Sugar Utilization
A) There are basically 4 disaccharides we are concerned about
1) Maltose
a) Cleaved by maltase into 2 glucose
b) glucose enters glycolysis directly
2) trehalose
a) Cleaved by trehalase into 2 glucose
b) glucose enters glycolysis directly
3) Sucrose
a) Cleaved by sucrase into fructose and glucose
b) glucose enters glycolysis directly
c) fructose must undergo distinct metabolic modification
for utilization in glycolysis
4) lactose
a) cleaved by lactase into galactose and glucose
b) glucose enters glycolysis directly
c) galactose must undergo distinct metabolic modification
for utilization in glycolysis
d) lactase defficiency is the most common defect in disaccharide
metabolism
d1) 10% of public is affected to some degree
d2) lactase is in the brush border of the small intestine
d3) undigested lactose has an osmotic effect causing
cramps and diarhea
d4) undigested lactose is utilized by bacteria to generate
methane, CO2 and H2
B) Fructose metabolism
1) Fructose has a non insulin depedent uptake by cells.
This uncontrolled utilization can lead to increased glycolytic activity: lactate
acidosis.
2) Hexokinase: works on fructose trapping it in the cell
Fructose + ATP--> Fructose 6 Phosphate (F6P) + ADP
a) This is the same hexokinase discussed in glycolysis
b) F6P generated in this way enters glycolysis directly
at F6P
c) However, hexokinase has a very low affinity for fructose
(high Km) so this is not the major mechanism of fructose utilization.
3) Fructokinase: traps fructose in the cell.
Fructose + ATP --> Fructose 1 phosphate (F1P) + ADP
4) F1P Aldolase: cleaves 6C sugar into 2 3C sugars
F1P ---> DHAP + glyceraldehyde
a) DHAP is the dihydroxy acetone phosphate generated
in glycolysis. It can enter glycolysis directly at thatstep and is converted to
glyceraldehyde 3 phosphate.
5) triose kinase: converts glyceraldehyde to a usable
form
glyceraldehyde + ATP --> Glyceraldehyde 3 phosphate (G3P) + ADP
a) G3P enters glycolysis directly at that step.
6) Since 1 ATP was used at fructose kinase and one was
used at triose kinase, there is a net utilization of 2ATP per 6C sugar identical
to Glucose.
C) Galactose Metabolism
1) Hexokinase: traps galactose in cell
Galactose + ATP --> Galactose 1 phosphate + ADP
2) Galactose 1P uridyl transferase (Galactose 1P UT):
transfers a UDP to galactose
Galactose 1P + UDP-glucose --> UDP-Galactose + Glucose 1P
a) UDP-glucose is the substrate for glycogen synthesis
b) Glucose 1P is generated normally in glycogenolysis
c) UDP-galactose can be used directly in lactose synthesis
in the mammary glands
d) mutations in Galactose 1P UT cause hereditary galactosemia:
vomitting, diarrhea, liver enlargement and retardation
3) UDP-glacatose epimerase: converts glactose to its epimer
glucose
UDP Galactose --> UDP-Glucose
a) UDP-glucose can be used in the formation of glycogen
b) UDP-glucose can be converted to glucose 1 phosphate
for conversion to glucose or utilization in glycolysis (from G6P).
c) ATP was expended at hexokinase and another high energy
bond was used in the formation of UDP-glucose used at Galactose 1P UT (UTP -->
UDP) so a Net of 2 high energy bonds was used before galactose even enters glycolysis.
Pentose Phosphate Shunt
-occurs in cytosol
A) Glucose 6 Phosphate Dehydrogenase (G6PD)
G6P + 2NADP + H2O --> ribose-5-phosphate (R5P) + CO2 + 2NADPH + 2H+
1) G6P is generated in the first step of glycolysis
2) R5P is required for nucleotide synthesis
3) NADPH are the major cytosolic reducing equivalents
while NADH are the major mitochodrial reducing equivalents
a) NADP/NADPH = 0.01 in fed hepatocytes
b) NAD/NADH = 700 in cytosol
B) NADPH is is used in numerous synthetic and maintenace functions
1) Mixed functin oxidases (p450 mono-oxygenases)
a) liver enzymes which use NADPH as cosubstrates for
hydroxylation
RH + O2 + NADPH + H --> NADP + H20 + ROH
b) when R is saturated (-CH2-), hydroxylation is the
first step in drug detoxification
c) -OH is then esterified to glucuronic acid or sulfate
which makes the compounds soluble and easily excreted
2) Fatty acid biosynthesis
8 Acetyl CoA + 7ATP + 14 NADPH + 6 H --->
Palmitate (16C) + 14 NADP + 8 CoA + 7 ADP + 6 H20 + 7
Pi
3) Hydroxylations which require molecular oxygen use NADPH
progesterone --> cortisol
progesterone --> corticosterone
a) Pentose phosphate pathway is elevated in tissues which
carry out these reactions
a1) gonads --> testosterone
a2) adrenal cortex --> corticoids
a3) adipocytes --> FA biosynthesis
4) Maintenance of reduced glutathione (GSH)
a) GSH is used to eliminate or repair oxidative damage
GSH + Ox --> GSSG
b) oxidized glutathione GSSG is not usable so GSH must
be regenerated using NADPH
GSSG + NADPH + H --> GSH + NADP + H20
c) GSH is critical for maintaining the integrity of RBC
membrane because it repairs its continual oxidative damage. In addition GSH is
required to maintain Heme iron in the ferrous state.
