iii. metabolism glucose catabolism – part...
TRANSCRIPT
Biochemistry 3300 Slide 1
III. Metabolism
Glucose Catabolism – Part II
Department of Chemistry and BiochemistryUniversity of Lethbridge
Biochemistry 3300
Biochemistry 3300 Slide 2
Metabolic Fates of NADH and Pyruvate
Cartoon:
Fate of pyruvate, theproduct of glycolysis.
+O2 TCA cycle
-O2
Fermentation
Biochemistry 3300 Slide 3
Metabolic Fates of NADH and Pyruvate
Pyruvate is a central branch point inMetabolism.
Aerobic pathway:Citric acid cycle and then respiration;- yields far more energy (discussed later) than glycolysis
- relatively slow (limited by O2 transport)
NADH + O2 → NAD+ + Energy
Pyruvate + O2 → 3 CO2 + Energy
Biochemistry 3300 Slide 4
Metabolic Fates of NADH and Pyruvate
Pyruvate is a central branch point inMetabolism.
Two anaerobic pathways:-Pyruvate is converted to lactate via lactate dehydrogenase (ie. muscle cells)
-Pyruvate is converted to ethanol via ethanol dehydrogenase (ie. yeast)
Both pathways use the NADH (produced in glycolysis): Overall: Glucose → 2 lactate + 2 ATP
Anaerobic pyruvate utilization = Fermentation
Biochemistry 3300 Slide 5
Lactate Fermentation
Pyruvate + NADH + + H+ L-Lactate + NAD+
Enzyme = Lactate Dehydrogenase
Regenerates NAD+ from NADH (reducing equivalents) produced in glycolysis.Essential as NAD+ is required for glycolysis (step 6 -GAPDH)
Lactate fermentation is important in red blood cells, parts of the retina and in skeletal muscle cells during extreme high activity.
Also important in plants and microbes growing in absence of O2.
∆G’° = -25.1 kJ/mol
Biochemistry 3300 Slide 6
Lactate Dehydrogenase (LDH)
In mammals two different types of LDH subunits are found: the M type and the H type.
Five forms of the tetrameric isozymes are possible:
M4, M3H1, M2H2, M1H3, H4
H-type predominates aerobic tissues (ie. heart muscle) H4 LDH has a low KM for pyruvate and is allosterically inhibited by it.
M-type predominates in tissue subject to anaerobic conditions (ie. liver and skeletal muscle) M4 LDH has a low KM for pyruvate and is NOT allosterically inhibited by it.
Biochemistry 3300 Slide 7
Lactate Dehydrogenase (LDH)
NADH
LDH monomer
NADH shown as sticksCatalytic site circled
Redox reaction involvingelectron transfer fromNADH to pyruvate.
Biochemistry 3300 Slide 8
Reaction Mechanism of Lactate Dehydrogenase
Biochemistry 3300 Slide 9
Pyruvate: Terminal Electron Acceptorof Lactic Acid Fermentation
Corey Cycle:
Most lactate is exported from the muscle cell via the blood to the liver↓
Liver converts lactate (back) to glucose↓
Glucose is transported from liver cells via the blood to the muscle(stored as glycogen)
The process of transporting lactate to the liver and its conversion to glucose takes from hours to days to complete.
Fate of Lactate (from fermentation)
Biochemistry 3300 Slide 10
Alcoholic Fermentation
Two enzymes involved: Pyruvate decarboxylase irreversible
Alcohol dehydrogenase reversible
Pathway is active in yeast
Regenerates NAD+ from NADH (reducing equivalents) produced in glycolysis.
Second step is reversible Ethanol can be further metabolised via oxidation that ultimately produces acetate and enters fat biosynthesis pathways
Biochemistry 3300 Slide 11
Pyruvate Decarboxylase(Alcohol Fermentation)
Yeast produces CO2 and ethanol in two consecutive reactions
Decarboxylation of pyruvate to acetaldehyde is catalyzed bypyruvate decarboxylase (PDC) (not present in animals).
PDC contains a tightly non covalently bound coenzyme:
Thiamin pyrophosphate (TPP)
Catalytically active
Biochemistry 3300 Slide 12
TPP Cofactor (Pyruvate Decarboxylase)
Decarboxylation of α-keto acidsbuilds up negative charge on the carbonyl carbon.
Transition state is stabilized bydelocalization of the developingneg. charge into a “electron sink”.
The dipolar carbanion (ylid) is theactive form
Biochemistry 3300 Slide 13
TPP Cofactor (Pyruvate Decarboxylase)
Thiamine Pryophosphate(TPP)
TPP of Pyruvate Decarboxylase:
Two views related by 90º rotationabout a vertical axis
Biochemistry 3300 Slide 14
TPP Cofactor (Pyruvate Decarboxylase)
How is TPP deprotonated toits the ylid form?
