Minimum competence in energy metabolism

overall gain in entropy (2. law of thermodynamics)

but living cell maintain entropy constant / gain complexity

life = semi-closed system

1. initial oxidation: oxidation of substrates (glucose, fatty acids, AA, lactate) and transfer of electron to cofactor

2. intermediate redox reactions: reoxidation of the cofactors (NAD, FAD, NADP, FMN)

3. final reduction: of O2 —> H2O (respiration), pyruvate —> lactate or ethanol (fermentation)

starch and glycogen most important substrates for metabolism

glucose polymer derived from plants

less branched

hydrolysed to glucose dimers = maltose

glucose polymer from animals and mushrooms

more branched

carbohydrates

proteins

lipids

Glucose and galactose disaccharide

galactose can be isomerized to glucose

fructose and glucose dimer

fructose can be isomerized to glucose

Blood glucose is kept within physiological limits

physiological constant

0.7 g/L < fasting state < 1.1 g/L

3.9 mM - 5.83 mM

distant from meal

< 1.8 g/L

10 mM

after meal rich in fatty acids and carbohydrates

blood glucose < 0.7 g /L

pathophysiologica

1.1 g/L < fasting state < 1.26 g/L

not physiopathologic

1.26 g/L > fasting blood glucose

1.4 > postprandial blood glucose

defect in glucose homeostasis

gut endothelium takes up glucose & exports it under fasting conditions

kidney epithelial cells filtrate plasma glucose to produce glucose-free urine

import into glucose-consuming cells (muscle, neuron)

stored in adipocytes (stored as lipids)

liver cell: transforms glucose into glycerol / lipids but also release of glucose under certain conditions

no —>polar molecule can’t diffuse through apolar membrane

SGLTs

Sodium-glucose transporters

12 genes

6 transporters linked to plasma membrane: SGLT 1-6

secondary active transport

—> transport of glucose against its gradient powered by transport of sodium with its gradient (symporter)

—> sodium reexported with Na+/K+ ATPase (antiporter)

—> K+ exported via K+ channels

low Km —> high affinity for Glc and Gal

in small intestine

high Km (6-10 mM)—> low affinity for Glc, Gal & fructose

in kidney

high Km (20mM)

low affinity & high specificity for Glc

no transport —> glucose sensing

in intestinal cholinergic neurons & sensory neurons of the hypothalamus—>signal blood glucose level to the brain, contributes to satiety

binding of Glc triggers influx of Na+ —>depolarization and rate of action potential proportional to Glc conc

low specificity for Glc

small intestine

absorption/reabsorption of mannose, fructose (& glucose)

low specificity for glucose

in kidney

glucose, mannose & fructose transport

no affinity for glucose

myo-inositol absorption/reabsorption

involved in signal transduction cascade

via GLUTs

Glucose transporters

channels that can be open or closed

according to physicochemical forces transport in different directions possible —> bidirectional transport

saturable transport (maximum speed of transport —>if above increase in [Glc] doesn’t change the speed)

Class I: thoroughly characterized, GLUT 1-4, 13 (duplicon of GLUT 3)

Class II: fructose transport, GLUT 5, 7, 9, 11

Class III: oldest transporters ( GLUT 6, 8, 10, 12, 13)

GLUT 13 H+/myo-inositol transporter in the brain

12 highly conserved transmembrane segments (M1 - M12 from Nter)

poorly conserved intracellular loop M6/M7, Nter and Cter tails

physiological consequences of the biochemical characteristics of the different transporters

GLUT1, 3, 4, 14

expressed in specific tissues

under physiological values always under Vmax

flux independent of [Glc]

for constant rate of Glc in placenta, brain

GLUT 2

in liver, ß-cell, basolateral membrane of enterocyte, kidney

V more than doubled between fasting and postprandial state—>glucose-sensing

triggers insulin production in ß- cells

low affinity—>no influx of Glc

low ATP production by mitochondria

K+ATP channel open and efflux K+

reinforces gradient—>polarization

voltage operated Ca2+ channels closed

insulin in intracellular droplets

influx of Glc at high conc

stimulation of metabolic activity and [ATP] increases

closes K+ ATP channel

depolarization of the membrane

opening VOC —>influx Ca2+

second messenger mediates fusion of insulin-containing droplets with membrane

mainly GLUT4

in muscle, adipose tissue, heart

works at Vmax—>translocation of intracellular transporter to membrane upon insulin stimulus to increase Glc uptake

low Km: efficiently converts Glc into G6P and maintains gradient constant after Glc influx

high Km: accumulation of Glc in the cell impairs gradient—>allows Gluconeogenesis in liver cells in fasting conditions and inverse transport (export)

GLUT and HK type coupled

co-expression (GLUT2 with HK IV (high Km)

HK III ubiquitous, HIF inducible and poorly expressed, inhibited by glucose (only works at low [Glc])

fasting: no Glc uptake via SGLT-1 from intestine but transported from internal medium with GLUT2

