Download 19_Glycolysis, aerobic oxidation of glucose

METABOLISM OF CARBOHYDRATES:
GLYCOLYSIS
For centuries,
bakeries and
breweries have
exploited the
conversion of
glucose to ethanol
and CO2 by
glycolysis in yeast
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.
-amilase hydrolyzes the -1-4-glycosidic
bonds randomly to produce smaller subunits like
maltose, dextrines and unbranched
oligosaccharides.
-amilase
The intestinal juice contains enzymes hydrolyzing
disaccharides into monosaccharides (they are produced
in the intestinal wall)
Sucrase hydrolyses sucrose into glucose and fructose
Glucose
sucrase
Fructose
Sucrose
Galactose
lactase
Glucose
Lactase hydrolyses
lactose into glucose
and galactose
Lactose
Glucose
maltase
Maltase hydrolyses
maltose into two
glucose molecules
Maltose
Glucose
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+
Protein
Glucose
Protein
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.
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.
The fate of glucose molecule in the cell
Glucose
Glycogenogenesis
(synthesis of
glycogen) is
activated in well
fed, resting state
Glucose-6phosphate
Pentose phosphate
pathway supplies
the NADPH for lipid
synthesis and
pentoses for nucleic
acid synthesis
Ribose,
NADPH
Glycogen
Pyruvate
Glycolysis
is activated if
energy is required
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.
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
• Catabolism of glucose in
aerobic conditions via
glycolysis and the citric
acid cycle
The glycolytic pathway consist of ten enzymecatalyzed reactions that begin with a glucose and
split it into two molecules of pyruvate
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)
4 ATP are
produced per
glucose
• Net: 2 ATP
produced per
glucose
Stage 1, which is the conversion of glucose into fructose
1,6-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
phosphorylated
threecarbon units.
Stage 2 is the cleavage of the fructose
1,6-bisphosphate into two three-carbon fragments
dihydroxyacetone phosphate and glyceraldehyde 3phosphate.
Dihydroxyacetone phosphate and glyceraldehyde 3phosphate are readily interconvertible.
In stage 3,
ATP is
harvested
when the
threecarbon
fragments
are
oxidized to
pyruvate.
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
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.
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
3. Phosphofructokinase-1 (PFK-1)
• Catalyzes transfer of a phosphoryl group from ATP to the
C-1 hydroxyl group of F6P to form fructose 1,6bisphosphate (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)
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
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.
Fate of carbon atoms from hexose stage
to triose stage
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 acidanhydride linkage of 1,3BPG
• Next step of glycolysis uses the high-energy phosphate
of 1,3BPG to form ATP from ADP
7. Phosphoglycerate Kinase (PGK)
• Transfer of phosphoryl group from the energy-rich mixed
anhydride 1,3BPG to ADP yields ATP and
3-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
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
9. Enolase: 2PG to PEP
• 2-Phosphoglycerate (2PG) is dehydrated to
phosphoenolpyruvate (PEP)
• Elimination of water from C-2 and C-3 yields the enolphosphate PEP
• PEP has a very high phosphoryl group transfer potential
because it exists in its unstable enol form
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
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