Metabolism of saccharides

Sources of glucose (Glc)

● from food (4 hours after meal) ● from glycogen (from 4 to 24 hours after meal) ● from gluconeogenesis (days after meal, during starvation)

Figure was assumed from Devlin, T. M. (editor): Textbook of Biochemistry with Clinical Correlations, 4th ed. Wiley-Liss, Inc., New York, 1997

Glycemia

• glucose concentration in the blood

• physiological range of fasting glycemia 3,3 – 5,6 mmol/L

• is regulated by hormones (insulin, glucagon, epinephrine,

kortisol, …)

Glucose can enter into cells:

a) by facilitative diffusion (GLUT 1 – 7)

• GLUT 1 – blood-brain barrier, erythrocytes

• GLUT 2 – liver, β-cells in pancreas

• GLUT 3 – neurons

• GLUT 4 – skeletal muscles, heart muscle, adipose tissue

b) by cotransport with Na+ ion (SGLT-1 and 2)

Figure was assumed from textbook: Devlin, T. M. (editor): Textbook of Biochemistry with Clinical Correlations, 4th ed. Wiley-Liss, Inc., New York, 1997.

small intestine, kidneys

An effect of insulin on insulin-sensitive cells

Transport of Glc into cells is dependent on insulin effect (GLUT-4) in the following tissues: skeletal and heart muscle and adipose tissue

Figure is found on http://www.mfi.ku.dk/ppaulev/chapter27/Chapter%2027.htm

Metabolic pathways included in utilization of Glc – glycolysis, pentose cycle, glycogen synthesis

Phosphorylation of glucose

after enter into cell Glc is always phosphorylated to form Glc-6-P

enzyme hexokinase catalyzes esterification of Glc

ATP is a donor of phosphate group!

enzyme is inhibited by excess of Glc-6-P

2 isoenzymes of hexokinase exist: hexokinase and glucokinase

hexokinase has a higher affinity to glucose than glucokinase

Hexokinase vs. glucokinase

Figure is found on http://web.indstate.edu/thcme/mwking/glycolysis.html

KM hexokinase = 0,1 mM KM glucokinase = 10 mM

Glycolysis

• substrate: Glc-6-P • product: pyruvate • function: source of ATP • subcellular location: cytosol • organ location: all tissues • regulatory enzymes: hexokinase/glucokinase, 6-phosphofructokinase-1 (main regulatory enzyme), pyruvatekinase

Regulatory enzymes are activated by hormone insulin!

Glycolysis

Figure is found on http://web.indstate.edu/thcme/mwking/glycolysis.html

Production of ATP in glycolysis

conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate

conversion of phosphoenolpyruvate (PEP) to pyruvate

These reactions are examples of substrate level

phosphorylation!

Regulation of glycolysis

Regulatory enzymes ● Hexokinase – inhibited by Glc-6-P ● Glucokinase – activated by insulin

– inhibited by Fru-6-P

● 6-phosphofructokinase-1 (PFK-1)

– activated by insulin, ↑AMP / ATP

- inhibited by ↑ ATP /AMP, citrate

● Pyruvatekinase

– activated by insulin, Fru-1,6-bisP

- inhibited by glucagon, ↑ ATP /AMP, acetyl-CoA

Pentose phosphate pathway

• substrate: Glc-6-P • product: CO2, NADPH + H+

• function: gain of NADPH + H+, production of rib-5-P for nucleotide synthesis, mutual conversions of monosacharides

• subcellular location: cytosol • organ location: all tissues • regulatory enzyme: glucose 6-phosphate dehydrogenase

Pentose phosphate pathway – oxidative stage produces rub-5-P

Figure is found on http://web.indstate.edu/thcme/mwking/pentose-phosphate-pathway.html

Pentose phosphate pathway – non-oxidative stage includes interconversions of monosaccharides

Figure is found on http://web.indstate.edu/thcme/mwking/pentose-phosphate-pathway.html

Glycogen synthesis (glycogenesis)

• substrate: Glc-6-P • product: glycogen • function: glucose storage in the form of glycogen • cellular location: cytosol • organ location: especially in the liver and skeletal

muscles, other tissues have lower glycogen storage • regulatory enzyme: glycogen synthase Enzyme glycogen synthase is inhibited by phosphorylation

(glucagon in liver and epinephrine in muscles)!

