic acid,

Coenzyme A (from

pantothenate, B5), FAD (from

riboflavin, B2) and NAD (from

nicotinamide, B3).

http://www.rcsb.org/pdb/101/

motm.do?momID=153

NAD+

NADH+H+

• The Pyruvate Dehydrogenase

complex (PDH) is inactivated by

PDH kinase, which in turn is

activated by high ATP/ADP and

high NADH/NAD+ but

inactivated by pyruvate.

• (May also be responsible for

Warburg effect: Mol. Cell

61(5):705-19 (March 03 2016)

PDH

Acetyl-CoA


Isocitrate

α-Ketoglutarate


Fumarate

Malate

CO2

CO2

The TCA cycle

• The TCA cycle (or Krebs cycle, or citric acid cycle) is a central “metabolic roundabout” with

multiple entry and exit points. Several of the intermediates are involved in gluconeogenesis, amino

acid and heme metabolism.

• The oxidative catabolism of carbohydrates, lipids and amino acids comes together here.

• All TCA cycle reactions happen in mitochondria and require oxygen to recycle the reduced

coenzymes NADH+H+ and FADH2..

Acetyl-CoA


Isocitrate

α-Ketoglutarate


Fumarate

Malate

CO2

TCA cycle function

• The TCA cycle provides for full

oxidation of acetyl-CoA to 2 CO2

• …and generation of reducing

equivalents, which upon oxidation

in the mitochondrial electron

transport chain generate 28 ATP

per molecule glucose (6 from

PDH, 22 from cycle) and 2 GTP

NAD+

CO2

NAD+

GDP + Pi GTP,

Co-A

NAD+

FAD FADH2

NADH+H+

NADH+H+

NADH+H+

Ox

ida

tive

ph

os

ph

ory

lati

on

• Four intermediates of the TCA

cycle are amino acid

metabolites. This allows their

conversion to glucose by

gluconeogenesis.

Carbohydrates Amino acids Fatty acids

28

AT

P

Amino

acids

Glucose

The Normal Liver:

Basic metabolic liver function

Part II: Lipids and amino acid metabolism

Review of fasting and absorptive state

G-6-P Glucose

F-6-P

F-1,6-BP

GA-3-P Dihydroxyacetone-P

1,3-BPG

3-PG

2-PG

PEP

Pyruvate

Acetyl-CoA


Isocitrate

α-Ketoglutarate


Fumarate

Malate

CO2

CO2

CO2

Acetoacetate

Hydroxybutyrate

Malonyl-CoA


Glycerol Glycerol-P

Triacylglycerol

Methylmalonyl-CoA

Propionyl-CoA

• Most of the fatty acids needed by the body

are provided with a normal diet. Any

carbohydrates or proteins in excess of the

body’s needs can be converted to fatty

acids by the liver and ultimately stored as

fats (triacylglycerols) in adipocytes.

• The process starts with cytoplasmic

acetyl-coA.

• Since most acetyl-Co-A is

generated in mitochondria

and cannot cross the

membrane, a shuttle is

needed.

Fatty acid synthesis

Malonyl-CoA

• The next step, catalysed by Acetyl-

CoA carboxylase (ACC) is rate-

limiting and regulated:

• It is activated by citrate.

• The enzyme is active as a multi-

subunit polymer stabilised by citrate.

• ACC is inactivated directly by fatty

acyl-CoA and by phoshorylation by

AMPK.

• Via regulation of ACC phosphorylation,

insulin indirectly activates ACC;

glucagon and AMP inactivate ACC.


Acetyl-CoA

ATP ADP CO2

+

Acetyl-CoA

carboxylase

Citrate

CH3

S

O

CoA

CoA S

O O -

O

- Fatty acyl-CoA

+ Insulin

- Glucagon

- AMP

Fatty acyl-CoA

Triacylglycerol

• The β-oxidation of fatty acids produces large amounts of energy:

• Per 2-carbon unit, one FADH2, one NADH and one acetyl-CoA are

produced. Ultimately, these produce 2, 3 and 12 ATP, respectively. Per

16-carbon (palmitoyl-) CoA, that’s 129 ATP!

