Glucose Oxidation
major Pathway
I. Glycolysis
Definition:
Glycolysis means oxidation of glucose to give:
• Pyruvate (in the presence of oxygen) or,
• Lactate (in the absence of oxygen)
Site:Cytoplasm of all tissue cells, but it is of physiological importance
in: 1. Tissues with no mitochondria: mature RBCs, cornea and
lens. 2. Tissues with few mitochondria: Testis, leucocytes, medulla
of the kidney, retina, skin and gastrointestinal tract. 3. Tissues undergo frequent oxygen lack: skeletal muscles
especially during exercise.
Glycolysis
What are the possible fates of glucose?
Glycolysis
All the intermediates in glycolysis have either 3 or 6 carbon atoms
All of the reactions fall into one of 5 categories phosphoryl transfer phosphoryl shift isomerization dehydration aldol cleavage
Glycolysis
Entire reaction sequence may be divided into three stages glucose is trapped and destabilized six carbon molecule is split into two three carbon
molecules ATP is generated
Glycolysis – Stage 1
glucose converted to glucose-6-PO4ATP is neededcatalyzed by hexokinase or glucokinase
Glycolysis – Stage 1
phosphoglucoisomerasealdose is converted to ketose
Glycolysis – Stage 1
rate limiting enzyme – phosphofructokinase inhibited by high ATP, citric acid, long-chain fatty
acids stimulated by ADP or AMP
Glycolysis
Glycolysis – Stage 2
six carbon molecule split into 2- 3 carbon molecules aldose and ketose
Glycolysis – Stage 3
Glycolysis – Stage 3
redox reactionenergy from redox used to form acyl
phosphate
Glycolysis – Stage 3
Consists of two coupled processes
Glycolysis – Stage 3
formation of ATP – substrate level phosphorylation
Glycolysis – Stage 3
phosphoryl shift – uses 2,3 bisphosphoglycerate
Glycolysis
• In the energy investment phase, ATP provides activation energy by phosphorylating glucose.– This requires 2 ATP per glucose.
• In the energy payoff phase, ATP is produced by substrate-level phosphorylation and NAD+ is reduced to NADH.
• 2 ATP (net) and 2 NADH are produced per glucose.
Energy Investment Phase
Energy-Payoff Phase
Fate of Pyruvate
Alcoholic FermentationWhich organisms carry out this process?
yeast other microorganisms
PDC requires thiamine pyrophosphate as coenzyme
NAD+ is regenerated
Lactic Acid Fermentation
Occurs in muscle cells, microorganismsRegenerates NAD+
Substrate level phosphorylation:
• This means phosphorylation of ADP to ATP at the reaction itself .
• In glycolysis, there are 2 examples: - 1.3 Bisphosphoglycerate + ADP 3
Phosphoglycerate + ATP - Phospho-enol pyruvate + ADP Enolpyruvate +
ATP.
Special features of glycolysis in RBCs
1. Mature RBCs contain no mitochondria, thus:
a) They depend only upon glycolysis for energy production (=2 ATP).
b) Lactate is always the end product.
2. Glucose uptake by RBCs is independent on insulin hormone.
3. Reduction of met-hemoglobin: Glycolysis produces NADH+H+, which
used for reduction of met-hemoglobin in red cells
Biological importance (functions) of glycolysis:
• Energy production: a) anaerobic glycolysis gives 2 ATP. b) aerobic glycolysis gives 8 ATP.• Provides important intermediates: a) Dihydroxyacetone phosphate: can give glycerol-3phosphate,
which is used for synthesis of triacylglycerols and phospholipids (lipogenesis).
b) 3 Phosphoglycerate: which can be used for synthesis of amino acid serine.
c) Pyruvate: which can be used in synthesis of amino acid alanine.
• Aerobic glycolysis provides the mitochondria with pyruvate, which gives acetyl CoA Krebs' cycle.
Glycolysis
How can fructose be used for energy?
Glycolysis
To use galactose it must be converted to glucose-6-PO4
Glycolysis
Glycolysis
What causes lactose intolerance?
Reversibility of glycolysis (Gluconeoqenesis): 1. Reversible reaction means that the same enzyme can catalyzes the reaction in both directions. 2. all reactions of glycolysis -except 3- are reversible. 3. The 3 irreversible reactions (those catalyzed by kinase enzymes) can be reversed by using other enzymes.
Glucose-6-p Glucose F1, 6 Bisphosphate Fructose-6-pPyruvate Phosphoenol pyruvate
4. During fasting, glycolysis is reversed for synthesis of glucose from non- carbohydrate sources e.g. lactate. This mechanism is called: gluconeogenesis.
