Intermediary metabolism

Eva Samcová

Metabolic roles of tissues • Four major tissues play a dominant role in fuel

metabolism : liver, adipose, muscle, and brain.

• These tissues do not function in isolation.

• Communication between tissues is mediated by the nervous systém, by the availability of circulating substrates, and by variation in the levels of plasma hormones.

• The integration of energy metabolism is controlled by the actions of two peptide hormones, insulin, and glucagon (response to changing substrate levels) with catecholamines epinephrine and norepinephrine (response to neural signals).

Liver

Liver lies immediately under the diaphragm. It is supplied

with blood from below through two major vessels: the

hepatic artery (20% of blood) and the hepatic portal vein

which brings the substrates (soluble in water) absorbed

from the intestinal tract including stomach into the blood

and then directly to liver.

Pancreatic vein (insulin, glucagon)

Liver consumes 20 – 30% of total oxygen consumption

Functions of the liver

The uptake of nutrients delivered from the

digestive tract via portal vein

The synthesis, storage, interconversion and

degradation of metabolite

The regulated supply of energy-rich

intermediates

The detoxification of harmful compounds by

biotransformation

The excretion of substances with the bile;

synthesis and degradation of many blood plasma

constituents

Carbohydrate metabolism in the liver- fed

conditions

Concentration in portal vein after a meal up to 10 mmol/l – GLUT-2 –

type glucose transporter not responsive to insulin, relatively high Km

(rate and direction of movement of glucose through hepatocyte

membrane are determined by concentration inside and outside the cell)

Glucokinase (Km= 12 mmol/l) x hexokinase 0.1 mmol/L

Any increase in glucose concentration against blood conc. leads to

proportional increase in the rate of phosphorylation by glucokinase

.Likewise any decrease in glucose conc. leads to proportional decrease

in the rate of phosphorylation.

Thus liver uses glucose at significant rate only when blood glucose level

is greatly elevated.

The overall result is that when glucose conc. outside the hepatocyte

rises, glucose will be rapidly taken into cells and phosphorylated.

Carbohydrate metabolism in the liver-

fed conditions

The presence of high-Km glucose transporter and

high-Km glucokinase do not enable the hepatocyte

to take up unlimited quantities of glucose as G-6-P

There are specific mechanisms for stimulating the

disposal of Glu-6-P

Glycogen synthesis (activation of glycogen

synthase by insulin and glucose)

Glycolysis metabolizes glucose to pyruvate

TCA, some released after conversion to lactate.

But minor energy source for liver.

Metabolic Fate of G6P

Carbohydrate metabolism –

overnight fasted conditions

Glycogen breakdown (glycogenolysis), controlled by

reciprocal activation of glycogen phosphorylase by

glucagon, adrenalin, noradrenalin, catecholamines.

Glu-1-P produced by glycogenolysis is in equilibrium

with Glu-6-P (enzyme phosphoglucomutase).

Formation of glucose from Glu-6-P is produced by

enzyme Glu-6-phosphatase (membrane ER)

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Carbohydrate metabolism in the liver

Synthesis of glucose – gluconeogenesis

Substrates : lactate, alanine, glycerol

Hepatic gluconeogenesis can be also stimulated by

increase in the supply of substrate from other tissue

(after physical exercise-lactate, starvation-glycerol)

and by hormones (glucagon)

Glucose paradox (gluconeogenesis after meal)

The pentose phosphate pathway – alternative fate for

Glu-6-P, conversion to five-carbons sugars (ribose-5-

P for synthesis of nucleic acids)

Formation of NADPH for reductive synthesis

Fat metabolism in the liver

The metabolism of lipids in the liver is closely

linked to metabolism of carbohydrates and

amino acids.

The pathwayof FA oxidation diverges from that

of glycerolipid synthesis when acyl-CoA enters

the mitochondrion for oxidation.

Carnitine-palmitoyl transferase-1 (CPT-1).