Fatty acid catabolism: β-oxidation

Acetyl-CoA Fatty acyl-CoA Fatty acids

Synthesis Degradation

Greatest Flux through pathway After carbohydrate-rich meal In starvation

Hormonal state favouring pathway High insulin/ glucagon ration Low insulin/ glucagon ration

Major tissue site Primarily Liver Muscle, Liver

Subcellular localisation Cytosol Primarily mitochondria

Redox coenzymes NADPH NAD+, FADH2

Product Palmitate (C16) Acetyl-CoA

• Ketone bodies are an “emergency fuel” that the liver can produce to preserve glucose. The

liver itself cannot use ketone bodies, though!

• During starvation, the ability of the liver to oxidise fatty acids released from adipocytes may

be limited. The liver produces ketone bodies and releases them into the blood for

peripheral tissues.

Ketone bodies

2 Acetyl-CoA

Acetoacetyl-CoA 3-Hydroxy-3-methylglutaryl-CoA

Acetoacetate

3-Hydroxybutyrate

CoA Acetyl-CoA CoA

Acetyl-CoA

NADH + H+

NAD+

CH3

O

O-

O

CH3

OH

O-

O

Acetone

CH3CH3

O

Aceto-

acetyl

-CoA

CoA Thiaphorase

Ketone bodies

2 Acetyl-CoA

Acetoacetyl-CoA 3-HMG-CoA

Acetoacetate

3-Hydroxybutyrate

CoA Acetyl-CoA

CoA

Acetyl-CoA

NADH + H+

NAD+

CH3

O

O-

O

CH3

OH

O-

O

Acetone

CH3CH3

O

• Ketone bodies are an “emergency fuel” that the liver can produce to preserve glucose. The

liver itself cannot use ketone bodies, though!

• During starvation, the ability of the liver to oxidise fatty acids released from adipocytes may

be limited. The liver produces ketone bodies and releases them into the blood for

peripheral tissues.

• Ketone bodies are highly soluble and unlike lipids can be transported without carriers.

• Increased levels of ketone bodies in blood (ketonemia) and urine (ketonuria) are observed

in uncontrolled type 1 diabetes mellitus. The acidity of ketone bodies lowers blood pH

(ketoacidosis).

Pyruvate

Acetyl-CoA


Isocitrate

α-Ketoglutarate


Fumarate

Malate

CO2

CO2

CO2

Acetoacetate

Hydroxybutyrate

Leu

Phe

Tyr

Trp

Lys

Glutamine

Glutamate

Pro

His

Arg

Ile


Met

Val

Thr Propionyl-CoA

Ala

Cys

Gly

Ser

Thr

Phe

Tyr

Arginino-

succinate

Arginine

NH3

Ornithine

Citrulline

CO2

Carbamoyl-P

Urea

Aspartate

Asparagine

Amino acid metabolism

• Unlike carbohydrates and fatty acids, amino acids have no storage form. All must be taken

up with the diet or recycled via regular turnover of body proteins (about 400g/ day).

• Excess amino acids are degraded, and the generated nitrogen excreted largely as urea.

• Most amino acids can be used in gluconeogenesis (they are glucogenic), but some are

partially or fully ketogenic: they only form acetyl-CoA or acetoacetate.

• The catabolism of most amino acids begins with the removal of the α-amino group.

• The amino group is transferred to α-ketoglutarate in a transaminase reaction:

α-Ketoglutarate Glutamate

O O

O

OHOH

O O

NH2

OHOH

O

NH2

CH3

OH

O

O

CH3

OH

Alanine Pyruvate


• Most transaminases are quite specific for one or few amino acids

and transfer their amino group to α-ketoglutarate.

• Alanine transaminase (ALT) is a typical enzyme in that is fully

reversible and does not strongly favor one direction:

• Aspartate transaminase (AST) is an exception. The α-amino

group of glutamate that has come from many other amino acids is

passed on to oxaloacetate to form aspartate, and from there is fed

into the urea cycle.

• Aminotransferases are cytoplasmic enzymes. Increased levels of

these enzymes in blood plasma, especially of ALT and AST, are

diagnostic of cell/ tissue damage. ALT is more specifically

indicative of liver damage, but serum AST is more sensitive

because the liver contains larger amounts of AST than ALT.

