CARBOHYDRATE METABOLISM
Carbohydrates include the sugars (monosaccharides) and their polymers (oligosaccharides and
polysaccharides).
Repeat classification and structures of carbohydrates – monosaccharides, oligosaccharides (sucrose,
lactose, maltose), polysaccharides (cellulose, starch, glycogen), glycoproteins, glycosaminoglycans.
Glucose is a universal substrate for gain of energy. Most tissues are partially or totally dependent on
glucose for ATP generation and for production of precursors of other pathways. Insulin and glucagon
maintain blood glucose levels near 3.3 to 6.1 mmol/L.
In the digestive tract, dietary polysaccharides and disaccharides are converted to monosaccharides by
glycosidases. Monosaccharides are transported from intestine into blood and circulate to the liver and
peripheral tissues, where they are taken up by facilitative transporters.
Glucose transport proteins (GLUT 1 – 7) are responsible for facilitative diffusion of glucose across cell
membranes.
GLUT 1 – erythrocytes, blood-brain barrier, independent on insulin
GLUT 2 – liver, kidney, pancreatic β-cells, independent on insulin
GLUT 3 – brain (neurons)
GLUT 4 – adipose tissue, skeletal muscle, heart muscle. It is a insulin-sensitive transporter. In
presence of insulin the number of GLUT 4 increases on the cell surface.
SGLT = Sodium Glucose Transporters (bind Na+) are located on intestinal epithelial cells and tubular
cell in kidneys. Glucose (Glc) moves via the facilitative transporters against its concentration gradient.
Utilization of Glc
Glycolysis is a catabolic pathway occuring in the cytoplasm of the cell.
Glc is oxidized:
a) to pyruvate (Pyr) CH3-CO- COO- under aerobic conditions = aerobic glycolysis
b) to lactate CH3-CHOH - COO- when O2 is depleted = anaerobic glycolysis
The individual reactions of glycolysis (see Fig. 1):
1. Phosphorylation of Glc to Glc-6-P is the first reaction of glycolysis and it is a regulatory step of
glycolysis. It is an ATP-dependent reaction and it is catalyzed by tissue specific isoenzymes
known as hexokinases (four mammalian isoenzymes of hexokinase are known - types I - IV,
type IV = glucokinase is found in hepatocytes and in β-cells of pancreas).
2. Isomeration of Glc-6-P to Fru-6-P by enzyme glucose-6-phosphate isomerase. This reaction
is freely reversible.
3. Phosphorylation of Fru-6-P to Fru-1,6-bisP - ATP is consumed in this reaction and reaction is
catalyzed by 6-phosphofructo-1-kinase. This is a key regulatory enzyme in glycolysis.
4. Hydrolysis of Fru-1,6-bisP is catalyzed by enzyme aldolase into two C3 products:
dihydroxyacetone-P (DHA-P) and glyceraldehyde-P (G-3-P).
5. G-3-P and DHA-P are rapidly interconverted by triose phosphate isomerase. G-3-P is utilized
as a substrate.
6. G-3-P is oxidized
by enzyme glyceraldehyde-3-P dehydrogenase to yield 1,3bisphosphoglycerate (1,3-BPG) and NADH + H+. It is only one oxidation reaction in
glycolysis. In this reaction, inorganic phosphate is incorporated into a high energy bond (=
substrate level phosphorylation).
7. The cleavage of anhydride bond in 1,3-BPG is energetically coupled to the formation of ATP.
Product of this reaction is 3-phosphoglycerate (3-PG). This rection is catalyzed by
phosphoglycerate kinase.
8. 3-PG is converted to 2-PG by phosphoglycerate mutase.
9. Elimination of water by enzyme enolase from 2-PG  product of this reaction is
phosphoenolpyruvate (PEP).
10. Enzyme pyruvate kinase catalyzes the synthesis of pyruvate (Pyr) and ATP from PEP and
ADP. This reaction is strongly exergonic, the high energy phosphate of PEP is conserved as
ATP = substrate level phosphorylation. This reaction is irreversible it is regulatory site of
glycolysis.
