Module 7: Carbohydrate Metabolism Flashcards
Carbohydrates storage
Carbohydrates not required immediately for energy are stored as glycogen in our muscle and liver
Once glycogen stores are filled, excess carbohydrates are converted to fat for storage
Glycogen
highly branched structure with a single reducing end and several non- reducing ends
alpha-1,4 bonds link the glucose subunits linearly, while the alpha- 1,6 bonds form the branch points
Glycogen phosphorylase
catalyzes the phosphorolysis of glycogen with the addition of Pi to form glucose-1-phosphate, which can be converted by phosphoglucomutase to glucose-6-phosphate
Glycogen synthase
catalyzes the synthesis of glycogen from glucose-1-phosphate, using UTP
What causes Glycogen synthase to be activated?
activated by high concentrations of glucose-6-phosphate (signals plenty of carbohydrate available)
What causes Glycogen phosphorylase to be activated/inhibited?
activated by high concentrations of AMP
(signals energy status of the cell is quite low)
inhibited by high concentrations of ATP
(signals ample energy supply)
Glycogen synthase (phosphorylation/dephosphorylation)
Phosphorylation by kinase =inactive form
Removal of the phosphate group by phosphatase=active form
Glycogen phosphorylase
phosphorylation/dephosphorylation
Phosphorylation by kinase =active form
Removal of the phosphate group by phosphatase=inactive form
Which of these hormones: insulin, glucagon, epinephrine, lead to phosphorylation/dephosphorylation?
Insulin leads to dephosphorylation
Glucagon leads to phosphorylation
Epinephrine leads to phosphorylation
When is insulin release and what enzyme is activated/inactivated?
released when blood glucose levels elevated
activate glycogen synthase (abundance of glucose that can be used to replenish the glycogen stores)
inactivate glycogen phosphorylase (glucose can be used for energy)
When is glucagon release and what enzyme is activated/inactivated?
released when blood glucose levels drop
inactivates glycogen synthase
activates glycogen phosphorylase
Glycolysis
occurs in cytosol
one 6-carbon glucose molecule is split into two three-carbon pyruvate molecules, in addition to 2 ATP molecules and 2 NADH molecules
Pyruvate further oxidized in the mitochondria
NADH further oxidized by ETC inside the mitochondria
What is special about rxns 1 & 3 of glycolysis?
Enzymes that catalyze reactions 1 and 3 are targets for the control of flux through glycolysis –have a large change in free energy and therefore irreversible
Step 1 of glycolysis
regulations
catalyzed by hexokinase
inhibited by its product, glucose-6- phosphate (signal enough product to proceed)
Step 3 of glycolysis
regulations
catalyzed by phosphofructokinase
inhibited by high concentrations of ATP (signal adequate energy levels) and citrate (signals enough substrate to proceed)
stimulated by high concentrations of AMP and ADP (signal low energy)
and fructose-2,6-bisphosphate
Fermentation
pathway that produces ATP with no net oxidation of carbon (no O2, no ETC to regenerate NAD+)
Strategy 1: Formation of Lactate
employed by microorganisms and some eukaryotic cells
enzyme lactate dehydrogenase oxidizes NADH, reducing pyruvate to lactate, and regenerating NAD+
Glucose+2ADP+2Pi–>2Lactate +2ATP
Strategy 2: Formation of Ethanol
occur in yeast
ethanol is produced rather than lactate
Glucose+2ADP+2Pi–>2Ethanol+2CO2 +2ATP
Gluconeogenesis
synthesis of glucose that occurs when the body’s stores of glycogen are low
occurs in liver (and kidneys)
Precursors such as some amino acids, some citric acid cycle intermediates and lactate via pyruvate can be converted to oxaloacetate–>converted to glucose
2Pyruvate+4ATP+2GTP+2NADH–>
Glucose+4ADP+2GDP+6Pi+2NAD+
Given the large changes in free energy that we saw in glycolysis, how can the reverse pathway of gluconeogenesis proceed?
Gluconeogenesis is not exactly the reverse of glycolysis
Steps 1, 3 and 10, catalyzed by glucokinase, phosphofructokinase and pyruvate kinase must be by-passed as these reactions are irreversible
By-passed by glucose-6- phosphatase, fructose bisphosphatase, PEP carboxykinase and pyruvate carboxylase
What inhibits fructose bisphosphatase?
inhibited by high concentrations of AMP and fructose-2,6-bisphosphate
When is gluconeogenesis/glycolysis stimulated?
when the energy state of the cell is high (high ATP, low AMP), gluconeogenesis is stimulated
when the energy state of the cell is low (lowATP, high ADP and AMP), glycolysis is stimulated
Pentose Phophate Pathway
alternate pathway by which glucose is broken down to generate NADPH
also provides ribose-5-phopshate for the biosynthesis of nucleotides
3Glucose-6-phosphate+6NADP+–>
2Fructose-6-phosphate (glycolysis)+
glyceraldehyde-3-phosphate (glycolysis)+
3CO2 (waste)+6NADPH (reducing power)
