1ROLES OF CARBOHYDRATES IN BIOLOGY

Carbohydrates serve as information-rich molecules that guide many biological processes.
Examples include:
1) Asialoglycoprotein receptor
Present in liver cells; binds to asialoglycoproteins to remove them from circulation
Presence of sialoglycoprotein prevents glycoproteins such as antibodies and peptide
hormones from being internalized
Presence of sialic acid on terminal galactose on these proteins mark the passage of
time; when they are removed (usually by the protein itself), the glycoproteins are
removed from circulation
2) Lectins
Carbohydrate-binding proteins of plant origin.
Contain 2 or more binding sites for carbohydrate units --> cross-link or agglutinate
erythrocytes and other cells.
3) Many viruses and bacteria can gain entry into host cells via carbohydrates displayed on
cell surface.
Influenza virus contains a hemagglutinin protein that recognizes
sialic acid residues on cells lining respiratory tract.
Neisseria gonorrhoeae infects human genital or oral epithelial cells
because of recognition of cell surface carbohydrates; other cells
lack these carbohydrates.
4) Interaction of sperm with ovulated eggs
Ovulated eggs contain zona pellucida, an extracellular coat made of O-linked
oligosaccharides.
Sperm cells have receptor for these carbohydrates.
Binding of sperm to egg causes release of proteases and hyaluronidase, which
dissolve zona pellucida to allow sperm entry.
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5) Selectins
Carbohydrate-binding adhesion proteins that mediate binding of neutrophils and
other leukocytes to sites of injury in the inflammatory response.
6) Homing receptor of lymphocytes
Homing is phenomenon in which lymphocytes tend to migrate to lymphoid sites from
which they were originally derived.
Mediated by carbohydrates on lymphocyte surface and endothelial lining of lymph
nodes.

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Chapter 11 - Glycolysis
Purpose: catabolism of glucose to provide ATPs and NADH molecules
Also provides building blocks for anabolic pathways.
Sequence of 10 enzyme-catalyzed reactions:
glucose pyruvate 2 ATPs and 2 NADH produced
All enzymes (and reactions) are cytosolic.
Net reaction:
glucose + 2ADP + 2NAD
+
+2Pi 2 pyruvate + 2ATP + 2NADH +2H
+
+2H2O
Can catabolize sugars other than glucose:
e.g. fructose ----> 2 glyceraldehyde 3-phosphate
e.g. lactose --> glucose + galactose
galactose --> glucose 1-phosphate --> glucose 6-phosphate
e.g. mannose ---> mannose 6-phosphate --> fructose 6-phosphate
Ten Steps of Glycolysis
1) glucose --> glucose 6-phosphate by hexokinase G = -8.0 kcal/mole
Hexokinase also works on mannose and fructose at increased [ ].
Serves to trap glucose in the cell --> a phosphorylated molecule cannot leave
2) glucose 6-phosphate --> fructose 6-phosphate by glucose 6-phosphate isomerase
Example of aldose--> ketose isomerization.
Enzyme is very stereospecific.
Reaction is near equilibrium in cell --> not a control point in glycolysis
3) fructose 6-phosphate --> fructose 1,6-bisphosphate by phosphofructokinase-1 (PFK-1)
Reaction has G = -5.3 kcal/mole and is metabolically irreversible.
Represents the first committed step in glycolysis.
4) fructose 1,6-bisphosphate --> dihydroxyacetone phosphate + glyceraldehyde 3-phosphate by
fructose 1,6 bisphosphate aldolase.
5) DHAP --> glyceraldehyde 3-phosphate by triose phosphate isomerase
Also catalyzes aldose--> ketose conversion.
Rate is diffusion controlled (substrate is converted to product as fast as substrate is
encountered).
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6) glyceraldehyde 3-phosphate --> 1,3-bisphosphoglycerate by glyceraldehyde 3-phosphate
dehydrogenase
One molecule of NAD+ is reduced to NADH --> respiratory chain
7) 1,3 bisphosphoglycerate --> 3-phosphoglycerate
Phosphoryl group transfer to ADP to form ATP.