Glycolysis consumes 2 ATP per glucose for activation. 2 ATP are formed per C3 - fragment  total:
gain of 2 mol ATP per mol Glc.
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Fig. 1: Scheme of glycolysis
Figure is found on http://web.indstate.edu/thcme/mwking/glycolysis.html
Regulation of glycolysis:
Enzymes hexokinase, 6-phosphofructo-1-kinase (PFK) and pyruvate kinase are regulatory points of
glycolysis. Major control point in glycolysis is PFK (allosteric enzyme).
ATP is a substrate and an allosteric inhibitor of PFK. ↑ ATP / AMP ratio inhibits PFK. AMP activates
this enzyme. Futher inhibitor of PFK is citrate.
Enzyme hexokinase is inhibited by its product – Glc-6-P.
Enzyme pyruvate kinase is inhibited by ATP (allosteric inhibitor).
Hormones regulate the process of glycolysis. Glycolysis is activated by insulin and inhibited by
glucagon and catecholamines (epinephrine, norepinephrine).
Metabolic fates of Pyr:
Pyr is the branch point of glycolysis. The final fate of Pyr depends on the oxidation state of the cell.
a) under aerobic conditions – Pyr enters into mitochondria and undergoes pyruvate dehydrogenase
reaction (PDH) → acetyl-CoA → CAC.
b) under anaerobic conditions – Pyr undergoes lactate dehydrogenase reaction (LDH) → lactate + NAD+
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Pentose phosphate pathway is an oxidative metabolic pathway of Glc. This pathway is located in all
tissues in the cytoplasm of the cell (especially in erythrocytes, liver, mammary gland, testis, adrenal
cortex). Glc-6-P enters into and this pathway and pentose cycle supplies two important precursors for
anabolic pathways:
● NADPH + H+ (it is used in synthesis of fatty acids and steroids)
● ribose-5-phosphate (Rib-5-P) - it is used in synthesis of nucleic acids
pentose phosphate pathway has 2 phases:
A. oxidative phase converts Glc-6-P to ribulose-5-P. One CO2 and 2 NADPH are produced in the
process.
B. regenerative phase may convert a fraction of pentose phosphates back to hexose phosphates.
Reactions of oxidative phase: (see Fig. 2)
Glc-6-P is oxidized and decarboxylated to a pentose phosphate:
1. Glc-6-P enters to the pentose phosphate pathway and it is oxidized to 6-phosphogluconolactone
by enzyme glucose-6-P dehydrogenase. NADPH + H+ is released. Reaction is irreversible.
Enzyme Glc-6-P dehydrogenase is the major regulatory site for the pathway (it is regulated by
NADPH / NADP+ ratio).
2. 6- phosphogluconolactone is cleaved to 6-phosphogluconate by hydration.
3. 6-phosphogluconate is oxidized and decarboxylated to ribulose-5-P (ketopentose) by
6-phosphogluconate dehydrogenase. NADPH + H+ and CO2 are released.
4. Isomeration of ribulose-5-P to ribose-5-P by enzyme ribose isomerase.
Fig. 2: Oxidative phase of pentose phosphate pathaway.
Figure is found on http://web.indstate.edu/thcme/mwking/glycolysis.html
Interconversions of pentose phosphates lead to glycolytic intermediates. These interconverions are
catalyzed by enzymes transketolase and transaldolase. Both enzymes catalyze chain cleavage and
transfer reactions of substrates. (see Fig. 3)
Transketolase transfers a C2 unit from one sugar to another. Transaldolase transfers a C3 units from
sedoheptulose-7-P to the aldehyde group of glyceraldehyde-3-P. These reactions lead to form fructose6-P and glyceraldehyde-3-P.
3 Rib-5-P  2 Fru-6-P + glyceraldehyde-3-P
Fru-6-P is converted to Glc-6-P and Glc-6-P can be completely oxidized to CO2.