Alanine

Pyruvate

α-KG

Glutamate

ALT

Aspartate

Oxaloacetate

α-KG

Glutamate

AST


Oxaloacetate

α-Ketoglutarate

Glutamine

Glutamate

Arginino-

succinate

Arginine

NH3

Ornithine

Citrulline

CO2

Carbamoyl-P

Urea

Fumarate

NH3 + NADH

NAD+

Glutamate Dehydrogenase

NH3

H2O

Glutaminase

AST

NH2

R

NH

R

CNH2O

Transaminase

NH

R

CNH2 N

O

O

O-

O-

C

NH

NH

NH2

RC

NH2

NH2

O

C NH2

O

OP

O-

O

O-


Aspartate

O

ONH2

O-

O-

α-amino

acid

α-keto

acid

Glutamine

synthetase

NH3 + ATP

ADP + Pi

NH3 + NADH

NAD+

Glutamate Dehydrogenase

Oxaloacetate

α-Ketoglutarate

Glutamine

Glutamate

Aspartate

NH3

H2O

Glutaminase

AST

O

ONH2

O-

O-


In peripheral tissues, excess ammonia

is converted to glutamine and shuttled

to the liver.

In the liver, two molecules NH3 can be

released from glutamine by glutaminase

and then glutamate dehydrogenase.

Glutamine

synthetase

NH3 + ATP

ADP + Pi

Alanine

Pyruvate

ALT

Ammonia can also be transferred to

oxaloacetate by aspartate transaminase. The

resulting aspartate feeds into the urea cycle.

A second route for delivering ammonia to the liver is via the alanine-

glucose shuttle: alanine from muscle delivers NH3 via ALT; resulting

pyruvate goes into gluconeogenesis; glucose is returned to muscle.

Oxaloacetate Arginino-

succinate

Arginine

NH3

Ornithine

Citrulline

CO2

Carbamoyl-P

Urea

Aspartate

Fumarate

NH2

R

NH

R

CNH2O

O

ONH2

O-

O-

NH

R

CNH2 N

O

O

O-

O-

C

NH

NH

NH2

RC

NH2

NH2

O

C NH2

O

OP

O-

O

O-


In liver mitochondria, ammonia and CO2 are joined

by carbamoyl phosphate synthetase I. Two

molecules of ATP are required to drive the process.

In the urea cycle, a total two molecules ammonia

and one bicarbonate are converted to urea, the

major nitrogen waste product. The total cost of one

round of the urea cycle is 4ATP.

Urea is shuttled from

the liver to the kidney

for excretion.

Liver function in the absorptive state

Glucose

Acetyl-CoA

Amino acid

Glucose-6-P

Glycogen

Pyruvate

Fatty acid

TAG VLDL

PPP

Fro

m t

he

gu

t

TCA

cycle

NH3

protein

NADPH

Chylomicron remnants

VLDL To adipose tissue

Glucose

Acetyl-CoA

Amino acid

Glucose-6-P

Glycogen

Fatty acid

TAG VLDL

PPP

Fro

m t

he

gu

t

TCA

cycle

NH3

protein



Although the liver

normally produces

glucose rather than

consume it, after a meal

the low-affinity GLUT2

transporter takes up

glucose from the blood.

The low-affinity, high-capacity

glucokinase can channel glucose into

glycolysis only when glucose is

abundant.

High levels of glucose-

6-phosphate promote

glycogenesis.

High levels of glucose-6-

phosphate promote NADPH

production in the Pentose

phosphate pathway.

Glycolysis is promoted by

a high insulin/glucagon ratio:

PFK-1 activation by F-2.6-

BP; dephosphorylation

(=activation) of pyruvate

kinase and PDH.

Pyruvate

NADPH

Glucose

Acetyl-CoA

Amino acid

Glycogen

Fatty acid

TAG VLDL

PPP

Fro

m t

he

gu

t

NH3


Surplus carbohydrates from a meal are converted to acetyl-CoA and then

mainly channelled into fatty acid synthesis:

High ATP inhibits isocitrate dehydrogenase, leading to an accumulation

of citrate in mitochondria and export to the cytoplasm. ATP-citrate lyase

restores acetyl-CoA in the cytoplasm, and ACC is activated by

dephosphorylation and by citrate.

Pyruvate

NADPH TCA

cycle

Glucose-6-P

TAG synthesis is promoted

by the high availability of

fatty acyl-CoA both from de

novo fatty acid biosynthesis

and from dietary fats.