1) TTP’s aminopyradine ring (subunit 1) is deprotonated by Glu51 (subunit 2) of the PDC dimer. 2) amine of aminopyradine deprotonates thiazolium ring producing ylid form TPP
Note: PDC is a dimer of dimers.Note: TPP ylid form circled in red
Biochemistry 3300 Slide 15
TPP Cofactor (Pyruvate Decarboxylase)
Glu51
Thiamine Pyrophosphate(TPP)
Biochemistry 3300 Slide 16
Thiamine Deficiency
TPP addition to carbonyl groups and its ability to act as an“electron sink”(electron withdrawl) makes it the coenzyme most utilized in α-keto acid decarboxylations.
Thiamin (vitamin B1) is not synthesized or stored in significant amounts by vertebrates. Deficiency in humans results in an ultimately fatal condition known as beriberi.
Biochemistry 3300 Slide 17
Alcoholic Fermentation (step II)
Reduction of acetaldehyde to ethanol and regeneration of NAD+
by alcohol dehydrogenase (ADH)
Each subunit of the tetrameric yeast ADH binds one NADH and one Zn2+.
Biochemistry 3300 Slide 18
Alcoholic Fermentation Part II
Zn2+ polarises the carbonyl oxygenof acetaldehyde
Hydride ion is transferred fromNADH to the carbonyl carbon
Reduced intermediate acquires aproton from the medium to formethanol.
Biochemistry 3300 Slide 19
Glycolysis: Substrates other than glucose
Glycogen / Starch
Dietary Polysaccharides Maltose (Glu-Glu) Lactose (Glu-Gal) Sucrose (Glu-Fru)
Biochemistry 3300 Slide 20
Feeder Pathways for Glycolysis
Glycogen metabolism
Glycogen storage granulesin liver
Enzymes of 'feeder pathways'are underlined in red
Biochemistry 3300 Slide 21
Phosphorolysis:glycogen / starch degradation
Glycogen phosphorylase / Starch phosphorylase
- attack of Pi on the (α1→4) glycosidic linkage of the last two glucose residues.
Phosphorolysis generatesG1P which must be converted to G6P (phosphoglucomutase)to enter glycolysis
Biochemistry 3300 Slide 22
Phosphorolysis: glycogen / starch degradation
Phosphorylase - repetitively breaks (α1→4) linkages until it reaches an (α1→6)- produces glucose-1- phosphate
Debranching enzyme - required to break (α1→6) linkages - produces glucose
Biochemistry 3300 Slide 23
Phosphoglucomutase mechanism
Glucose 1-phosphatehas to be converted into glucose 6-phosphateto enter glcolysis
Where have we previouslyseen this type of mechanism?
Biochemistry 3300 Slide 24
Phosphoglycerate Mutase – Reaction 8
Similar mechanism tophosphoglycerate mutase(glycolysis)- different catalytic residue
Biochemistry 3300 Slide 25
Complication! – The Liver
Glycogen is primarily stored in the liver and is used to maintain bloodglucose levels between meals
But … neither G1P nor G6P can be transported out of liver cells
Require separate pathway (below) to convert G6P to glucose for transport
Biochemistry 3300 Slide 26
Dietary Polysaccharides
Dextrin + n H20 → n D-glucose Dextrinase
Maltose + H20 → 2 D-glucose Maltase
Lactose + H20 → D-galactose + D-glucose Lactase
Sucrose + H20 → D-fructose + D-glucose Sucrase
Di- and polysaccharides are converted to monosaccharides, then funneled into the glycolytic sequence
Biochemistry 3300 Slide 27
Fructose entry into Glycolysis
Two routes for fructose entryinto glycolysis - tissue specific
Biochemistry 3300 Slide 28
Fructose entry into Glycolysis
Non-LiverD-Fructose is phosphorylated by hexokinase and F6P enters glycolysis:
Fructose + ATP → fructose 6-phosphate + ADPMg2+
Liver D-Fructose phosphorylated by fructokinase (at C1):
Fructose + ATP → fructose 1-phosphate + ADPMg2+
Fructose 1-phosphate is then cleaved to glyceraldehyde and dihydroxyacetone phosphate (DHAP) by fructose 1-phosphate aldolase.
Glyceraldehyde is phosphorylated by triose kinase and ATP to glyceraldehyde-3-phosphate.DHAP and glyceraldehyde-3-phosphate
Are both glycolytic intermediates
Biochemistry 3300 Slide 29
Galactose entry into the Glycolysis
Galactose entry into glycolysis ismore complex than for other dietarysugars
Biochemistry 3300 Slide 30
Galactose conversion to Glucose-1-phosphate
Metabolism of Galactose involvesthree enzymes and a sugar nucleotide.
C1 carbon is activatedas phosphate ester
Textbook (3rd Edition) has typo that is corrected here
Glycolysis
Biochemistry 3300 Slide 31
Galactose conversion to glucose-1-phosphate
Activation of C1 phosphatevia formation of phosphateester with UDP
→ glycolysis
UDP-glucose + Galactose-1-phosphate↓
UDP-galactose + Glucose-1-phosphate
Must regenerate UDP-glucose tocontinue cycle
Biochemistry 3300 Slide 32
Conversion of UDP-galactose to UDP-glucose
Textbook (3rd Edition) has typo that is corrected here