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Carbohydrate Catabolism IChapter 14 and parts of 15
March 5, 2015
BC368Biochemistry of the Cell II
BC368Biochemistry of the Cell II
FeederPathways
Fig 14-9
glycerol
Glycerol 3-P
All carbohydrates enter glycolysis
In muscle, often via hexokinase
Case Study
A 9-month-old is brought to your clinic with recurrent bouts of sweating and vomiting. Symptoms began shortly after weaning and introduction to solid foods. Testing reveals hypoglycemia and lactic acidosis after consumption of milk formula or fruit. Enzyme activity testing reveals a deficiency in fructose 1-phosphate aldolase.
Notably, her 3-year-old brother has a marked aversion to fruit.
Fructose intolerance
Hereditary fructose intolerance results from a defect in fructose breakdown in the liver, usually in aldolase.
GlycogenPhosphorylase
Glycogen phoshorylase catalyzes the simultaneous phosphorylation and cleavage of an -1,4 linked glucose from a non-reducing end of glycogen.
This reaction is called “phosphorolysis.”
GlycogenBreakdown
G6P fate depends on tissue.
In muscle, G6P proceeds through glycolysis.
In liver, G6P is converted to glucose.
• Lactic acid from skeletal muscle is sent into the bloodstream.
• Lactate threshold occurs when production exceeds clearance. Glycolysis cannot continue.
Fermentation in Animals
Irreversible steps are regulated:
Hexokinase/Glucokinase
Phosphofructokinase I
Pyruvate Kinase
Regulation of glycolysis
ATP is an allosteric inhibitor of PFK-1.
Two binding sites: substrate and allosteric site.
Control of PFK-1
Control of glycogen phosphorylase
phosphorylase b (inactive)
phosphorylase a (active)
phosphorylation
glycogen breakdown
Phosphorylase kinase is activated upon phosphorylation by protein kinase A (PKA).
Glycogen phosphorylase is activated upon phosphorylation by phosphorylase kinase.
PKA is activated by cyclic AMP, which is produced by a G-protein in response to epinephrine/glucagon.
Phosphorylase kinase is activated upon phosphorylation by protein kinase A (PKA).
Glycogen phosphorylase is activated upon phosphorylation by phosphorylase kinase.
Transketolase requires thiamine pyrophospate (TPP) as a coenzyme
NADPH is necessary to protect against reactive oxygen species
Ribose 5-P is necessary in rapidly dividing cells
•Rxns 1 and 3 produce NADPH
•Rxn 4 produces ribose-5- phosphate
Glucose 6-P + 2 NADP+ + H2O Ribose 5-P + 2 NADPH + 2 H+ + CO2
Oxidative phase
From C1
Case StudyOmar’s mother noticed that every time she served falafel, her son complained of feeling tired, hot, headachy, and sick to his stomach. At first she thought he was just being fussy, but sometimes he would actually look yellow. Medical testing confirmed hemolytic anemia. What’s up with Omar?
A deficiency in G6PDH is the most common human enzyme defect, affecting more than 400 million people worldwide. Protective against malaria.
Divicine leads to reactive oxygen species
Favism!
Case StudyOmar’s mother noticed that every time she served falafel, her son complained of feeling tired, hot, headachy, and sick to his stomach. At first she thought he was just being fussy, but sometimes he would actually look yellow. Medical testing confirmed hemolytic anemia. What’s up with Omar?
X
Glucose 6-P + 2 NADP+ + H2O Ribose 5-P + 2 NADPH + 2 H+ + CO2
Oxidative Phase
Some cells need NADPH but not ribose 5-P
Ribose 5-P can be recycled in the nonoxidative phase