Surplus amino acids are

recycled, redistributed or

degraded into pyruvate, TCA

cycle intermediates or

acetyl-CoA.

Branched-chain amino acids

are only used by muscle.

protein


Liver function during fasting

Glucose

Acetyl-CoA

Glucose-6-P

Glycogen

Pyruvate

Fatty acids

Blo

od

TCA

cycle

Ketone bodies


The #1 priority for the liver during a fast is to

maintain blood glucose levels for the glucose-

dependent tissues.


Glucose

Acetyl-CoA

Glucose-6-P

Glycogen

Fatty acids

Blo

od

TCA

cycle

Ketone bodies

Glucagon stimulates the activation

of glycogen phosphorylase via

PKA-mediated activation of

phosphorylase kinase.

The liver-specific enzyme

Glucose-6-phosphatase produces

glucose from G-6-P. Glucose is

released into the bloodstream.

Glucagon also triggers a reduction

in the concentration of Fructose-2,6

bisphosphate by shifting PFK-2

towards phosphatase activity.

The reduction in F-2,6-BP means that

gluconeogenesis is favoured over

glycolysis: the key enzyme Fructose-

1,6-bisphosphatase is no longer

inhibited by F-2,6-BP.

Pyruvate


The main sources of carbon for

gluconeogenesis are lactate

from muscle and glucogenic

amino acids.


Glucose

Acetyl-CoA

Glucose-6-P

Glycogen

Fatty acids

Blo

od

TCA

cycle

Ketone bodies

Hydrolysis of

TAGs in adipose

tissue supplies the

liver with fatty acids.

Pyruvate


The key enzyme of fatty acid

biosynthesis, ACC, is inhibited

directly by abundant fatty acyl CoA

and indirectly by phosphorylation

(mediated by AMPK and glucagon).

ACC inhibition lowers malonyl-CoA,

which in turn activates β-

oxidation.

Abundant acetyl-CoA activates

pyruvate carboxylase (for

gluco-neogenesis) and inhibits

degradation of pyruvate

(inhibition of PDH)

During prolonged fasting,

the liver produces the

ketone bodies aceto-

acetate and 3-hydroxy-

butyrate that can be

used as emergency fuel

by all tissues, even the

brain.

Glucose-1-P Glucose-6-P Glucose

Glycogenesis

• Glycogen biosynthesis starts with the activation

of glucose with UTP to form UDP-glucose.

• UDP-glucose is used to extend existing

glycogen structures, but if stores are depleted:

• De-novo synthesis of glycogen requires

glycogenin transferring glucose units to itself,

thus forming a primer.

• Glycogen synthase extends the α-1,4-linked

backbone chains

• Branching enzyme transfers 6-8 glucose

residues from one end to a 6’ hydroxyl group to

form a branch.

UTP

PPi

UDP-Glucose

Glycogenin

Glycogenin

primer Glycogen

synthase

Branching

enzyme

Glucose-1-P

Glucose-6-P Glucose

Glycogenolysis

• Glycogen degradation starts with glycogen

phosphorylase, an enzyme that can

sequentially cleave α-1,4-linkages until four

glucose residues are left on a branch.

• Debranching enzyme has two separate

enzymatic activities that together trim off the α-

1,6-linked branches.

• The resulting glucose-1-phosphate is converted

to G-6-P by phosphoglucomutase.

• Glucose-6-phosphate enters glycolysis (in

muscle) or is (uniquely in the liver) shuttled into

the ER, dephosphorylated to glucose and

released into the bloodstream.

Debranching

enzyme

Glycogen

phosphorylase

Glucose-6-

phosphatase PGM

Fructose-6-P

Fructose-1,6-bis-P



3-Phosphoglycerate

2-Phosphoglycerate

Phosphoenolpyruvate

Pyruvate

Oxaloacetate

CO2

Glucose-6-P Glucose

Gluconeogenesis

Of the ten steps of glycolysis,

seven are fully reversible. For

gluconeogenesis, a Plan B is

needed to reverse the other 3:

• Pyruvate is converted

“back” to PEP in two

steps, involving Pyruvate

carboxylase and PEP

carboxykinase involving

a complex coordination of

mitochondrial and

cytoplasmic steps. 2ATP

are needed..