The reactions of regenerative phase are freely reversible, and can be used to convert hexose
phosphates into pentose phosphate.
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Fig. 3: Regenerative phase of pentose phosphate pathway.
Figure is found on http://web.indstate.edu/thcme/mwking/glycolysis.html
Some tissues (brain, erythrocytes) are dependent on the constant supply of glucose. When the amount
of carbohydrates taken up with the diet is insufficient, the concentration of Glc in the blood (= blood
glucose level) can be maintained by degradation of liver glycogen. The glycogen reserves are already
depleted after one day, and the blood glucose level begins to fall. This is the sign for de novo synthesis
of glucose = gluconeogenesis.
Gluconeogenesis is a synthesis of glucose from various precursors: amino acids, lactate, glycerol =
sources of carbon for the pathway. The main precursors of gluconeogenesis are amino acids derived
from the muscles. A futher important precursor is lactate, which is formed in erythrocytes and in muscles
when O2 is in short supply. Glycerol is produced by the degradation of fats.
Gluconeogenesis occurs predominantly in the liver (90%) and tubule cells of the kidney (10%).
The human body can synthesize several hundred grams of glucose per day by gluconeogenesis.
Hormones cortisol, glucagon and epinephrine promote gluconeogenesis, hormone insulin inhibits it.
Reactions of gluconeogenesis: (see Fig. 4)
Many reactions of gluconeogenesis are catalyzed by enzymes that are also involved in glycolysis
(gluconeogenesis uses glycolytic enzymes in the reverse direction). Other enzymes are specific for
gluconeogenesis. Glycolysis occurs in the cytoplasm, gluconeogenesis occurs in the mitochondria,
endoplasmatic reticulum and cytoplasm.
Gluconeogenesis from lactate  lactate is converted to pyruvate by enzyme lactate dehydrogenase.
Pyruvate cannot be converted directly to PEP by pyruvate kinase because the reaction is irreversible 
bypass.
1. First step of the reaction chain occurs in mitochondria. Pyruvate is initially carboxylated to
oxaloacetate, reaction is catalyzed by enzyme pyruvate carboxylase = 1st bypass.
Oxaloacetate can leave the mitochondria via transport system of the inner mitochondrial
membrane.
2. In the cytoplasm, oxaloacetate is converted to phosphoenolpyruvate (PEP) by PEP
karboxykinase (GTP-dependent enzyme).
3. The steps from PEP to Fru-1,6-bisP are steps of the glycolytic pathway in reverse.
4. Phosphofructokinase catalyzes an irreversible step in glycolysis and cannot be used for
conversion of Fru-1,6-bisP to Fru-6-P. A way around this step is provided by enzyme fructose4
1,6-bisphosphatase = 2nd bypass. This enzyme catalyzes irreversible hydrolysis of Fru-1,6-bisP
to Fru-6-P and it cannot be used in glycolysis to produce Fru-6-P.
5. Isomeration of Fru-6-P to Glc-6-P is catalyzed by phosphoglucose isomerase is freely
reversible and functions in both glycolysis and gluconeogenesis
6. Enzyme glucose 6-phosphatase catalyzes the hydrolysis of Glc-6-P to Glc = 3rd bypass 
phosphorylation of Glc to Glc-6- P is irreversible in glycolysis.
Glucose 6-phosphatase is located in endoplasmatic reticulum of the liver  from there, Glc is finally
released into the blood.
Fig. 4: Gluconeogenesis
Figure is found on http://web.indstate.edu/thcme/mwking/gluconeogenesis.html
Regulation of gluconeogenesis:
Regulatory enzymes of gluconeogenesis: pyruvate carboxylase, PEP carboxykinase, Fru-1,6bisphosphatase and Glc-6-phosphatase.
The Cori cycle depends on gluconeogenesis in liver. Glucose from liver is used in a peripheral tissue.
Function of the Cori cycle is described in Fig. 5.
Fig. 5: The Cori cycle.