d) in G6PD defficiency, NADPH is not made in RBCs, so
GSSG accumulates and RBC membrane integrity falls leading to hemolytic anemia.
d1) G6PD defficiencys are one of the most common metabolic
defects in humans: 200 million affected.
C) transketolases and transaldolases allow non oxidative interconversion of sugars
C7 + C3 --> C4 + F6P
C5 + C4 --> Glyceraldehyde 3 P + F6P
1) These pathways allow "odd" sugars to enter glycolysis
2) Ribose 5 phosphate would be a C5 example. RBCs must
use pentose phosphate shunt to maintain membrane integrity. However, they don't
have a nucleus, so they don't need to synthesize nucleotides. The R5P formed can
be converted to G3P and F6P (with the input of a 4C sugar) and used in glycolysis.
Glycogen Metabolism
A) Glcogen Synthesis
1) Insulin triggers glycogen synthesis and glucagon inhibits
it.
2) Phosphoglucomutase: conversion of G6P
Glucose 6 Phosphate --> Glucose 1 Phosphate
a) G6P is from glycolysis
3) UDP Glucose pyrophosphorylase: UDP is attatched to
G1P
G1P + UTP --> UDP-Glucose + Pi
a) UDP-Glucose is an essential substrate leading to the
formation of UDP Glucuronate which is used in numerous pathways of synthesis and
detoxification.
4) Glycogen synthase: attaches glucose monomers alpha1,4
UDP-Glucose + n residue glycogen --> n+1 residue glycogen + UDP
a) Glycogen synthase is the regulated step in glycogen
synthase.
b) Phosphorylation of Glycogen Synthase a (active) by
cAMP dependent protein Kinase forms Glycogen Synthase b (inactive). Glucagon or
epinephrine trigger cAMP cascade which inhibits glycogen synthesis.
c) Glycogen synthase b can be partially activated by
elevated G6P. If it is not being used to make ATP we should store it.
5) Branching enzymes (4,6 tansferases) form an alpha1,6
branch every 6-8 alpha 1,4 additions. Increases number of "ends" available, accelerating
synthesis and breackdown.
a) Andersens disease: a1,4 --> a1,6 branching enzyme
mutation
a1) liver and spleen affected
a2) normal glycogen level with long outer branches
a3) progressive cirhossis of liver, liver failure and
death by 2
B) Glycogenolysis
1) Glucagon triggers glycogenolysis
2) Glycogen Phosphorylase: cleaves glycogen into monomers
at alpha 1,4 bonds.
N+1 residue glycogen --> Glucose 1 Phosphate + n-residue glycogen
a) Glycogen phosphorylase is the regulated step in glycogenolysis.
b) phosphorylation of Glycogen phosphorylase b (inactive)
by glycogen phosphorylase kinase forms glycogen phosphorylase a (active). Glucagon
or epinephrine triggers phosphorylation leading to activation of glycogen phosphorylase.
c) AMP can partially activate Glycogen Phosphorylase
b. Low energy, make glucose.
d) ATP is a competitive inhibitor of AMP activation.
Enegy status is high, glucose must be available, don't make more.
e) G6P is a competitive inhibitor of AMP activation.
