Download Welcome to Class 8

Announcements / Reminders!
•  Midterm TA led Review Sessions!
Welcome to Class 8!
Sunday, February 23 from 8-10pm!
Location: Science Center Main Room (315)!
•  Office Hours!
Prof Salomon: SFH 270 on Thursday Feb 20,
2:30-4:30!
TAs: by appointment!
Introductory Biochemistry!
•  Sapling problem set 3 due Monday, February 24, 5pm!
•  Midterm 1 is Tuesday February 25 at 1 pm!
!
Location: Last names beginning with A-M Macmillan 117!
Last names beginning N-Z BERT 130!
(BERT= Building for Enviromental Research and Teaching, with
greenhouse on top. Previously called Hunter labs)!
Energy released by hydrolysis of biological
phosphate compounds!
figure 13-19!
1!
ATP can provide energy by group transfer even when there
is no net transfer of P!
Hydrolysis of phosphocreatine!
Derivation of energy from ATP hydrolysis!
generally involves covalent!
participation of ATP in the reaction.
Formation of glutamine by
condensation of glutamate with NH3
is endergonic (positive ΔG' º).!
Formation of γ-glutamyl P by transfer
of P from ATP is exergonic (negative
ΔG' º).!
Phosphocreatine has a high phosphoryl group transfer potential.!
It can drive the formation of ATP from ADP.
figure 13-15!
Class 8: Outline and Objectives
Formation of glutamine by
displacement of P from γ-glutamyl P
by NH3 is exergonic (negative ΔG' º).!
The net coupled reaction is
exergonic (negative ΔG' º).!
figure 13-18!
Many enzyme reactions involve a change
in the oxidation state of the substrate!
Redox reactions !
l  Oxidation states of carbon!
l  Relationship between ∆G and ∆E!
l  Electron carriers NADH, NADPH, FAD, FMN!
l  Where does the energy come from?!
l  Overview metabolism, catabolism, anabolism!
l  Glycolysis!
l  Pathway!
l  Regulation!
l  Substrate channeling!
l  Fermentation!
l  Lactate!
l  Ethanol!
l  Pentose phosphate pathway!
l  Fructose and galactose metabolism !
l 
2!
e- affinity!
stronger!
oxidants!
Redox Reactions!
Electrons are transferred in reduction-oxidation reactions.!
Redox reactions require an electron donor (reducing agent)!
and an electron acceptor (oxidizing agent).!
Therefore, two simultaneous reactions occur in a redox process.!
stronger!
reductants!
The reduction potentials of redox half-reactions under standard conditions
can be tabulated, like ΔG' º values for other reactions.!
!
The strongest oxidant is O2 and the strongest reductant is a small Feprotein called ferredoxin.!
!
!
Measurement of the standard reduction potential of a redox pair!
stronger!
oxidants!
stronger!
reductants!
The E' º value is for a half reaction under standard conditions (all components
at 1 M except H+ and H2O). The units of E' º is volts (V).!
!
Standard reduction potentials, E' º, are relative to that of hydrogen (E' º ≡ 0).!
Figure 13-23!
3!
Standard Reduction Potentials E’°!
The reduction potentials of redox couples under standard conditions can be
tabulated, like ΔG' º values for other reactions.!
!
The E' º value is for a half reaction under standard conditions (all components
at 1 M except H+ and H2O). The units of E' º is volts (V).!
!
The tabulated reduction potentials are for the reduction reaction:!
!
Oxidized form + electrons → Reduced form !
!E' º (Volts)!
!
Standard reduction potentials, E' º, are relative to that of hydrogen (E' º ≡ 0).!
!
For an actual redox reaction, a reduction must be accompanied by a oxidation
so that electrons are not “left over” or deficient.!
!
For any complete redox reaction, ΔE' º = E' º for the substance being reduced
(the oxidant) minus E' º for the substance being oxidized (the reductant).!
stronger!
oxidants!
stronger!
reductants!
The tabulated reduction potentials are for the reduction reaction: Oxidized form +
electrons → Reduced form!
Example:!
2Fe3+ + Ethanol →2Fe2+ + Acetaldehyde + 2H+ ΔE' º = 0.771 – (–0.197)= 0.968 V!
!
Note that the number of atoms or molecules involved, and the number of electrons
transferred, don’t enter into these calculations.!
Oxidation states of carbon in the biosphere
C bonding
electrons!
