2. CONTENTS
• Glucose Transport
• Glycolysis
• Irreversible enzymes
• Phosphofructokinases
• Glycolysis in RBCs
• Effect of 2,3-BPG
3. INTRODUCTION
• Carbohydrates are aldehyde or ketone
compounds with multiplehydroxyl
groups.
• They serve as energy stores, fuels, and
metabolicintermediates,structural
framework of RNAand DNA, structural
elements in the cell walls of bacteria and
plants, linkedto many proteinsand lipids
in mediatinginteractions among cells and
between cells.
5. GLUCOSE
TRANSPORTERS
They mediate the thermodynamically downhill movement of glucose across the
plasma membranes of animal cells.
They can be divided into two classes: the sodium-glucose cotransporters or
symporters (SGLTs) and the facilitative glucose transporters (GLUTs).
Members of GLUT family consists of a single polypeptide chain about 500 residues
long with 12 transmembrane segments.
Members of SGLT family consists of a 660 - 680 amino acid residues with 14
transmembrane segments.
6. Sodium-glucose co-transporters (SGLTs)
SGLTs are expressed by cells in the small intestine and in the
renal proximal tubules.These proteins mediate the active
transport of glucose against an electrochemical gradient by
another transportmechanism,where glucose uptake is coupled
with the uptake of sodium ions. SGLT1 serve as the primary
transporter of glucose in the intestine.SGLT2 is located in cells
that line the proximal tubule,where it aids reabsorption of
glucose from renal fluid,to prevent glucose being eliminated in
the urine.
Facilitative glucose transporters (GLUTs)
These are responsible for the bidirectional transport of glucose
in tissues and cells.This involves using facilitative diffusion to
carry glucose down a concentration gradient,into the
cell.These proteins have substrate binding sites. Binding of glucose
to one site induces a conformational change that results in glucose
being transported from one side of the membraneto the other.
7. GLUT1 and GLUT3,present in nearly all mammalian cells,
are responsible for basal glucose uptake.
Their KM value for glucose is about 1 mM, significantly less
than the normal serum-glucose level,which typically
ranges from 4 mM to 8 mM.
Hence,GLUT1 and GLUT3 continually transport glucose
into cells at an essentially constant rate.
GLUT1 is highly abundant in foetal tissue and in adults, it
is most highly expressed in red blood cells and in barrier
tissues such as the blood brain−barrier.
GLUT3 is mostly found in nerve cells,where it is thought
to be responsible for the majority of glucose transport. It
is also found in the placenta.
Normal blood glucose concentration:100mg/dl (5.6mM)
8. GLUT2,present in liver and pancreatic β cells,is
distinctive in having a very high KM value for glucose
(15-20 mM).
Hence, glucose enters these tissues at a biologically
significant rate only when there is much glucose in
the blood.
The pancreas can thereby sense the glucose level and
accordingly adjust the rate of insulin secretion.
Insulin signals the need to remove glucose from the
blood for storage as glycogen or conversion into fat.
The high KM value of GLUT2 also ensures that
glucose rapidly enters liver cells only in times of
plenty.
Normal blood glucose concentration:100mg/dl (5.6mM)
9. GLUT4,which has a KM value of 5 mM, transports
glucose into muscle and fat cells.
The presence of insulin,which signals the fed state,
leads to a rapid increase in the number of GLUT4
transporters in the plasma membrane.
Hence, insulin promotes the uptake of glucose by
muscle and fat.
The amount of this transporter present in muscle
membranes increases in response to endurance
exercise training.
GLUT 4 is expressed in adipose tissue, cardiac
muscle and skeletal muscle.
Normal blood glucose concentration:100mg/dl
(5.6mM)
10. GLYCOLYSIS
Glycolysis is also called as EMP pathway. It is
after the name of the discoverers - Embden,
Meyerhof and Parnas
Derived from the Greek stem glyk-,"sweet,"
and the word lysis, "dissolution."
This pathway is common to virtually all cells,
both prokaryotic and eukaryotic. In
eukaryotic cells, glycolysis takes place in the
cytosol.
It is a pathway that converts glucose into two
pyruvate molecules, releasing a modest
amount of energy.
12. Iodoacetate and arsenate
are the inhibitors of
glyceraldehyde-3-
phosphate dehydrogenase
Fluoride is the inhibitor
of enolase.