Activity of this enzyme is strictly regulated by

means of compound malonyl-CoA (potent

inhibitor). This role of malonyl-CoA provides a

vital link between carbohydrate and fat

metabolism.

Fat metabolism in the liver

The liver converts glucose (Glc) via Acetyl-CoA into

fatty acids (FA) - cytosol. FA and chylomicrons are

used as a sources – neutral fats and phospholipides. In

humans FA synthesis from other molecules (Glc) is

usually small in comparison with dietary fatty acid

intake.

VLDL are formed in smooth ER of hepatocytes.

High concentration of acetyl-CoA (postabsorptive

state, starvation) as a result of β-oxidation of FA in

mitochondrion great amount of ketone bodies :

acetoacetate, 3-hydroxybutyrate and acetone.

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Fat metabolism in the liver

Cholesterol has two sources, the diet and de

novo synthesis (in liver significant amount).

Some cholesterol is required for synthesis of bile

acids, some for cell membranes, some is stored

in the form of lipids droplets in esterified form.

The rest in free and esterified form in VLDL (to

supply another tissues)

The liver also degrades lipoprotein complexes

(with cholesterol and cholesterol esters) taking

up from the blood.

Amino acid metabolism in the liver

Our bodies do not continuously accumulate or

lose protein in a net sense. The rate of AA

oxidation in the body must therefore balance the

rate of entry of dietary protein (70-100g per day)

Catabolism of AA occurs predominantly in the

liver with exceptions (branched chain amino

acids in muscles)

AA oxidation provides ½ of the liver´s energy

requirements

It is also the only organ capable of eliminating

the nitrogen from amino acids by urea cycle

Starve-Feed Cycle

• The starve-feed cycle allows a variable fuel and nitrogen consumption to meet a variable metabolic and anabolic demand. Feed refers to intake of meals (variable fuel) after which we store the fuel in the form of glycogen and fat, to meet our metabolic demand while we fast. ATP is energy-transferring agent in this cycle.

Well-Fed State – Amino acids Dietary proteins are hydrolyzed in the intestine (some

of them are used like energy source here : Asp, Asn, Glu, Gln→Ala, Lac, citrul, Pro into the portal blood)

Liver lets most of AA coming from intestine pass through, for synthesis of proteins in peripheral tissue , thanks to high Km .High Km allows to AA to be in excess without catabolism.

Utilization of AA for proteosynthesis (much lower

Km for tRNA-charging enzymes)

Excess of AA can be oxidized to CO2 , water, urea, or metabolites can be used as substrates for lipogenesis

Well-Fed State - glucose

Glucose → glycogen (glycogenesis), pyruvate, lactate

(glycolysis), for pentose phosphate pathway

(NADPH)

Much of glucose from intestine passes through liver

to reach other organs (brain, testis, RBC, renal

medulla, AT)

Number of tissues produce lactate and pyruvate from

circulating glucose, which are taken up by liver , and

fat is formed lipogenesis)

In well-fed state liver does not engage in

gluconeogenesis

Cori cycle is interrupted

Well-βFed State – fat

Glucose, lactate, pyruvate and AA support hepatic lipogenesis.

Fat formed from these substrates is released in the form of VLDL

Chylomicrons, VLDLs circulate in the blood until they meet lipoprotein lipase (near AT), hydrolysis of TAG (FA taken up adipocytes, reesterified with glycerol-3-phosphate to form TAG)

During well-fed state insulin from cells of the pancreas is in high concentration. These cells are very responsive to the influx of glucose and AA in the fed state.