Concentration dependence of E
C bonding
electrons!
Like ∆G, the reduction potential E of a half cell is concentration dependent:
[electron acceptor A+]!
RT!
E = E' º + !
ln!
nF!
[electron donor A]!
ΔE = (E2 – E1) V!
E = E1 V!
2H+/H2!
Figure 13-22!
increasing electronegativity: H < C < S < N < O!
the more electronegative atom “owns” the bonding electrons!
+
–
E = E2 V!
2A+/2A!
If [A+] = [A], E = E' º.!
If [A+] > [A], E > E' º.!
If [A+] < [A], E < E' º.!
Example: The ratio NAD+/NADH is kept high in the cell. This makes E more positive
than E' º (makes the couple a stronger oxidant than if [NAD+] = [NADH]) and favors
the reaction direction NAD+ + H+ → NADH (ΔG is more negative than ΔG' º)!
4!
Standard Reduction Potential Differences
are Mathematically Related to the
Standard Free Energy for the Reaction!
2Fe3+ + Ethanol → 2Fe2+ + Acetaldehyde + 2H+!
!
ΔE' º = 0.771 – (–0.197) = 0.968 V!
!
ΔG = –nFΔE!
!
n = the number of electrons transferred in the reaction!
!
F = The Faraday Constant: 96.5 kJ/V-mol!
!
For the above reaction, ΔG' º = –(2)(96.5 kJ/V-mol)(0.968 V) = –186.8 kJ/mol.!
!
Note that the number of electrons exchanged does enter into the calculation of
ΔG' º from ΔE' º.!
!
Note that a positive ΔE' º corresponds to a negative ΔG' º (exergonic).!
stronger!
oxidants!
stronger!
reductants!
Electrons are transferred by cofactors acting as electron carriers!
• Very important biological redox cofactors are the pyridine nucleotides and flavin
nucleotides.!
• The reduced forms of these cofactors are relatively strong reductants.!
Pyridine Nucleotide Coenzymes!
Flavin nucleotide cofactors
Niacin (vitamin B3)!
!
Nicotinamide Adenine Dinucleotide (NAD)!
Nicotinamide Adenine Dinucleotide !
Phosphate (NADP)
!
NAD(P)+ = oxidized form!
NAD(P)H = reduced form!
!
Figure 13-24!
Riboflavin (vitamin B2)!
Nicotinamide ring!
Flavin Mononucleotide (FMN) and !
Flavin Adenine Dinucleotide (FAD)!
!
FMN, FAD = oxidized forms!
FMNH2, FADH2 = reduced forms
Figure 13-27!
5!
Four ways to transfer electrons
Coenzymes NAD and NADP
l 
l 
l 
l 
Directly as electrons (Fe2+ + Cu2+ ↔ Fe3+ + Cu+)!
As hydrogen atoms (AH2 + B ↔ A + BH2)!
—
As a hydride ion :H (CH3CH3 + NAD+ ↔ CH2=CH2 + NADH + H+)!
Through direct combination with oxygen (RCH3 + 1/2 O2 ↔ RCH2OH)!
For most biological molecules, the unit of oxidation and reduction is two
reducing equivalents, i.e., two electrons, i.e., pairs of electrons are gained
or lost in each redox reaction.!
In biological systems, oxidation is often synonymous with dehydrogenation
(loss of hydrogen, note that there is no oxygen involved),!
i.e. gain of a double bond between carbon atoms or change of an alcohol to
a carbonyl oxygen in an organic molecule.
The reduced nicotinamide ring has a characteristic near-UV
absorption near 340 nm, which can be used to follow the
progress of the enzymatic reaction in kinetic experiments.!
2 Fe3+ +!
2 Fe2+ +!
+ 2 H+!
Figure 13-24b!
Several ways of expressing redox reactions!
NAD+
+
H+
+
2e–
NAD+ + 2[H]
NADP+ + 2[H]
NAD(P)+ + 2[H]
FMN + 2H+ + 2e–
FAD + 2[H]
NADH!
Overview: Catabolism and anabolism
metabolism: meta = change; bolism ≈ throwing
NADH + H+!
NADPH + H+!
NAD(P)H + H+!
ana = up, back, again!
reduction!
cata = down!
oxidation!
FMNH2!
FADH2!
Part II, Figure 3!
6!
