1. METABOLISM OF CARBOHYDRATES: GLYCOLYSIS
For centuries, bakeries and breweries have exploited the conversion of glucose to ethanol and CO2 by glycolysis in yeast

2. DIGESTION OF CARBOHYDRATES
Glycogen, starch and disaccharides (sucrose, lactose and maltose) are hydrolyzed to monosaccharide units in the gastrointestinal tract.
The process of digestion starts in the mouth by the salivary enzyme –amilase.
The time for digestion in mouth is limited.
Salivary -amilase is inhibited in stomach due to the action of hydrochloric acid.
Another -amilase is produced in pancreas and is available in the intestine.

3. -amilase hydrolyzes the -1-4-glycosidic bonds randomly to produce smaller subunits like maltose, dextrines and unbranched oligosaccharides.
-amilase

4. The intestinal juice contains enzymes hydrolyzing disaccharides into monosaccharides (they are produced in the intestinal wall)
Sucrase hydrolyses sucrose into glucose and fructose
sucrase
Glucose
Fructose
Sucrose

5. Galactose Glucose Glucose Glucose lactase
Lactase hydrolyses lactose into glucose and galactose
Lactose
Glucose
Glucose
Maltose
maltase
Maltase hydrolyses maltose into two glucose molecules

6. ABSORPTION OF CARBOHYDRATES
Only monosaccharides are absorbed
The rate of absorption: galactose > glucose > fructose
Glucose and galactose from the intestine into endothelial cells are absorbed by secondary active transport
Na+
Glucose
Protein
Protein

7. Carrier protein is specific for D-glucose or D-galactose.
L-forms are not transported.
There are competition between glucose and galactose for the same carrier molecule; thus glucose can inhibit absorption of galactose.
Fructose is absorbed from intestine into intestinal cells by facilitated diffusion.
Absorption of glucose from intestinal cells into bloodstream is by facilitated diffusion.

8. Transport of glucose from blood into cells of different organs is mainly by facilitated diffusion.
The protein facilitating the glucose transport is called glucose transporter (GluT).
GluT are of 5 types.
GluT2 is located mainly in hepatocytes membranes (it transport glucose into cells when blood sugar is high);
GluT1 is seen in erythrocytes and endothelial cells;
GluT3 is located in neuronal cells (has higher affinity to glucose);
GluT5 – in intestine and kidneys;
GluT4 - in muscles and fat cells.

9. The fate of glucose molecule in the cell
Pentose phosphate pathway supplies the NADPH for lipid synthesis and pentoses for nucleic acid synthesis
Glycogenogenesis (synthesis of glycogen) is activated in well fed, resting state
Glucose-6-phosphate
Ribose, NADPH
Glycogen
Pyruvate
Glycolysis is activated if energy is required

10. Glycolysis is the earliest discovered and most important process of carbohydrates metabolism.
Glycolysis – metabolic pathway in which glucose is transformed to pyruvate with production of a small amount of energy in the form of ATP or NADH.
Glycolysis is an anaerobic process (it does not require oxygen).
Glycolysis pathway is used by anaerobic as well as aerobic organisms.
In glycolysis one molecule of glucose is converted into two molecules of pyruvate.
In eukaryotic cells, glycolysis takes place in the cytosol.

11. Pyruvate can be further metabolized to:
Pyruvate can be further metabolized to: (1) Lactate or ethanol (anaerobic conditions) (2) Acetyl CoA (aerobic conditions)
Acetyl CoA is further oxidized to CO2 and H2O via the citric acid cycle
Much more ATP is generated from the citric acid cycle than from glycolysis
Acetyl CoA

12. Catabolism of glucose in aerobic conditions via glycolysis and the citric acid cycle

13. The glycolytic pathway consist of ten enzyme-catalyzed reactions that begin with a glucose and split it into two molecules of pyruvate

14. Glycolysis (10 reactions) can be divided into three stages
In the 1st stage (hexose stage) 2 ATP are consumed per glucose
In the 3rd stage (triose stage) ATP are produced per glucose
Net: 2 ATP produced per glucose

15. Stage 1, which is the conversion of glucose into fructose ,6-bisphosphate, consists of three steps: a phosphorylation, an isomerization, and a second phosphorylation reaction.
The strategy of these initial steps in glycolysis is to trap the glucose in the cell and form a compound that can be readily cleaved into phospho-rylated three-carbon units.