Intemediates are accumulating in glycolysis, don't feed it more glucose.
f) Pompes Disease: lysosomal alpha 1,4 glucosidase mutation
f1) all organs affected
f2) increased glycogen
f3) Cardiorespiratory failure, death before 2
g) McArdles Disease: mutation in phosphorylase
g1) muscle
g2) moderately increased amount of glycogen
g3) limited ability tdo perform excersize due to cramping
g4) require continuous food intake
h) Hers Disease: mutation in phosphorylase muatation
h1) liver
h2) similar to McArdles disease
3) Debranching enzymes (4,4 transferase and 1,6 glucosidase)
cleaves alpha 1,6 bonds.
a) Coris disease: Amylo-1,6glucosidase mutation
a1) muscle and liver affected
a2) increased glycogen with short outer branches
a3) similar to but milder than Von Gierkes disease
4) Phosphoglucomutase
G1P --> G6P
a) G6P can enter glycolysis directly at that step
5) Glucose 6 Phosphatase: formation of glucose
G6P --> Glucose + Pi
a) if glucose 6 phosphatase and glucokinase both worked
in the cytosol, a futile cycle of glucose --> G6P --> glucose would be established.
Therefore, the two activities are separated with glucokinase in the cytosol and
G6 phosphatase in the ER.
b) Von Gierke' disease: Glucose 6 Phosphatase mutation
b1) liver and kidney
b2) increased glycogen
b3) enlarged liver, hypoglycemia, ketosis, hyperurecemia,
hyperlipemia
C) Differential regulation of Glycogen synthesis and Glycogenolysis by phosphorylation.
1) Glycogen Synthesis and Glycogenolysis have opposite
effects so one is turned on when the other is turned off.
2) The primary regulatory factors for these enzymes are
glycogen synthase and glycogen phosphorylase.
a) "a" forms of both enzymes are active and "b" forms
are inactive.
b) phosphorylated glycogen synthase is glycogen synthase
b. It is off and glycogen is not ynthesized.
c) phosphorylated glycogen phosphorylase is glycogen
phosphorylase a. It is on and glycogneolysis ensues.
3) Both glycogen synthase b and glycogen phosphorylase
a are dephosphorylated by Protein Phosphotase I (PPI).
a) dephosphorylated glycogen synthase is glycogen synthase
a. It is on and glycogen is synthesized.
c) dephosphorylated glycogen phosphorylase is glycogen
phosphorylase b. It is off and glycogneolysis does not occur.
4) Inhibitor I, inhibits PPI only when Inhibitor I is
phosphorylated.
a) cAMP Protein Kinase phosphoryates Inhibitor I.
b) if Inhibitor I is phosphorylated, PPI is inactive
so the phosphorylation status of glycogen synthase b (off) and glycogen phosphorylase
a (on) is maintained.
c) if inhibitor 1 is dephosphorylated, PPI is active
and the phosphorylation of glycogen synthase and glycogen phosphorylase is eliminated.
Glycogen synthase a (on) and glycogen phosphorylase b (off) are formed.
5) Glucagon binds to its receptor:
a) adenylate cyclase induced to produce cAMP.
b) rising cAMP activates cAMP dependent Protein Kinase
(PK)
c) PK:
c1) phosphorylates glycogen phosphorylase kinase turning
it on.
c2) phosphorylates glycogen synthase turning it off (b
form)
c3) phosphorylates inhibitor 1 turning it on. PPI is
inhibited so phosphoryation status of cascade enzymes are maintained.
d) glycogen phosphorylase kinase phosphorylates glycogen
phosphorylase turning it on (a form).
e) glycogen synthesis halted, glycogenolysis activated.
6) Glucagon receptor not occupied (insulin and glucose
high):
a) adenylate cyclase does not produce cAMP.
b) falling cAMP levels no longer activate cAMP dependent
Protein Kinase (PK)
c) low PK activity:cannot maintain inhibitor 1 phosphorylation
so it turns off.
d) PPI is no longer inhibited so it becomes active and
reverses the phosphoryation status of cascade enzymes.
d1) Protein Kinase dephosphorylated and inactivaated
d2) glycogen phosphorylase kinase dephosphorylates turnig
it off.
c3) glycogen synthase is dephosphorylated turning it
on (a form)
d4) glycogen phosphorylase dephosphorylated turning it
off (b form).
e) glycogen synthesis begins, glycogenolysis halted.
7) High glucose can rapidly activate glycogen synthesis
and inhibit glycogenolysis even before cAMP falls.
a) glucose binds glycogen phosphorylase a.
b) phosphorylase a conformation altered and phosphorylated
serines exposed.
c) PPI is associated with phosphorylase a and rapidly
dephosphorylates it to glycogen phosphorylase b.
d) PPI cannot associate with phosphorylase b so it dissociates
and binds glycogen synthase b.
e) glycogen synthase b is dephosphorylated to glycogen
synthase a.
8) Muscle glycogen
a) Ca2+ increase triggers contraction and binds to Ca*ý