Three stages of cellular respiration
Catabolic pathways converge, anabolic pathways diverge
Glycolysis “splits” sugars and partially
oxidizes the products, generating substrates
for complete oxidation to CO2, and also
generates some ATP and reduced cofactors. !
!
(In fermentations, the reduced cofactors are
re-oxidized in non-energy-generating
reactions.)!
Glycolysis: glyco = sugar; lysis = splitting!
The citric acid cycle completely oxidizes the
products of glycolysis to CO2 and generates
reduced cofactors as well as some ATP.!
Respiratory re-oxidation of the reduced
cofactors generated in the above stages is
coupled to the synthesis of large amounts of
ATP.!
Part II, Figure 4!
Figure 16-1!
Oxidation states of carbon in the biosphere
C bonding
electrons!
Class 8: Outline and Objectives
l  Redox reactions !
l Oxidation states of carbon!
l Relationship between ∆G and ∆E!
l  Electron carriers NADH, NADPH, FAD, FMN!
l Where does the energy come from?!
l  Overview metabolism, catabolism, anabolism!
l  Glycolysis!
l Pathway!
l Regulation!
l Substrate channeling!
l  Fermentation!
l Lactate!
l Ethanol!
l  Pentose phosphate pathway!
l  Fructose and galactose metabolism !
5! 4!
4!
4!
4!
C bonding
electrons!
3!
Sugars have a composition
like formaldehyde, (CH2O)n!
CH2O + H2O → CO2 + 4H+ + 4e–!
Figure 13-22!
7!
Fire: NOT stepwise oxidation of glucose
Glycolysis: stepwise oxidation of glucose
● 
Oxygen has a higher affinity for electrons than glucose and the various
electron carriers (e.g. NADH).!
● 
Therefore, the transfer of electrons from these molecules to O2 is
energetically favorable.!
● 
Glucose + 6 O2
!
!
6 CO2 + 6 H2O
!
!
!
∆G´o!= –2840 kJ/mol
! = –473.3 kJ/mol C!
● 
Only 30-50 kJ/mol are required to form one molecule of ATP from ADP + Pi!
● 
Stepwise oxidation of glucose converts electron flow into usable energy.
The energy released in some steps can be captured by coupling the step to
ATP synthesis, or by temporarily storing the electrons in a molecule (e.g.,
NADH) whose re-oxidation can be coupled to ATP synthesis.!
Glycolysis: Part II
Glycolysis: Part I
Figure 14-2-2!
Figure 14-2-1!
8!
1. Formation of glucose 6-phosphate
1
Glucose + Pi ↔ Glucose 6-P
ATP ↔ ADP + Pi !
!
!
Glucose + ATP ↔ Glucose 6-P + ADP
1. Formation of glucose 6-phosphate
Importance of phosphorylated intermediates:!
●  Negative charge traps intermediates inside the cell, even if the concentration
outside is much lower. !
●  Phosphoryl groups conserve energy.!
●  The binding energy resulting from enzyme interactions with phosphate
groups helps to lower the activation energy and increases the specificity of
reactions.!
!ΔG'0 = 13.8 kJ/mol!
!ΔG'0 = –30.5 kJ/mol!
!ΔG'0 = –16.7 kJ/mol!
The large negative ΔG’° for ATP hydrolysis drives the reaction.!
Coupled reactions drive endergonic processes. !
2. Isomerization and second phosphorylation
2
Aldose!
Enzyme-limited vs. substrate-limited reactions
Ketose
1
2
3
3
Enzyme-limited!
reaction (far from!
equilibrium)!
Substrate-limited!
reactions (at or!
near equilibrium)!
For some steps in glycolysis, the substrate/product
ratios are near the equilibrium ratios, because the
involved enzymes are relatively fast or abundant.!
→ flux is substrate-limited.!
Example: steps 2, 4!
!
For a step that is catalyzed by a relatively slow or
scarce enzyme, the substrate/product ratio is
greater than the equilibrium ratio.!
→ flux is enzyme-limited.!
Example: steps 1, 3!
4
Lehninger 3rd Ed., Figure 15-16!
9!
Regulation of metabolic pathways
Regulation of metabolic pathways
Most enzymes in a pathway operate near their equilibrium, but some
enzymes that are stategically located operate far from equilibrium.!
Metabolic flux must be controlled:!
● 
the demand for ATP production in muscle may increase 100-fold in a
few seconds in response to exercise!
● 
relative proportions of carbohydrate, fat and protein in the diet vary
from meal to meal!
