Regulation of metabolism on the cellular level Vladimíra Kvasnicová

General Principles of Regulation catabolic / anabolic processes last step of each regulation mechanism: change of a concentration of an active enzyme (= regulatory or key enzyme) regulatory enzymes  often allosteric enzymes  catalyze higly exergonic reactions (irreverzible)  low concentration within a cell

I. Regulation on the organism level 1.signal transmission among cells (signal substances) 2.signal transsduction through the cell membrane 3.influence of enzyme activity:  induction of a gene expression  interconversion of existing enzymes (phosphorylation / dephosphorylation)

II. Regulation on the cell level 1.compartmentalization of mtb pathways 2.change of enzyme concentration (on the level of synthesis of new enzyme ) 3.change of enzyme activity (an existing enzyme is activated or inactivated)

1. Compartmentalization of mtb patways transport processes between compartments various enzyme distribution various distribution of substrates and products (  transport) transport of coenzymes subsequent processes are close to each other

Compartmentalization of mtb pathways The figure is found at (May 2007)

Cytoplasm glycolysis gluconeogenesis (from oxaloacetate or glycerol) metabolism of glycogen pentose cycle synthesis of fatty acids synthesis of nonessential amino acids transamination reactions synthesis of urea (a part; only in the liver!) synthesis of heme (a part) metabolism of purine and pyrimidine nucleotides

Mitochondrion pyruvate dehydrogenase complex (PDH) initiation of gluconeogenesis  -oxidation of fatty acids synthesis of ketone bodies (only in the liver!) oxidation deamination of glutamate transamination reactions citrate cycle respiratory chain (inner mitochondrial membrane) aerobic phosphorylation (inner mitoch. membrane) synthesis of heme (a part) synthesis of urea (a part)

Endoplasmic Reticulum Smooth ER synthesis of triacylglycerols and phospholipids elongation and desaturation of fatty acids synthesis of steroids biotransformation of xenobiotics glucose-6-phosphatase Rough ER proteosynthesis (translation and posttranslational modifications)

Golgi Apparatus posttranslational modification of proteins protein sorting export of proteins (formation of vesicules) Ribosomes proteosynthesis Nucleus replication and transcription of DNA synthesis of RNA

Lysosomes hydrolysis of proteins, saccharides, lipids and nucleic acids Peroxisomes oxidative reactions involving O 2 use of hydrogen peroxide degradation of long chain FA (from C 20 )

2. Synthesis of new enzyme molecules: induction by substrate or repression by product (on the level of transcription) examples:  xenobiotics  induction of cyt P450  heme  repression of delta-aminolevulate synthase

3. Change of activity of an existing enzyme a)in relation to an enzyme kinetics  concentration of substrates (  K m )  availability of coenzymes  consumption of products  pH changes  substrate specificity - different K m

b)activation or inactivation of the enzyme covalent modification of the enzymes  interconversion: phosphorylation/dephosphorylation)  cleavage of an precursore (proenzyme, zymogen) modulation of activity by modulators (ligands):  feed back inhibition  cross regulation  feed forward activation 3. Change of activity of an existing enzyme

Phosphorylation / dephosphorylation some enzymes are active in a phosphorylated form, some are inactive phosphorylation:  protein kinases  macroergic phosphate as a donor of the phosphate (ATP!) dephosphorylation  protein phosphatase  inorganic phosphate is the product!

The figure is found at: 13/05jpeg/05_jpeg_HTML/index.htm (December 2006) 13/05jpeg/05_jpeg_HTML/index.htm Reversible covalent modification: A) phosphorylation by a protein kinase dephosphorylation by a protein phosphatase B) phosphorylated enzyme is either active or inactive (different enzymes are influenced differently)

Modulators of enzyme activity (activators, inhibitors) isosteric modulation: competitive inhibition allosteric modulation:  change of K m or V max  T-form (less active) or R-form (more active) important modulators: ATP / ADP