Glycogen synthesis

• Glc-6-P → Glc-1-P • Glc-1-P + UTP → UDP-Glc + PPi

Glycogen synthase catalyzes the formation of (1→4) glycosidic bonds. Branching (formation of (1→6) glycosidic bonds) is performed by enzyme amylo-(1,4 – 1,6)-transglycosylase („branching enzyme“). Figure is found on http://en.wikipedia.org/wiki/Glycogen

Metabolic pathways serving to supplementation of Glc into the bloodstream –

glycogen degradation and gluconeogenesis

Glycogen degradation (glycogenolysis) ● substrate: glycogen

• product: Glc-6-P

• function: releasing of Glc from glycogen

• subcellular location: cytosol

• organ location: liver, skeletal muscles, but also other tissues

• regulatory enzyme: glycogen phosphorylase

Enzyme glycogen phosphorylase is activated by phosphorylation which is induced by hormones glucagon and epinephrine. Insulin inhibits

enzyme phosphorylation.

Glycogen degradation

Glycogen (n Glc) + Pi → Glc-1-P + glycogen (n - 1 Glc) Enzyme glycogen phosphorylase catalyzes the cleavage of 1→4 bonds. Enzyme amylo-1→6-glucosidase („debranching enzyme“) cleaves 1→6 bonds. Glc-1-P ↔ Glc-6-P phosphoglucomutase Glc-6-P glucose-6-phophatase (liver, kidneys, enterocytes)

Glc

Gluconeogenesis

• substrates: lactate, pyruvate, glycerol, amino acids – Ala, Asp, Gln etc.

• product: glucose

• function: synthesis of Glc from non-sugar precursors

• subcellular location: mitochondrial matrix + cytosol

• organ location: liver + kidneys

• regulatory enzymes: pyruvate carboxylase and PEP carboxykinase

Regulatory enzymes are activated by hormones glucagon and cortisol. Insulin inhibits them.

Scheme of gluconeogenesis

Figure is found on http://web.indstate.edu/thcme/mwking/gluconeogenesis.html

Gluconeogenesis

Synthesis of PEP is divided into 2 steps:

• Pyr → matrix of mitochondria → Pyr is carboxylated to oxaloacetate (OA) by pyruvate carboxylase

CH3-CO-COO- + CO2 + ATP → -OOC-CH2-CO-COO- + ADP + Pi

• OA is transported to the cytosol and decarboxylated to PEP by PEP carboxykinase

-OOC-CH2-CO-COO- + GTP → PEP + CO2 + GDP

Synthesis of 1 mol Glc consumes 4 mol ATP and 2 mol GTP!

Figure was assumed from http://www.biochem.arizona.edu/classes/bioc462/462b/glycolysis.html

Regulation of gluconeogenesis

Hormones:

• activation: cortisol, glucagon, epinephrine

• inhibition: insulin

Enzyme pyruvate carboxylase

• activation: acetyl-CoA from β-oxidation of FA → source of ATP

Enzyme fructose-1,6-bisphosphatase

• activation: citrate, starvation

• inhibition: AMP, Fru-2,6-bisP

Enzyme glucose-6-phosphatase (in ER of liver, kidneys and enterocytes !)

Cori cycle

Figure was assumed from http://web.indstate.edu/thcme/mwking/gluconeogenesis.html

Glucose-alanine cycle

Figure is found on http://web.indstate.edu/thcme/mwking/gluconeogenesis.html

Fructose metabolism

• Fru is a component of sucrose (Glc + Fru) • part of Fru in converted to Glc in enterocytes: Fru-6-P → Glc-6-P → Glc • part of Fru is absorbed and it is transferred via blood into liver: Fru + ATP → Fru-1-P + ADP by enzyme fructokinase • Fru-1-P is broken down to glyceraldehyde (GA) and dihydroxyacetonephosphate (DHAP) by aldolase • DHAP enters glycolysis and GA → glyceraldehyde-3-P → glycolysis

Galactose metabolism

• Gal is a component of lactose (Glc + Gal)

• Gal is absorbed by the same mechanism in enterocytes like Glc → liver

• Gal is phosphorylated in liver to form Gal-1-P:

Gal + ATP → Gal-1-P + ADP by enzyme galactokinase

• Gal-1-P is converted to UDP-Gal:

Gal-1-P + UTP → UDP-Gal + PPi by uridyltransferase

• UDP-Gal is used to lactose synthesis in mammary gland during lactation

• epimerization of UDP-Gal to UDP-Glc → glycogen synthesis / synthesis of glucuronic acid / glycoprotein synthesis