Fates of pyruvate

1
Fates of pyruvate
28-18
1. Need to regenerate NAD from NADH produced in
glycolysis 2. Under aerobic conditions, NADH
reoxidized by passage of its e- to O2
pyruvate oxidized further ÆÆÆ CO2 H2O 3. Under
anaerobic conditions, NADH used to reduce
pyruvate.
to lactate (eg. in muscle)
to ethanol (alcoholic fermentation in yeast)
THIAMINE PYROPHOSPHATE COFACTOR
2
29-1
Entry of other carbohydrates into glycolysis
3
29-2
Glycogen phosphorylase
REMOVES SUCCESSIVE Glc-1-Ps FROM NON-REDUCING END
- Glc is phosphorylated no ATP needs to
be consumed to permit entry into glycolysis -
Ionized Glc-1-P cant diffuse out of cell
Note glycogen synthesized and degraded by
different pathways
4
29-3
Debranching of glycogen
Glc-1-P Ø Glc-6-P
phosphoglucomutase
ACTIVITY 1
phosphohexose isomerase, etc.
ACTIVITY 2
5
29-4
Regulation of glycolysis
1. Cell must maintain ATP at nearly constant
level, regardless of fuel rate of ATP
consumption 2. Biosynthetic precursors derived
from CH2O (eg. aa, other sugars, long chain
FA) must be produced as needed 3. Fuel reserves
(eg. glycogen) must be mobilized if needed 4.
Glucose catabolism must be reciprocally regulated
w. glucose synthesis Required changes in
catabolic patterns are accomplished by the
regulation of glycolysis In muscle and liver
tissue, 4 enzymes have a regulatory role
phosphofructokinase, hexokinase, pyruvate kinase
ALL IRREVERSIBLE (also glycogen
phosphorylase)
6
29-5
Regulation of glycolysis
REGULATED
MOST IMPORTANT, REGULATES COMMITMENT
TO GLYCOLYSIS
REGULATED
7
29-6
Regulation of phosphofructokinase essentially
irreversible, commits cell to channeling Glc to
glycolysis
1. Activity inhibited by high ATP (
end-product inhibition) 2. PFK tetramer 3.
ATP can bind at catalytic site, and also at
separate regulatory (allosteric) site 4. Binding
of ATP at regulatory site lowers affinity for
F-6-P (ie. high ATP, higher K0.5 for
F-6-P) 5. ATP allosteric inhibitor (Æ
sigmoidal F-6-P vs PFK activity plot) 6. ADP
and AMP reverse allosteric inhibition by ATP so
when cellular ATP falls PFK channels more Glc
into glycolysis Further regulation by
citrate and fructose-2,6-bisphosphate..
HIGHER K0.5
8
29-6A
Reminder/revision of how an allosteric inhibitor
might work allosteric inhibition of tetrameric
phosphofructokinase by ATP
Binding of allosteric inhibitor ATP to allosteric
site on monomer
PFK conformation with LOW affinity for F-6-P
PFK conformation with HIGH affinity for F-6-P
ATP
ATP
transition in the PFK tetramer is
increasingly favored as successive PFK monomers
bind ATP
ATP
ATP
ATP
F-6-P
F-6-P
F-6-P
F-6-P
ATP
ATP
Most active PFK
Less active PFK
9
29-7
Regulation of PFK, continued.
7. Citrate allosteric inhibitor of
PFK produced during aerobic oxidation of
pyruvate 8. Strongly activated by
fructose-2,6-bisphosphate - F-2,6-bisP
increases affinity of PFK for F-6-P, decreases
affinity for ATP ( allosteric activator that
shifts conformational equilibrium of tetrameric
PFK from T-state to R-state) - F-2,6-bisP
levels rise in response to high blood Glc and
so accelerate glycolysis, fall in response to
low Glc. (When blood Glc scarce, rise in blood
levels of the hormone glucagon results in
lowered levels of F-2,6-bisP in the liver,
slowing Glc consumption by glycolysis, and
stimulating Glc synthesis)
10
29-8
Regulation of hexokinase
Glc Glc-6-P F-6-P F-1,6-bisP
hexokinase
ALLOSTERIC INHIBITION of muscle H-K
When PFK inactive, F-6-P accumulates is
converted to Glc-6-P
phosphohexose isomerase
phosphofructokinase
There are different isoforms of hexokinase with
differing affinities for Glc 1. Muscle H-K has
high affinity for Glc and is inhibited by
Glc-6-P 2. Liver has H-K isoform called
glucokinase, with low affinity for
Glc phosphorylates Glc only when its
abundant. -role of glucokinase is to provide
Glc for synthesis of glycogen
NOT INHIBITED BY Glc-6-P
IN LIVER
Note b/c Glc-6-P is also needed for glycogen
synthesis, for pentose synthesis, and for
NADPH generation, regulation of glycolysis
through H-K alone would affect these
processes too. PFK, the first committed step to
glycolysis, is the best control site.
11
29-9
Regulation of pyruvate kinase
ACTIVATION BY - F-1,6-bisP
allows pyruvate K to keep pace w. generation of
earlier glycolytic intermediates
INHIBITION BY - ATP (allosteric)
decreases affinity for PEP, slows glycolysis
when ATP abundant - Alanine (allosteric). Ala
made from pyruvate this inhibition signals
building block aa are abundant - Fuels for
citric acid cycle (in which ATP formed),
acetyl CoA, long chain FA
12
29-10
Other fates of glucose
1. Glucose used to generate NADPH a carrier of
chemical energy in the form of reducing
power, - used as reductant in anabolic
pathways, eg. to introduce double bonds in fatty
acid synthesis 2. Glucose needed to produce
pentoses especially D-ribose - component of
ATP, DNA, RNA, NAD, FAD 3. NADPH and ribose
generated in the pentose phosphate pathway which
starts with Glc-6-P
13
29-11
Pentose phosphate pathway-1
1. Dehydrogenation of Glc-6-P - by
glucose-6-phosphate dehydrogenase - oxidation at
C-1 - NADP e--acceptor - product
6-phospho-d-gluconolactone (C1-C5
intramolecular ester) - allosterically inhibited
by NADPH
2. Hydrolysis of lactone to 6-phosphogluconate -
by lactonase - free acid formed
14
29-12
Pentose phosphate pathway-2
3. Dehydrogenation decarboxylation
of 6-phosphogluconate - by 6-phosphogluconate
dehydrogenase - C-1 COO- removed - NADP
e--acceptor - product D-ribulose-5-P
(ketopentose)
C-6
4. Isomerization of ribulose-5-P to ribose-5-P -
by phosphopentose isomerase - ketopentose
converted to aldopentose
C-5
Overall equation for pentose phosphate
pathway Glc-6-P 2 NADP H2O Æ ribose-5-P
CO2 2 NADPH 2 H
Pathway can end here, or, if NADPH, not
ribose-5-P is required, ribose-5-P is
recycled back to Glc-6-P in a series of
rearrangements of the carbon skeletons