● 
the supply of fuels obtained in the diet is intermittent (between meals,
starvation)!
The two ATP-requiring steps of glycolysis are regulated
Characteristics of enzymes that are regulated:
● 
They control the rate of the respective pathway and whether it is
turned on or shut off. !
● 
They catalyze reactions that are out of equilibrium and which are
enzyme-limited (valve function).!
● 
They catalyze highly exergonic (effectively irreversible) reactions, thus
driving the pathway forward. !
● 
They differ in a catabolic vs. anabolic pathway.!
Allosteric control of enzyme activity: Hexokinase!
Allosteric
site!
Glucose-bound form: induced fit
Hexokinase is allosterically inhibited by its own product, glucose 6-phosphate.
Figure 6-25!
Figure 14-2-1!
10!
PFK-1 regulation is complex
4. Aldol cleavage (the literal glycolytic reaction)
ATP is a required substrate, but at
high concentration, ATP can also
bind to an allosteric site and inhibit
PFK-1. This inhibition is relieved by
AMP, which competes with ATP for
binding at the allosteric site.!
Even though ∆G' º > 0, the reaction proceeds in the forward direction
because the reaction products are removed quickly by later steps, “pulling”
the reaction in the direction of cleavage. ∆G < 0.!
Figure 15-16b,c!
Triosephosphate isomerase: a “perfect enzyme”
5. Isomerization
Ketose
Aldose!
•  Limitation 1: a catalyst cannot affect the position of the reaction equilibrium!
•  TIM accelerates the isomerization by a factor of 1010 compared to the
chemical reaction!
Even though ∆G' º > 0, the reaction proceeds in the forward direction
because the reaction products are removed quickly by later steps, “pulling”
the reaction in the direction of cleavage. ∆G < 0.!
•  Limitation 2: a catalyst cannot catalyze an interconversion faster
than the substrate can ‘find’ the enzyme in solution!
•  the kcat/KM ratio for TIM is 2 x 108 M-1 s-1, which is close to the
diffusion limit of a small molecule in solution!
11!
Glycolysis: Summary of Part I
The fate of the hexose carbon atoms
Figure 14-7!
In this phase, a glucose molecule is converted to two triose phosphate
molecules, at the expense of (driven by) the hydrolysis of two ATP molecules.!
Figure 14-2-1!
Glycolysis: Part II
The first substrate-level phosphorylation uses two enzymes
6
7
Figure 14-2-2!
12!
6
Oxidative Phosphorylation of Glyceraldehyde 3-P
7
The first substrate-level ADP phosphorylation
1,3-bis-P-Glycerate ↔ 3-P-Glycerate + Pi
ADP + Pi ↔ ATP
1,3-bis-P-Glycerate + ADP ↔ 3-P-glycerate + ATP
!ΔG' 0 = –49.0 kJ/mol!
! ΔG' 0 = 30.5 kJ/mol!
! ΔG' 0 = –18.5 kJ/mol!
The large negative ΔG' 0 for 1,3-bis-P-glycerate hydrolysis drives the reaction !
3-P-glycerate + 2 H+ + 2 e– ↔ Glyceraldehyde 3-P
E'o = -0.55 V !!
NAD+ + H+ + 2 e– ↔ NADH
E'o = –0.32 V !ΔG' 0 = -2(96.5)(-0.32+0.55) !
!
!
!
!
!
!
! ΔG' 0 = - 44.4kJ/mol!
3-P-glycerate + Pi ↔ 1,3-bis-P-glycerate
!ΔG' 0 = 50.7 kJ/mol!
Glyceraldehyde 3-P + Pi + NAD+ ↔ 1,3-bis-P-glycerate + NADH + H+ !ΔG' 0 =
6.3 kJ/mol!
ΔG'
o
= –nFΔE' o!
The very large negative ΔG' 0 for glyceraldehyde 3-P oxidation drives the reaction !
Why is hydrolysis of 1,3-BPG so exergonic?
1,3-BPG is a high-energy compound
Figure 13-19!
1. 
The reaction product 3-P-glyceric acid is a strong acid, which immediately
ionizes at physiological pH.!
2. 
Resonance of the ionized reaction product 3-P-glycerate distributes the
negative charge over 2 oxygen atoms, which stabilizes the ionized form
and effectively lowers the concentration of the immediate hydrolysis
product.!
Figure 13-14!