HKIII active (only at low conc) and transformation into G6P, HK IV inactive

postprandial: SGLT-1 Glc uptake from intestine & HK III inactive—> HK IV has low affinity —> low rate of G6P increases free Glc —> exported into internal medium with GLUT2

passively uptaken with GLUT5 in endothelial cells

exported by GLUT 2

or transformed with Ketohexokinase KHK into F1P

chain reaction: active gene response elements, increase expression of GLUT 5 —>better uptake fructose

or isomerization to Glc

in enterocytes & hepatocytes

5% of exported Glc crosses basolateral membrane by exoxytosis

Glc uptake into intestine via SGLT1

HK IV to G6P

some enter ER with specific Glc transporter SWEET1

loaded into intracellular compartments, release of Glc via fusion with membrane

Glucose + ATP —> G6P

G6P + ATP + 2 NAD —> 2NADH2 + 4 ATP + 2 pyruvate

net result : 2 NADH2 + 2 pyruvate + 2 ATP

Pyruvate dehydrogenase transports pyruvate into mitochondria and transforms it to Acetyl-CoA (decarboxylation & oxidation)

oxidative phosphorylation in Krebs: produces CO2 and reduced cofactors

reoxidation of cofactors in respiratory electron transfer chain—>reduction of O2 and 34 ATP produced

malate aspartate shuttle also transfers reduced cofactors from cytosolic part into mitochondria for reoxidation

regulated/ inactivated by excess of Acetyl-CoA

activated when PDH inactive

in cytosol reduction of pyruvate for reoxidation of cofactors to allow continuation of glycolysis

anaerobic glycolysis —>fermentation

in goldfish and crucian carp by pyruvate decarboxylase PDC to acetaldehyde

produce acetaldehyde with alcohol dehydrogenase reduced to ethanol & reoxidation of cofactors

used as anti-freezing system of the blood (hypoxia induced by decreased movement of the fish)

lactate transported to liver and transformed into glucose (gluconeogenesis)

ATP provided by fatty acids

can take several days

99% of total energy storage

in adipose tissue as triglycerides

activation of fatty acid with thiokinase to acyl-CoA (ester)

ß-oxidation occurs in 4 steps

1. oxidation with Acyl-CoA dehydrogenase

2. hydration with enoyl CoA Hydratase

3. oxidation L-3-hydroxyacyl CoA dehydrogenase

4. Acyl-CoA thiolase

   no production of ATP without oxidative phosphorylation

   yields: 1 FADH2, 1 NADH2, 1 Acetyl-CoA. 1 Acyl CoA (n-2)

FA-CoA transported with carnitine transporter CPT-I / II (only works with <C18)

carnitine replaces CoA during transport

ß-oxidation to acetyl-CoA feeding the Krebs cycle

shorten long chain fatty acids

reoxidation of FADH2 by Acyl-CoA-oxidase ACO in peroxisome using partial reduction of O2 into H2O2 —>specific catalase present in peroxisomes

NADH2 transported with M/O and M/A shuttle first into cytosol and then into mitochondria

glucose & FA available and absence of insulin

FA converted into Acyl-Coa —>transported as Acylcarnitine into mitochondrial matrix—>high level of ß-oxidation—>high concentration Acetyl-CoA (inhibits pyruvate dehydrogenase, key enzyme glycolysis)—>ATP generation in oxidative phosphorylation

PFK & GAPDH inhibited by high levels of ATP

—>as long as the cell is fed with FA, glucose consumption is inhibited & FA used as a substrate

Acetyl-CoA the regulator: as long as Acetyl-CoA level is high, PDH is inhibited & FA used as a substrate

oxidative phosphorylation inhibits glycolysis because PFK is down-regulated by ATP, CP % citrate

Randle effect predicts that fatty acid utilization overrides glucose oxidation

however after a meal (providing fat & glucose), glucose is oxidized despite the presence of FA—>insulin inhibits the Randle effect

FA & glucose pathways cross in the mitochondria

AMPK acitve—>phosphorylates ACC—>inactive ACC prevents lipogenesis—>no production of malonyl-CoA from Acetyl-CoA (chain elongation)

FA is carried by carnitine transporter which is inhibited by malonyl-CoA

Acyl-CoA enters mitochondria as Acylcarnitine —>recycling carnitine and release acyl-CoA—>used in ß-oxidation—>converted into acetyl-CoA

PI3K activated by insulin—>inhibition AMPK—>ACC active—>increase in malonyl-CoA

inhibits the transfer of FA into mitochondria —>inhibits consumption of FA

—> Insulin triggers change in metabolic mode

triggers GLUT4 translocation—>increased glucose uptake & trapping inside the cell

due to lack of ß-oxidation decreased levels of acetyl-CoA—>activation PDH

low rate of Krebs cycle—>low ATP—>activation PFK & FAPDH

—>metabolic FFA overrides glucose utilization but insulin promotes glucose utilization by inhibiting FFA utilization & increasing glucose uptake in insulin-sensitive tissue