Download Biochem 2 Recitation #2 Spr 20152102105.pptx

Biochem 2 Recitation #2 Glycolysis &
Gluconeogenesis
How do
transporters
exert
control on
metabolic
pathways?
There are several fates for
glucose when it enters a cell;
•  These fates are all following
the initial rxn that forms
glucose 6-P04.
•  The pathways that claim G6P
are; pentose phosphate
pathway, glycogen synthesis
pathway as well as entering
glycolysis.
•  The final product of glycolysis
is pyruvate. In mammalian
cells it has two major fates,
homolactate fermentation or
aerobic respiration.
Glycolysis can be thought of
as consisting of three
separate portions.
•  Stage 1 is the
phosphorylation of glucose
and the isomerization to
Fr-6-P04; why is this step
necessary?
•  Stage 2 is the isomerization
of GAP and DHAP. Why is
DHAP the favored product?
•  Stage 3 is the utilization of
GAP to form pyruvate.
What are the controls of
stage 3?
Glucose + 2NAD à 2pyruvate + NADH
ΔG0’ = -146 kJ/mol
2ADP + 2Pià 2ATP + H2O
ΔG0’ = 61 kJ/mol
Overall free energy -85 kJ/mol
Glycolysis is not a reversible
pathway.
Gluconeogensis then must be a separate
pathway.
Glucoseà2 Lactate
ΔG0= -196 kJ/mol
But the lactate dh is reversible, there are 5
isoenzymes of Ldh in most mammals.
Why are
gluconeogensis and
glycolysis reciprocal
pathways?
•  Three major enzymes
are not reversible due to
their thermodymanics,
these are;
•  Hexokinase
•  PFK-1
•  Pyruvate kinase.
•  Gluconeogenesis is a
reciprocal pathway to
glycolysis, and not a mere
reversal of glycolysis.
At specific highly exothermic
reaction in glycolysis, the reverse
reaction is not possible. A second
enzyme is necessary that reverses
that reaction step.
There are three steps of this kind,
Pyruvate Kinase, PFK-1/PFK-2, and
Hexokinase/Glucokinase.
•  Non- carbohydrate precursors of
glucose via gluconeogenesis are
lactate, pyruvate, amino acids
(primarily alanine) and Krebs
cycle intermediates (especially
OAA/malate, and αKG).
Regulation of hexokinase IV (glucokinase) by
sequestration in the nucleus. The protein inhibitor of
hexokinase IV is a nuclear binding protein that draws hexokinase IV
into the nucleus when the fructose 6-phosphate concentration in liver
is high and releases it to the cytosol when the glucose concentration
is high.
•  This is the committed step to
glycolysis, the product
F16BP is the only molecule
that can be cleaved by the
reverse Claisen condensation
rxn, catalzyed by aldolase A
to form GAP + DHAP.
•  PFK-1 activity is controlled by
the product of PFK-2, the
homotropic activator F26BP.
Fr2,6BP is not a glycolytic
substrate, and found in µM
conc.
The glycerol-3-PO4
Shuttle
• 
the enzyme called cytoplasmic
glycerol-3-phosphate dehydrogenase
(cGPdh) converts DHAP to
glycerol 3-phosphate by oxidizing
one molecule of NADH to NAD+
• 
Glycerol-3-phosphate gets converted
back to DHAP by a membrane-bound
mGPdh, this time reducing one
molecule of enzyme-bound FAD to
FADH2. FADH2 then reduces
coenzyme Q (ubiquinone to
ubiquinol) which enters into
oxidative phosphorylation. This
reaction is irreversible
Regulation of PK. The enzyme is allosterically inhibited by ATP, acetyl-CoA, and long-chain
fa (all signs of an abundant energy supply), and the accumulation of fr 1,6-BP triggers its
activation. Accumulation of alanine, which can be synthesized from pyruvate in one step,
allosterically inhibits PK, slowing the production of pyruvate by glycolysis. The liver isozyme (L
form) is also regulated hormonally. Glucagon activates cAMP-dependent protein kinase (PKA),
which phosphorylates the PK L isozyme, inactivating it. When the glucagon level drops, a
protein phosphatase (PP) dephosphorylates PK, activating it. This mechanism prevents the
liver from consuming glucose by glycolysis when blood glucose is low; instead, the liver
exports glucose. The muscle isozyme (M form) is not affected by this phosphorylation
mechanism.
Transport of PEP and oxaloacetate from the mitochondrion to the cytosol. The participation of two second
messenger systems: the cAMP-mediated
stimulation of glycogenolysis and inhibition of
glycogen synthesis triggered by glucagon
and beta-andrenoreceptor activation; and the
IP3, DAG, and Ca2+-mediated stimulation of
glycogenolysis and inhibition of glycogen
synthesis triggered by alpha-adrenoreceptor
activation. IP3 stimulates the release of Ca2+
from the endoplasmic, whereas DAG,
together with Ca2+, activates protein kinase
C(PKC) to phosphorylate and thereby
inactivate glycogen synthase. G6Pase
occupies the endoplasmic reticulum.
Consequently, the cytosolically produced
G6P is transported into the endoplasmic
reticulum via the T1 G6P translocase, where
it is hydrolyzed to glucose and Pi. The
glucose and Pi are then returned to the
cytosol by the T2 and T3 transporters,
respectively, and the glucose is exported from
the cell via the GLUT2 glucose transporter.