Download Regulation of metabolic pathways at the cellular level

Regulation of metabolic
pathways at the cellular level
EC
Overview of the lecture
• General principles of regulation of
metabolic pathways at the cellular level
– regulation by the compartmentalization of
the metabolic processes
– changing the absolute concentration of
enzyme
– modulation of the activity of existing
enzyme
Overview of the lecture
• Overview of regulation of metabolic pathways
– regulation of respiratory chain and aerobic
phosphorylation
– regulation of the Krebs cycle
– regulation of the oxidative decarboxylation of pyruvate
– regulation of glycolysis and gluconeogenesis
– regulation of glycogen metabolism
– regulation of pentose phosphate pathway
– regulation of lipolysis and beta-oxidation
– regulation of ketogenesis
– regulation of synthesis of fatty acids
– regulation of urea cycle
General principles of regulation
of metabolic pathways at the
cellular level
General principles of regulation
• The activity of metabolic pathways is
constantly monitored and adjusted
• Synthesis and degradation of
metabolites satisfy the
physiological requirements of cells
and whole organism
Regulation of metabolic pathways
at the cellular level
• 1) Compartmentalization of
metabolic pathways
• 2) Change in the concentration of
active regulatory enzyme:
• a) changing the absolute concentration
of enzyme
• b) modulation of the activity of existing
enzyme
General principles of regulation
of metabolic pathways at the
cellular level
Regulation by the
compartmentalization of the
metabolic processes
Compartmentalization of
metabolic pathways
• Eukaryotic cell is separated by membranes
into individual compartments
• In each cell compartment take place only
certain metabolic pathways
• Different enzymatic equipment of each
compartment
• Specific carriers in the membranes
(various distribution of substrates and
products)
Compartmentalization of
metabolic pathways
• Regulation of opposing processes:
synthesis of FA takes place in the cytoplasm
and their degradation in the mitochondria
• Subsequent processes are close to each
other (KC and RC) - the local accumulation
of substrate
• Transport of excess citrate from MIT to the
cytoplasm - AcCoA transfer and regulation of
glycolysis and FA synthesis
General principles of regulation
of metabolic pathways at the
cellular level
Changing the absolute
concentration of enzyme
Changing the absolute
concentration of enzyme
• Induction (activation) or repression (inhibition) of
gene encoding the enzyme
• Transcription factor - inducer or repressor
• Effect of TF is usually reversible and often
dependent on ligand binding - ligand-dependent
TF
• Ligands - signal molecules, metabolites
• Takes longer time (hours to days) than the
regulation of activity of the existing enzyme
(seconds, minutes)
Changing the absolute
concentration of enzyme
• Enzymes of gluconeogenesis are
induced by glucocorticoids
• Induction of monooxygenase system
by xenobiotics
• Heme inhibits the synthesis of deltaaminolevulinic acid synthase
General principles of regulation
of metabolic pathways at the
cellular level
Modulation of the activity of
existing enzyme
Modulation of the activity of
existing enzyme
• 1) Covalent modifications of enzyme
molecule
• 2) Interventions that affect the
enzyme kinetics
Covalent modifications of
enzyme molecule
• A) Formation of active enzymes from
inactive precursors
• B) Interconversion of enzymes
Formation of active enzymes
from inactive precursors
• Conversion of inactive proenzymes into
active enzymes – partial proteolysis
• Rapid increase in the concentration of
active enzyme
• Digestive enzymes, enzymes of
coagulation cascade
• Following removal of the active enzyme can
be done by its proteolytic degradation
Interconversion of enzymes
• Enzyme is activated / inhibited by other
enzyme
• Reversible ATP-dependent phosphorylation
and dephosphorylation (Ser, Thr, Tyr)