13!
The coupled reaction [6 + 7] is overall exergonic
Substrate channeling between reactions 6 and 7 Glyceraldehyde 3-P + ADP + Pi + NAD+ → 3-P-Glycerate + ATP + NADH + H+!
Overall ∆G' º = 6.3 + (–18.5) = –12.2 kJ/mol!
Under cellular conditions, steps 6 and 7 are reversible.!
6
7
Protects 1,3-bisphosphoglycerate
from spontaneous hydrolysis!
!
Ensures that reactions 6 and 7
remain closely coupled!
Lehninger 3rd Ed., Figure 15-8!
2,3-BPG is an intermediate in step 8
8
The second substrate-level phosphorylation
10
9
14!
Like 1,3-BPG, PEP is a high-energy compound
PEP hydrolysis
PEP hydrolysis is driven by tautomerization of the immediate hydrolysis
product (enol form) to the more stable keto form, which lowers the effective
concentration of the immediate hydrolysis product.!
!
About half of the released energy (∆G' º = –61.9 kJ/mol) is captured in the
formation of ATP (∆G' º = –30.5 kJ/mol), the rest (net ∆G' º = –31.4 kJ/mol)
constitutes a driving force to “pull” the reaction forward.!
Figure 13-19!
Glycolysis: Summary of Part II
Figure 13-13!
Steady-state concentrations of glycolytic intermediates in erythrocytes!
In this phase, the initial
two ATP molecules that
were hydrolyzed in Part
I are regenerated, and
two more ATP plus two
NADH molecules are
formed for each
molecule of glucose.!
Figure 14-2-2!
Metabolite
!
!
Glucose !
!
Glucose 6-P
!
Fructose 6-P
!
Fructose 1,6-bis-P !
Dihydroxyacetone-P
Glyceraldehyde 3-P!
1,3-bis-P-glycerate !
2,3-bis-P-glycerate !
3-P-glycerate
!
2-P-glycerate
!
P-enol-pyruvate
!
Pyruvate !
!
Lactate !
!
ATP
!
!
ADP
!
!
Pi
!
!
!
!Concentration (mM)!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!5.0!
!0.083!
!0.014!
!0.031!
!0.14!
!0.019!
!0.001!
!4.0!
!0.12!
!0.030!
!0.023!
!0.0051!
!2.9!
!1.85!
!0.14!
!1.0!
From Minakami, S. and Yoshikawa, H. 1965. Biochem. Biophys. Res. Comm. 18: 545.!
15!
Calculation of the overall energetics of glycolysis
The standard free energies of
some of the steps of glycolysis
are positive, even though the
overall standard free energy is
negative.!
At the actual cellular concentrations
of the intermediates, the free
energies of all of the steps are
either negative or close to zero.!
First, calculate the energy released on complete oxidation of pyruvate:!
!
Glucose (C6H12O6) + 6 O2 ↔ 6 CO2 + 6 H2O !
!
!ΔG' 0 = –2840 kJ/mol!
2 Pyruvate (C3H4O3) + 2 NADH + 2 H+ ↔ Glucose + 2 NAD+
!ΔG' 0 = 146 kJ/mol!
2 NAD+ + 2 H+ + 4 e– ↔ 2 NADH !
!E'o = –0.320 V
!ΔG' 0 = 123.5 kJ/mol!
2 H2O ↔ O2 + 4 H+ + 4 e–
!
!E'o = –0.816 V
!ΔG' 0 = 315.0 kJ/mol!
2 Pyruvate + 5 O2 ↔ 6 CO2 + 4 H2O
!
!
! ΔG' 0 = –2255.5 kJ/mol!
Then use the value for the complete oxidation of 2 pyruvates to calculate the overall energetics of
glycolysis:!
!
Glucose + 6 O2 ↔ 6 CO2 + 6 H2O !
!
!
!
! ΔG' 0 = –2840 kJ/mol!
6 CO2 + 4 H2O ↔ 2 Pyruvate + 5 O2
!
!
!
! ΔG' 0 = 2255.5 kJ/mol!
2 ADP + 2 Pi ↔ 2 ATP + 2 H2O !
!
!
!
!
ΔG' 0 = 61 kJ/mol !
+
+
–
o
2 NAD + 2 H + 4 e ↔ 2 NADH !
!E' = –0.320 V
!
!
ΔG' 0 = 123.5 kJ/mol!