16. Stage 2 is the cleavage of the fructose ,6-bisphosphate into two three-carbon fragments dihydroxyacetone phosphate and glyceraldehyde 3-phosphate.
Dihydroxyacetone phosphate and glyceraldehyde 3-phosphate are readily interconvertible.

17. In stage 3, ATP is harvested when the three-carbon fragments are oxidized to pyruvate.

18. Glycolysis Has 10 Enzyme-Catalyzed Steps
Each chemical reaction prepares a substrate for the next step in the process
1. Hexokinase
Transfers the g-phosphoryl of ATP to glucose C-6 oxygen to generate glucose 6-phosphate (G6P)
Four kinases in glycolysis: steps 1,3,7, and 10
All four kinases require Mg2+ and have a similar mechanism

19. Properties of hexokinases
Broad substrate specificity - hexokinases can phosphorylate glucose, mannose and fructose
Isozymes - multiple forms of hexokinase occur in mammalian tissues and yeast
Hexokinases I, II, III are active at normal glucose concentrations
Hexokinase IV (Glucokinase) is active at higher glucose levels, allows the liver to respond to large increases in blood glucose
Hexokinases I, II and III are allosterically inhibited by physiological concentrations of their immediate product, glucose-6-phosphate, but glucokinase is not.

20. 2. Glucose 6-Phosphate Isomerase
Converts glucose 6-phosphate (G6P) (an aldose) to fructose 6-phosphate (F6P) (a ketose)
Enzyme preferentially binds the a-anomer of G6P (converts to open chain form in the active site)
Enzyme is highly stereospecific for G6P and F6P
Isomerase reaction is near-equilibrium in cells

21. 3. Phosphofructokinase-1 (PFK-1)
Catalyzes transfer of a phosphoryl group from ATP to the C-1 hydroxyl group of F6P to form fructose 1,6-bisphosphate (F1,6BP)
PFK-1 is metabolically irreversible and a critical regulatory point for glycolysis in most cells
A second phosphofructokinase (PFK-2) synthesizes fructose 2,6-bisphosphate (F2,6BP)

22. 4. Aldolase
Aldolase cleaves the hexose F1,6BP into two triose phosphates: glyceraldehyde 3-phosphate (GAP) and dihydroxyacetone phosphate (DHAP)
Reaction is near-equilibrium, not a control point

23. 5. Triose Phosphate Isomerase (TPI)
Conversion of DHAP into GAP
Reaction is very fast, only the D-isomer of GAP is formed
Reaction is reversible. At equilibrium, 96% of the triose phosphate is DHAP. However, the reaction proceeds readily from DHAP to GAP because the subsequent reactions of glycolysis remove this product.

24. Fate of carbon atoms from hexose stage to triose stage

25. 6. Glyceraldehyde 3-Phosphate Dehydrogenase (GAPDH)
Conversion of GAP to 1,3-bisphosphoglycerate (1,3BPG)
Molecule of NAD+ is reduced to NADH
Energy from oxidation of GAP is conserved in acid-anhydride linkage of 1,3BPG
Next step of glycolysis uses the high-energy phosphate of 1,3BPG to form ATP from ADP

26. 7. Phosphoglycerate Kinase (PGK)
Transfer of phosphoryl group from the energy-rich mixed anhydride 1,3BPG to ADP yields ATP and phosphoglycerate (3PG)
Substrate-level phosphorylation - Steps 6 and 7 couple oxidation of an aldehyde to a carboxylic acid with the phosphorylation of ADP to ATP

27. 8. Phosphoglycerate Mutase
Catalyzes transfer of a phosphoryl group from one part of a substrate molecule to another
Reaction occurs without input of ATP energy

28. 9. Enolase: 2PG to PEP
2-Phosphoglycerate (2PG) is dehydrated to phosphoenolpyruvate (PEP)
Elimination of water from C-2 and C-3 yields the enol-phosphate PEP
PEP has a very high phosphoryl group transfer potential because it exists in its unstable enol form

29. 10. Pyruvate Kinase (PK) PEP + ADP  Pyruvate + ATP
Catalyzes a substrate-level phosphorylation
Metabolically irreversible reaction
Regulation both by allosteric modulators and by covalent modification
Pyruvate kinase gene can be regulated by various hormones and nutrients

30. Net reaction of glycolysis
During the convertion of glucose to pyruvate:
Two molecules of ATP are produced
Two molecules of NAD+ are reduced to NADH
Glucose + 2 ADP + 2 NAD+ + 2 Pi
2 Pyruvate + 2 ATP + 2 NADH + 2 H+ + 2 H2O