Rate of insulin/glucagon

Well-Fed State

Early fasting state

Hepatic glycogenolysis

Lipogenesis is curtailed

Lactate, pyruvate and AA are diverted into formation

glucose completing Cori cycle (conversion glucose to

lactate, pyruvate in peripheral tissue, they are

substrates for gluconeogenesis in liver)

Alanine cycle, in which carbon and nitrogen return to

the liver in the form of alanine (muscle metabolizes

glucose to alanine, which is coming back to liver as a

substrate for gluconeogenesis)

ing State

• No fuels enters from the gut (duodenum) and little glycogen is left in the liver

• Tissues which require glucose are dependent on hepatic gluconeogenesis

• Cori and Alanine cycles play important role

• FAs can not be used for synthesis of glucose (acetyl-CoA can not be converted to glucose)

• Glycerol (by-product of lipolysis) becomes important substrate for gluconeogenesis

• AA, which are hydrolyzed in skeletal muscle (especially), supply most of the carbon atoms for net glucose synthesis mostly in the for of Ala and Gln

• Most of Gln released from muscle is converted (oxidized) into alanine and NH4

+ by intestinal epithelium and being released into bloodstream (glutaminolysis)

• Gluconeogenesis in the liver fasting is closely connected with to urea cycle (ornithine, carbamoyl phosphate, citrulline). Most AAs can give up the amino nitrogen by transamination with 2-oxo glutarate ….

• AT – lipolysis is activated (low blood insulin), blood level of fatty acids raises and are used by peripheral tissues (heart, muscle, liver – formation of glucose and ketone bodies)

• FA oxidation in liver provide most of ATP needed

for gluconeogenesis

• Acetyl~CoA is mostly converted to ketone bodies (small amount is oxidized completely)

• Ketone bodies and FA are preferred by many tissues over glucose; they can also suppress proteolysis and BCA oxidation in muscle

• Cooperation of tissues : liver synthesizes glucose, muscle and intestinal cells supply the substrate (alanine), and AT supplies the ATP (via FA oxidation in liver) needed for gluconeogenesis

• This cooperation is dependent on levels of hormones (insulin, glucagon, epinephrine)

• Reduction of triiodothyronine – reduces daily basal energy requirements by 25%

Fasting State

• After meal, fuel is again absorbed from gut

• Fat is metabolized like in well-fed state

• Glucose is poorly extracted by the liver, liver remains in the

gluconeogenic mode for a few hours after refed. Hepatic

gluconeogenesis provides glucose-6-phosphate for

glycogenesis. Rather, glucose is catabolized in peripheral

tissues to lactate which is converted in liver to glycogen

(glucose paradox)and substrates from it are used by liver for

gluconeogenesis and then glycogen.

• After the rate of gluconeogenesis declines, glycolysis becomes

the predominant means of glucose disposal in the liver

Metabolic interrelationship of major tissues

in the early refed state

Exercise

Anaerobic exercise : sprinting or weight lifting (very little organ

cooperation), muscle largely relies on its own stored glycogen

and phosphocreatine.

Aerobic exercise : long-distance running

is metabolically more interesting. For moderate exercise, much

of the energy is derived from glycolysis of muscle glycogen -

content of it can be increased by exhaustive exercise that

depletes glycogen, followed by rest and a high-carbohydrate

diet. It is not enough glucose and glycogen for endurance

running – switching to fatty oxidation

The respiratory quotient (the ratio of CO2 exhaled to oxygen

consumed) falls during running-this indicates the progressive

switch from glycogen to fatty acid oxidation during the race.

Aerobic exercise

• The liver is primarily responsible for the first two steps of

ethanol catabolism

• Alcohol dehydrogenase

• Aldehyde dehydrogenase

• Liver disposes of NADH generated by this reaction only in

mitochondrial electron transport chain – ethanol generates too

much NADH

• Some enzymes are inhibited by NADH (gluconeogenesis, -

oxidation) and also TCA is inhibited

• The result is fasting hypoglycemia and accumulation of TAG

• Fatty liver , cirhosis

• Redundant acetate is metabolized in peripheral tissue

• Formation of acetaldehyde adducts with proteins in the body

(control of abstinence)

Ethanol ingestion