Lactate

• The other two irreversible

steps are those that used

ATP in the glycolytic

pathway *. To make

gluconeogenesis

energetically favorable,

they are replaced with

F-1,6BP’ase (*1) and G-6-

P’ase (*2). ATP

GTP

*2

*1

Acetyl-CoA


Isocitrate

α-Ketoglutarate


Fumarate

Malate

+NADH+H+ + CO2

The TCA cycle

• Two carbon atoms enter the TCA cycle, and two carbon atoms leave the cycle in the form of CO2,

so there is no net consumption of oxaloacetate.

• One molecule of GTP (equivalent to ATP) is produced.

• Three molecules of NADH+H+ and one FADH2 are produced. Once entered into oxidative

phosphorylation, these produce 3 and 2 molecules of ATP, respectively. The oxidation of one molecule

of acetyl-CoA thus produces 12 molecules of ATP.

NAD+

+NADH+H+ + CO2

NAD+

GDP + Pi GTP,

Co-A

NAD+

NADH+H+

FAD

FADH2

Acetyl-CoA


Isocitrate

α-Ketoglutarate


Fumarate

Malate

+NADH+H+ + CO2

TCA cycle regulation

• The TCA cycle is not directly regulated by hormones and/ or protein phosphorylation. Instead, three

enzymes are regulated by direct product inhibition (citrate inhibits citrate synthase) or allosteric

control: Isocitrate dehydrogenase is activated by ADP and inhibited by ATP and NADH; α-

ketoglutarate dehydrogenase is inhibited by its products, NADH and succinyl-CoA.

NAD+

+NADH+H+ + CO2

NAD+

GDP + Pi GTP,

Co-A

NAD+

NADH+H+

FAD

FADH2

*

* *

Acetyl-CoA

• High energy levels (high ATP/ADP) inhibit isocitrate dehydrogenase (*) and lead to a

buildup of citrate in mitochondria.

• Citrate can be shuttled to the cytoplasm and converted “back” to Acetyl-CoA.



Isocitrate

α-Ketoglutarate


Fumarate

Malate

CO2

CO2

Pyruvate

CO2

Citrate

Acetyl-CoA

*

Oxaloacetate

ATP ADP

Co-A H2O

Malonyl-CoA

Fatty acyl-CoA

• Fatty acyl synthase (FAS) is a

multi-tasking enzyme that

catalyses multiple rounds of chain

elongation, reduction, dehydration

and reduction (actually a 7-step

reaction).

Fatty acid

synthesis

CH3 C

S

O

CH3C

S

OCO-

O

C

S

OCCH3

O

C

S

OCHCH3

OH

C

S

OCH

CH3

CH2 C

S

OCH2CH3

CH2C

S

OCO-

O

CH2 C

S

OCH2 CH2

CH2CH3

S

O

CH3

CH3

CO2

NADPH+H+

NADPH+

H2O

NADPH+H+

NADPH+

CO2

Acetyl-CoA

Malonyl-CoA

Malonyl-CoA

Palmitoyl-CoA

• The reducing equivalents used by FAS are NADPH

rather than NADH.

• NADPH is produced in an alternative pathway for

glucose oxidation, the pentose phosphate pathway.

• In the PPP, the first two irreversible oxidation steps

generate 2 NADPH per glucose. This is followed by

reversible sugar interconversion steps. No ATP is

generated.

• NADPH is needed for a number of interesting

purposes other than reductive biosynthesis, such as

drug metabolism and defence.

GA-3-P Dihydroxyacetone-P

Acetyl-CoA

Malonyl-CoA

Fatty acyl-CoA

Glycerol Glycerol-P

Triacylglycerol

• To produce triacylglycerols (TAG) as storage form of fatty acids, fatty

acyl-CoA need to be linked up (esterified) with glycerol-3-phosphate.

• Two reactions that produce glycerol-3-P are available: glycerol-3-P

dehydrogenase and (uniquely in the liver) glycerol kinase. The latter

allows the glycerol part of TAGs to be used in gluconeogenesis.

• Adipocytes do not express glycerol kinase and so cannot metabolise

glycerol produced during TAG mobilisation.

• The liver packages TAGs into VLDL (Very Low Density Lipoproteins)

for delivery to peripheral tissues.

Triacylglycerol synthesis

NADH

NAD+

ATP ADP

gly

ce

rol

the normal liver: basic metabolic liver function...the liver as nutrient distribution centre • the...

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