2 H2O ↔ O2 + 4 H+ + 4 e–
!
!E'o = –0.816 V
!
!
ΔG' 0 = 315 kJ/mol!
Glucose + 2 ADP + 2 Pi + 2 NAD+ ↔ 2 Pyruvate + 2 ATP + 2 H2O + 2 NADH + 2 H+ ΔG' 0 = –82 kJ/mol!
(Values from Lehninger)!
Of the energy released in the partial oxidation of glucose to pyruvate, 61/2840 = 2.1% is
captured as ATP. If O2 is available and can be used, another 438.5/2840 = 15.4% is
potentially obtainable from re-oxidation of NADH, and an additional 2255.5/2840 = 79.4%
is potentially obtainable from the complete oxidation of pyruvate.!
Garrett and Grisham, Figure 18-31!
Reoxidation of NADH is critical!
Glucose + 2 ADP + 2 Pi + 2 NAD+ → !
2 Pyruvate + 2 ATP + 2 H2O + 2 NADH + 2 H+!
ΔG'º = –82 kJ/mol!
Compared to the amount of glucose that is converted
to pyruvate, there are tiny amounts of NAD+ in cells.!
!
Glycolysis would quickly grind to a halt unless NADH
is re-oxidized to NAD+.!
!
Class 8: Outline and Objectives
l  Redox reactions !
l Oxidation states of carbon!
l Relationship between ∆G and ∆E!
l  Electron carriers NADH, NADPH, FAD, FMN!
l Where does the energy come from?!
l  Overview metabolism, catabolism, anabolism!
l  Glycolysis!
l Pathway!
l Regulation!
l Substrate channeling!
l  Fermentation!
l Lactate!
l Ethanol!
l  Pentose phosphate pathway!
l  Fructose and galactose metabolism !
16!
Fates of Pyruvate: Aerobic and anaerobic pathways
Lactic acid and alcohol fermentation
Both processes regenerate NAD+ to allow continued glycolysis.!
muscle,!
microorganisms!
muscle,!
microorganisms!
yeast!
Fermentation and the Pasteur Effect
In the absence of oxygen, the ATP generated during glycolysis is the sole energy!
derived from partial glucose oxidation to pyruvate.!
yeast!
Major pathways of glucose utilization
Problem that needs to be solved: !
NAD+ has to be regenerated to allow continued glycolysis.!
Solution: !
Reduce pyruvate, e.g. to lactate or ethanol,!
using the NADH that was generated during!
glycolysis. !
This does not require or generate energy (ATP).
Fermentation does not capture very much of the energy that is potentially available
from the complete oxidation of glucose. !
Energy available from complete glucose oxidation: –2840 kJ/mol!
Energy required to form two mols of ATP: 2 x 30.5 = 61 kJ/mol of glucose (2.1 %).!
!
In organisms that can ferment in the absence of O2 and respire in the presence of
O2, the rate of anaerobic glucose consumption is much higher (Pasteur effect in
yeast).!
and NADPH!
Figure 14-1!
17!
Pentose phosphate pathway
Pentose phosphate pathway - Overview!
Figure 14-21!
Figure 14-22!
Recycling of pentose phosphates in !
nonoxidative reactions
Recycling of pentose phosphates in !
nonoxidative reactions
(6)!
6!
2!
2!
6 → 5!
(2)!
(4)!
4!
(2)!
2!
(3)!
(2)!
2!
3!
(2)!
(2)!
(1)!
(2)!
2!
2!
Figure 14-23a!
Figure 14-23b!
18!
Feeder pathways (convergence)
Fructose catabolism
liver!
muscle, kidney!
glycerol!
triacylglycerides!
glycerol-3-phosphate!
phospholipids!
Figure 14-11!
①
Galactose catabolism
1. Galactokinase phosphorylates galactose!
to form galactose-1-P!
④
Phosphoglucomutase!
②
③
NADH!
NAD+!
Glucose 6-phosphate!
2. Galactose-1-P is exchanged for glucose-1P of UDP-glucose (a sugar-nucleotide).
UDP (uridine diphosphate) functions as a
coenzyme-like carrier of hexose groups!
3. UDP-galactose is isomerized to UDPglucose by an epimerase that contains a
bound NAD+. NADH and the 4-keto-UDPsugar are enzyme-bound intermediates!
4. Glucose-1-P is isomerized to glucose-6-P
Figure 14-13!
19!