13.
14. IRREVERSIBLE ENZYMES
Three enzymes in the pathway catalyze reactionsthat are
irreversible. This keeps the pathway moving in only one direction.
1. Glucokinase/hexokinase,
2. PhosphofructoKinase-1and
3. Pyruvatekinase.
In enzymology, the committed step is an effectivelyirreversible
enzymatic reactionthat occurs at a branch pointduring the
biosynthesisof some molecules.
As the name implies, after this step, the molecules are "committed"
to the pathway and will ultimatelyend up in the pathway's final
product.
The rate-determiningstep or rate limitingstep is the sloweststep in
a reactionor pathway. Here, the committedstep is in fact the rate-
determining step as well.
15. Phosphofructokinases (PFK-1 and PFK-2)
• Fructose 6-phosphate is phosphorylated to fructose 1,6-bisphosphate usingATP by PFK-1
• It is inhibited by ATP and citrate,and activated by AMP.
• This makes sense because the cell should turn off glycolysis when it has sufficient energy (high ATP) and
turn on glycolysis when it needs energy (high AMP).
• Citrate is an intermediate of the citric acid cycle,so high levels of citrate also imply that the cell is
producing sufficient energy.
• Insulin stimulates and glucagon inhibits PFK-1 in hepatocytes by an indirect mechanism involving PFK-2
and fructose 2,6-bisphosphate.
16. • Insulin activates Phosphofructokinase-2 (PFK-2), which converts a tiny amount of fructose 6-
phosphate to fructose 2,6-bisphosphate (F2,6-BP).
• F2,6-BP activates PFK1. On the other hand,glucagon inhibits PFK-2, lowering F2,6-BP and
thereby inhibiting PFK-1.
• PFK-2 is found mostly in the liver.By activating PFK-1, it allows these cells to override the inhibition
caused by ATP so that glycolysis can continue,even when the cell is energetically satisfied.
• The metabolites of glycolysis can thus be fed into the production of glycogen,fatty acids,and other
storage molecules rather than just being burned to produce ATP
17. GLYCOLYSIS IN ERYTHROCYTES
• In erythrocytes (red blood cells), anaerobic
glycolysis represents the only pathway for ATP
production, yielding a net 2 ATP per glucose.
• Red blood cells have bisphosphoglycerate mutase,
which produces 2,3- bisphosphoglycerate (2,3-BPG)
from 1,3-BPG in glycolysis.
• The phosphate is moved from the 1-position to the
2-position.
• 2,3-BPG binds allosterically to the β-chains of
hemoglobin A (HbA) and decreases its affinity for
oxygen.
18. Effect of 2,3-BPG on Heamoglobin
The oxygen binding curve for pure hemoglobin is markedly different than the oxygen binding
curve for hemoglobin found within red blood cells.If we examine and compare the two curves,we
will see that the curve for hemoglobin in red blood cells is shifted to the right with respect to the
pure hemoglobin curve.This implies that pure hemoglobin has a much higher affinity for oxygen
and will release much less (only 8%) of oxygen in exercising tissue (compared to 66% for
hemoglobin in RBCs).
19. Although 2,3-BPG binds to HbA, it does not bind well
to fetal hemoglobin (HbF), with the result that HbF has
a higher affinity for oxygen than maternal HbA.This
allows transplacental passage of oxygen
from mother to foetus.
It turns out that 2,3-biphosphoglycerate, or simply 2,3-BPG, acts as an allosteric effector to hemoglobin. 2,3-BPG is a
naturally occurring molecule that is produced as an intermediate in the glycolysis process. Deoxyhemoglobin in theT-state
is a very unstable molecule and this drives the equilibrium towards the R-state, which means that deoxyhemoglobin will
not exist for long and the majority of the hemoglobin will be bound to oxygen (i.e have a high affinity for oxygen).
That is, by binding to hemoglobin, 2,3-BPG
decreases hemoglobin's affinity for oxygen, thereby
shifting the entire oxygen-binding curve to the
right side.This is what allows the hemoglobin to act as
an effective oxygen carrier in the body, unloading about
66% of oxygen to exercising tissue.
However in the presence of 2,3 BPG, this molecule will bind to the center pocket found in hemoglobin, thereby stabilizing
theT-state of hemoglobin and allowing it to exist without quickly converting into the relaxed state.