Figure is found on http://web.indstate.edu/thcme/mwking/gluconeogenesis.html
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Metabolism of glycogen
Glycogen is a store of glucose. The human body can store up to 450 g of glycogen in the liver and in the
muscles. The glycogen content of the other organs is low.
Liver glycogen serves as a glucose reserve for the maintenance of blood glucose level.
Muscle glycogen serves as an energy reserve. It is a source of ATP for muscular activity. Exercise of
a muscle triggers mobilization of muscle glycogen for formation of ATP.
Glycogen is an animal branched homopolymer of glucose. Most glucose residues are linked by
α 14 bonds. Every 12th glucose is connected to a futher residue via α 16 bond. These branches are
extended by α 14- linked residues. The result is insoluble, tree-like molecules.
Glycogenesis (= glycogen synthesis)
1. Glc enters into hepatic tissue and it is phosphorylated by enzyme glucokinase  Glc-6-P.
2. Glc-6-P is converted to Glc-1-P by phosphoglucomutase (isomeration).
3. Glc-1-P is activated with UTP to form UDP-glucose by glucose-1-phosphate
uridylyltransferase. This reaction generates UDP-glucose = "activated glucose".
4. Enzyme glycogen synthase catalyzes transfer of UDP-glucose to a glycogen molecule to form a
new glycosidic bond. UDP is converted back to UTP: UDP + ATP  UTP + ADP
5. Glycogen synthase creates chains of glucose molecules with α 14 glycosidic linkages, but
does not form the α 16 glycosidic linkages. Form of α 16 glycosidic linkages is catalyzed by
"branching" enzyme (= glucan branching enzyme). This enzyme removes 6-7 Glc residues
from growing chain (> 11 Glc residues) and then adds to an internal part of the same chain.
These branches are extended by glycogen synthase.
Glycogen synthase is regulatory enzyme of glycogen synthesis. Glycogen synthase exists in two forms:
Glycogen synthase a (nonphosphorylated) = active form
Glycogen synthase b (phosphorylated) = inactive form
Phosphorylation of glycogen synthase is catalyzed by different kinases.
Glycogenolysis (= glycogen degradation)
Glycogen is never completely degraded. Enzyme glycogen phosphorylase catalyzes the first step in
glycogen degradation. This enzyme catalyzes phosphorolysis (Pi is used in the cleavage of an
α 14 glycosidic linkage) to yield Glc-1-P. The next step of glycogen degradation is catalyzed by
phosphoglukomutase: Glc-1-P  Glc-6-P. In liver, Glc-6-P produced by glycogenolysis is hydrolyzed
by glucose-6-phosphatase to give free Glc.
Debranching enzyme is required for complete
hydrolysis of glycogen, because glycogen
phosphorylase cannot cleave α 16 glycosidic linkages.
Glycogen phosphorylase is regulatory enzyme of glycogen degradation. Glycogen phosphorylase is
activated by AMP (allosteric activator) and it is inhibited by glucose and ATP. Glycogen phosphorylase
exists in two forms:
Phosphorylase a (phosphorylated) = active form
Phosphorylase b (nonphosphorylated) = inactive form
Phosphorylation of glycogen phosphorylase is catalyzed by phosphorylase kinase.
Hormones glucagon and epinephrine promote activation of glycogen phosphorylase. On the other
hand, hormone insulin has the opposite effect on phosphorylase.
Enzyme glucose-6-phosphatase is present only in liver, kidneys and enterocytes. Skeletal
muscles and brain have not this enzyme.
References:
Smith, C., Marks, A. D., Lieberman, M: Marks´ Basic Medical Biochemistry, A Clinical Approach, 2nd
edition, Lippincott Williams & Wilkins, Baltimore, USA (2005)
Koolman, J., Rőhm, K-H.: Color Atlas of Biochemistry, Thieme, Stuttgart (1996)
Devlin., T., M.: Textbook of Biochemistry with Clinical Correlations, 4th edition, Willey-Liss, New York,
USA (1997)
Pavla Balínová
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