Because phosphate group comes from a substrate molecule, called substrate level
phosphorylation
First ATP-generating step of glycolysis.
8) 3-phosphoglycerate --> 2-phosphoglycerate by phosphoglycerate mutase
Mutases are enzymes that transfer phosphoryl groups from one part of a substrate molecule
to another.
9) 2-phosphoglycerate --> phosphoenolpyruvate (PEP) by enolase (forms double bond)
10) PEP --> pyruvate
Second time for substrate level phosphorylation.
Reaction is metabolically irreversible.
FATE OF PYRUVATE
Under anaerobic conditions, cells must be able to regenerate NAD
+ or glycolysis will stop.
Usually regenerated by oxidative phosphorylation, but that requires O2.
There are 2 anaerobic pathways that use NADH and regenerate NAD+.
1) alcoholic fermentation
Conversion of pyruvate to ethanol
H
+
CO2 NADH NAD
+
pyruvate acetaldehyde ethanol
pyruvate alcohol
decarboxylase dehydrogenase
glucose +2Pi + 2ADP + 2H
+
---> 2 ethanol + 2CO2 + 2ATP + 2H2O
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2) lactate fermentation
NADH + H
+
NAD
+
pyruvate ------------------------> lactate
lactate
dehydrogenase
glucose +2Pi + 2ADP ---> 2 lactate + 2ATP + 2H20
Lactate causes muscles to ache.
Also produced by bacterial fermentation of lactose.
3) entry into citric acid cycle
REGULATION OF GLYCOLYSIS
First step possible is glucose transport into cells via glucose transporters.
Intestinal and kidney cells have Na
+-dependent cotransport system called SGLTI.
Move in by passive transport via facilitated diffusion.
Hexose transporters called GLUT family
GLUT1 and GLUT3 - present in nearly all mammalian cells; continually transport
glucose at a constant rate
GLUT2 - liver cells
GLUT4 - skeletal muscles cells and adipocytes; insulin promotes rapid uptake of
glucose by increasing number of GLUT4 receptors in the cell membrane
GLUT 5 - transports glucose in small intestine
GLUT 7 - transports glucose 6-phosphate from cytosol to ER
There are three enzymes that can be regulated:
1) hexokinase
Catalyzes the first irreversible reaction.
Inhibited by glucose 6-phosphate.
Not true controlling step because not a committed step.
Glucose 6-phosphate can be used elsewhere (i.e. pentose phosphate pathway
and glycogen synthesis).
2) phosphofructokinase-1
ATP is an allosteric inhibitor because it increases the Km of PFK-1 for fructose 6-
phosphate.
AMP is an allosteric activator; same for ADP
When [ATP] is low, [AMP] is high --> low [ATP]/[AMP] levels stimulate PFK-1.
Citrate is an allosteric inhibitor of PFK-1 (ample substrate entering citric acid
cycle).
pHi also regulates PFK-1 ( inhibition is due to excess H
+
due to lactic acid
accumulation --> no O2 present to continue).
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Fructose 2,6-bisphosphate is an allosteric activator.
ATP ADP
fructose 6-phosphate fructose 2,6-bisphosphate
PFK-2
activity
F 2,6-BPase activity
(2 active sites on enzyme)
Substrate is present in all cells but procaryotes.
PFK-2 activity stimulated by Pi and inhibited by citrate.
PFK-2 activity linked to action of glucagon due to adenylate cyclase
activation --> phosphorylation of serine residue of PFK-2 -->
inactivates kinase activity but activates phosphatase activity --
> [fructose 2,6 bisphosphate] decreases as it is converted to
fructose 6-phosphate , PFK-1 falls --> glycolysis decreases
Glucagon is made by pancreas and is secreted when blood sugars levels
fall --> mobilizes glycogen breakdown.
3) pyruvate kinase
Regulated by allosteric modulation and covalent modification.
Allosterically activated by fructose 1,6-bisphosphate.
Allosterically inhibited by [ATP].
Protein kinase A phosphorylates pyruvate kinase --> less active.