Importance of lactate production in anerobic glycolysis:
1. In absence of oxygen, lactate is the end product of glycolysis:
2. In absence of oxygen, NADH + H+ is not oxidized by the respiratory chain.3. The conversion of pyruvate to lactate is the mechanism for regeneration of NAD+. 4. This helps continuity of glycolysis, as the generated NAD+ will be
used once more for oxidation of another glucose molecule.
Glucose Pyruvate Lactate
What is galactosemia?
inability to metabolize galactose missing galactose 1-phosphate uridyl transferase
liver disease development of cataracts CNS malfunction
Control of Glycolysis
Of what value is glycolysis for cells? provides energy in form of ATP provides building blocks for synthetic reactions
Where are most control points found? enzymes that catalyze irreversible reactions
hexokinase phosphofructokinase pyruvate kinase
Phosphofructokinase
Most important control point in mammalian glycolytic pathway allosteric enzyme
activated by AMP and fructose 2,6 bisphosphate inhibited by high levels of ATP, citrate, fatty acids
Hexokinase
Hexokinase is inhibited by its product glucose-6-PO4 glucose remains in blood
Glucokinase, an isozyme of hexokinase is not inhibited by glucose-6-PO4 found in liver has lower affinity for glucose
Pyruvate Kinase
Pyruvate kinase exists as isozymes L form – predominates in liver M form – mostly in muscle and brain
PK is an allosteric enzyme activated by fructose 1,6 bisphosphate inhibited by ATP, alanine
L form of PK influenced by covalent modification inhibited by phosphorylation
Pyruvate Kinase
Gluconeogenesis
What is gluconeogenesis? synthesis of glucose from non-carbohydrate
precursorsWhy is this an important pathway?What are some of the major precursors?
lactate, amino acids, glycerolWhere does this process occur?
liver, kidney
Gluconeogenesis
If gluconeogenesis involves the conversion of pyruvate to glucose why is it not simply the reverse of glycolysis? glycolysis contains several irreversible reactions
Which reactions in glycolysis are irreversible? phosphoenolpyruvate to pyruvate fructose 6-phosphate to fructose 1,6-
bisphosphate glucose to glucose 6-phosphate
Gluconeogenesis
CH3 CCO2-
O
CH2 CCO2-
O
CO2-
+ CO2 + ATP
+ ADP + Pi
Pyruvate
Oxaloacetate
biotinpyruvate
carboxylas e
Gluconeogenesis
Pyruvate carboxylase is an allosteric enzyme activated by acetyl
CoA needed to form
carboxybiotin
Gluconeogenesis
Carboxylation of pyruvate occurs in the mitocondria but next step in reaction sequence occurs in cytosol
Gluconeogenesis
Decarboxylation of oxaloacetate is coupled with
phosphorylation by GTPenzyme is phosphoenolpyruvate
carboxykinase
CH2 = CCO2-
OPO32 -
CH2 CCO2-
O
CO2-
+ CO2
+ GTP
Phos phoenol pyruvate
Oxaloacetate + GDP
Gluconeogenesis
Which other steps in glycolysis are irreversible? conversion of fructose 1,6-bisphosphate to fructose 6-
phosphate conversion of glucose 6-phosphate to glucose
Gluconeogenesis
Fructose-6-phosphate
C
CH2 OP O32 -
OHHOOHHOHH
CH2 OH
Fructose-1 ,6-bisphosphate
C
CH2 OP O32 -
OHHOOHHOHH
CH2 OP O32 -
fructose-1 ,6 -bis-phosphatase
H2 O P i
G° = -16.7 kJ mol-1
fructose-1,6-bisphosphatase is an allosteric enzyme, inhibited by AMP and activated by ATP
Gluconeogenesis
Enzyme that catalyzes last reaction not found in all tissues liver and kidney cortex
Gluconeogenesis
Is gluconeogenesis an energetically favorable reaction in the cell?
What drives this reaction?
Are glycolysis and gluconeogenesis active at the same time?
Regulation of Glycolysis and Gluconeogenesis
What are some of the factors that ensure the reciprocal regulation of these processes? allosteric regulators of key enzymes energy charge fructose 2,6-bisphosphate hormones
Regulation of Glycolysis and Gluconeogenesis
Regulation of Glycolysis and Gluconeogenesis
fructose 2,6-bisphosphate stimulates PFK and inhibits fructose 1,6-bisphosphase controlled by insulin and glucagon and reflects the
nutritional status of the cell
Regulation of Glycolysis and Gluconeogenesis
How do hormones influence the enzymes associated with these processes? influence gene expression
change transcription rate influence degradation of m-RNA
insulin PFK, PK glucagon PEPCK, fructose 1,6-bisphosphatase
Regulation of Glycolysis and Gluconeogenesis
What are substrate cycles and why are they important? can amplify
metabolic signals can generate heat
Regulation of Glycolysis and Gluconeogenesis
What is the Cori cycle and why is it important?
Regulation of Glycolysis and Gluconeogenesis