• Proteinkinases and phosphatases
• Phosphorylation can activate or inhibit
• Fast switching between active and inactive
form of the enzyme
Interventions that affect the
enzyme kinetics
• 1) Effect of substrates and products
concentrations, pH and
temperature
• 2) Influence of modulators
(activators or inhibitors): feedback
regulation, cross regulation,
feedforward regulation
Effect of substrate concentration
Effect of product concentration
• 1) Removing of the product of one
reaction by subsequent reactions (metabolic
pathways)
• 2) Utilization of a product of one metabolic
pathways in other metabolic pathway
• 3) Transport of the product to another cell
compartment
• → acceleration of the first reaction
Obrázek převzat z http://web.indstate.edu/thcme/mwking/glycolysis.html
Activity modulators
• Activators and inhibitors
• 1) Accumulation of intermediate or final
product of a metabolic pathways leads to
inhibition - feedback regulation
• 2) Intermediate (or product) of one metabolic
pathway influences the rate of the other
pathway - cross regulation
• 3) Intermediate affects one of the following
enzyme - feedforward regulation
Overview of regulation of
metabolic pathways
Regulation of respiratory chain and
aerobic phosphorylation
• Availability of O2
• Ratio of NADH / NAD+
• Availability of ADP for ATP synthesis
• Uncoupling proteins (proton gradien
on an inner mitochondrial membrane)
Regulation of citric acid cycle
• Availability of substrates
• Consumption of products (NAD, FAD)
• Ratio ATP/ADP
• Citrate synthase
• Isocitrate dehydrogenase (key enzyme)
• 2-oxoglutarate dehydrogenase
Regulation of pyruvate
dehydrogenase reaction (PDH)
• Interconversion of PDH:
phosphorylated form is inactive,
dephosphorylated form is active –
insulin activates - principal regulatory
mechanism
• Competitive inhibition by products acetyl-CoA and ↑ NADH / NAD+
Regulation of glycolysis
• 1) Hexokinase / glucokinase
• 2) 6-phosphofructo-1-kinase (6-PFK-1) (key
enzyme)
• 3) Pyruvate kinase
• Regulation of PPK:
• A: ↑ ratio AMP / ATP, Fru-2,6-bisP (related to ↑
insulin / glucagon), insulin (induction)
• I: ↑ ATP / AMP, citrate, acidic pH
Regulation of gluconeogenesis
• 1) Pyruvate carboxylase
• 2) Phosphoenolpyruvate carboxykinase
• 3) Fructose-1,6-bisphosphatase
• 4) Glucose-6-phosphatase
• General regulation (Ad 1) - 4)):
• A: cortisol a glucagon (induction)
• I: insulin (repression)
•
• Ad 3): opposite to regulation of phosphofructokinase
• Ad 1): AcCoA activates
Regulation of glycogen metabolism
• 1) Glycogen degradation - glycogen
phosphorylase
• 2) Synthesis of glycogen - glycogen synthase
•
• Ad 1):
• A: glucagon, adrenaline (phosphorylation), ↑
AMP / ATP, Ca2+ (in muscle)
• I: insulin
•
• Ad 2):
• A: insulin
• I: glucagon, adrenaline (phosphorylation)
Regulation of pentose cycle
• ↑ NADPH / NADP+ inhibits regulatory
enzyme: glucose-6-phosphate
dehydrogenase
• Insulin activates
Regulation of lipolysis
• Hormone sensitive lipase
(adipocytes)
• A: catecholamines, glucagon
(phosphorylation)
• I: insulin
Regulation of beta-oxidation
• Supply of fatty acids (see regulation of
lipolysis)
• Velocity of transport of fatty acids into a
mitochondrion: carnitine
acyltransferase I is inhibited by
malonyl-CoA
• Consumption of AcCoA, FADH2 and
NADH by subsequent reactions
Regulation of ketogenesis
• Production of acetyl-CoA (=
substrate)
• Regulation is related to the regulation
of lipolysis and beta-oxidation
Regulation of fatty acid synthesis
• Acetyl-CoA carboxylase - key regulatory
enzyme
• I: palmitoyl-CoA and other acyl-CoA,
glucagon (phosphorylation, repression)
and AMP
• A: citrate, insulin, low-fat, energy rich,
high-saccharide diet (induction)
Regulation of urea cycle
• Induction of the enzymes by protein-rich
diet or by metabolic changes during
starvation
• Protonproductive reaction - inhibited during
acidosis
• Carbamoylphosphate synthetase I
• A: N-acetylglutamate
• N-acetylglutamate synthetase
• A: arginine