Module 10: Carbohydrate Metabolism (Part 01) Flashcards
This is the process of breakdown of food molecules by hydrolysis into simpler chemical units that can be used by cells in their metabolic needs.
Digestion
Carbohydrate digestion begins where?
Mouth
Which enzyme catalyzes the hydrolysis of alpha-glycosidic linkages of starch and glycogen to produce smaller polysaccharides and disaccharide (maltose)?
Salivary “Alpha-amylase”
What does Salivary “Alpha-amylase” catalyze?
hydrolysis of alpha-glycosidic linkages of starch and glycogen
How much carbohydrate digestion occurs in the mouth?
Only a small amount of carbohydrate digestion occurs in the mouth because food is swallowed so quickly into the stomach.
How much carbohydrate digestion occurs in the stomach?
In the stomach very little carbohydrate is digested because no carbohydrate digestion enzymes present in stomach
What happens to salivary amylase in the stomach?
Salivary amylase gets inactivated because of stomach acidity
The primary site for the carbohydrate digestion is within what?
the small intestine
This breaks down polysaccharide chains into disaccharide – maltose
Pancreatic alpha-amylase
The final step in carbohydrate digestion occurs where?
outer membranes of intestinal mucosal cells
These enzymes in the intestinal mucosa convert disaccharides (maltose, sucrose and lactose) to monosaccharides (glucose, fructose and galactose)
Disaccharidase enzymes
This enzyme converts maltose to glucose.
Maltase
This enzyme converts sucrose to glucose and fructose
Sucrase
This enzyme converts lactose glucose and galactose
Lactase
What are the three (3) carbohydrate digestion products?
(1) Glucose
(2) Galactose
(3) Fructose
These are are rich in blood capillaries into which the monosaccharides are actively transported and absorbed into the bloodstream to the intestinal wall.
Intestinal villi
These mediate the passage of the monosaccharides through cell membranes.
ATP Hydrolysis and Protein Carriers
These are converted to products of glucose metabolism in the liver.
Galactose and Fructose
This is an energy-consuming stage
Six-carbon stage of glycolysis
What happens in steps one to three in glycolysis (six carbon stage)?
Phosphate derivatives glucose and fructose are formed via a ATP coupling reactions.
What happens in step 01?
Phosphorylation of glucose - phosphate group from ATP is transferred to the hydroxyl group on carbon 6 of glucose.
What enzyme catalyzes the formation of glucose-6-phosphate?
Hexokinase
What kind of reaction is formation of glucose-6-phosphate?
Endothermic reaction where energy (ATP) is formed from ATP hydrolysis
What happens in step 02?
Glucose 6 phosphate is isomerized to Fructose -6-Phosphate.
What enzyme catalyzes the Formation of Fructose-6-phosphate?
Phosphoglucoisomerase
What happens in step 03?
Further phosphorylation of Fructose-6-bisphosphate to engender the formation of Fructose 1,6-bisphosphate
What enzyme catalyzes the reaction for the formation of Fructose 1,6-bisphosphate?
Phosphofructokinase
What kind of reaction is formation of fructose 1-6-phosphate?
Endothermic reaction where energy (ATP) is formed from ATP hydrolysis
In this, the Reaction intermediates are derivatives of glycerol and acetone
Three-Carbon Stage of Glycolysis (Steps 4-10)
All reaction intermediates are phosphorylated derivatives of what?
(1) dihydroxyacetone,
(2) glyceraldehyde,
(3) glycerate, or
(4) pyruvate
What happens in step 04?
C6 species is split into two C3 species. Two C3 species formed are dihydroxyacetone phosphate and glyceraldehyde 3-phosphate (formation of triose phosphates)
What enzyme catalyzes the formation of triose phosphates?
Aldolase
What happens in step 05?
Dihydroxyacetone phosphate is isomerized to glyceraldehyde 3-phosphate
What enzyme is responsible for the isomerization of Dihydroxyacetone phosphate is into glyceraldehyde 3-phosphate
Triosephosphate isomerase
What happens in step 07?
Diphosphate from step 6 is converted back to monophosphate species. It is an ATP producing step (C1 high energy phosphate group of 1,3-bisphosphoglycerate is transferred to an ADP molecule to form an ATP)
What enzyme is responsible for the Formation of 3-Phosphoglycerate?
phosphoglycerokinase
What happens in step 08?
Isomerization of 3-phosphoglycerate to 2-phosphoglycerate (Phosphate group moved from C-3 to C-2)
Which enzyme is responsible for the Formation of 2-phosphoglycerate?
Phosphoglyceromutase
What enzyme is responsible for the Formation of Phosphoenolpyruvate?
Enolase
What happens in step 09?
The formation of phosphoenolpyruvate (This is an alcohol dehydration reaction – results in another high energy phosphate group containing compound)
Which enzyme is responsible for the formation of Pyruvate?
Pyruvate kinase
What happens in step 10?
High energy phosphate is transferred from phosphoenolpyruvate to ADP molecule to produce ATP and pyruvate
How many ATP molecules are produced for each original glucose molecule?
Two ATP molecules are produced for each original glucose molecule
What steps are the control points of glycolysis?
Steps 1,3 and 10 are control points for glycolysis
How many net gain of ATP molecules are there for every glucose molecule processed?
There is a net gain of two ATP molecules in glycolysis for every glucose molecule processed
These are converted in liver to intermediates that enter into the glycolysis pathway
Fructose and Galactose
Entry of fructose into the glycolytic pathway involves:
(1) Phosphorylation by ATP to produce fructose 1-phosphate
(2) Fructose 1-phosphate is converted to two trioses, namely Glyceraldehyde and Dihydroxyacetone.
This is phosphorylated to enter into glycolysis.
Glyceraldehyde
This enters into glycolysis directly.
Dihydroxyacetone phosphate
What does the entry of galactose need?
The entry of galactose into glycolysis also needs phosphorylation by ATP to produce glucose 1-phosphate and is isomerized to glucose 6-phosphate
Hexokinase inhibited by what?
glucose 6-phosphate (feedback inhibition)
How is glycolysis regulated?
Step 1- Conversion of glucose to glucose 6-phosphate by hexokinase
Step 3: Fructose 6-phosphate converted to fructose 1,6-bisphosphate by phosphofructokinase
Step 10: Conversion of phosphoenolpyruvate to pyruvate by Pyruvate kinase
What inhibits phosphofructokinase?
High concentrations of ATP and citrate inhibit phosphofructokinase
What inhibits pyruvate kinase?
High concentrations of ATP
Both of these enzymes in glycolysis are allosteric enzymes.
Both pyruvate kinase (Step 10) and phosphofructokinase (Step 3) are allosteric enzymes.
Under aerobic (oxygen-rich) conditions, pyruvate is oxidized to what?
acetyl CoA by pyruvate dehydrogenase complex
What happens to acetyl CoA after it is oxidized from pyruvate?
Acetyl CoA thus formed enters the mitochondrial matrix for further processing through the citric acid cycle. Most pyruvate formed during glycolysis is converted to Acetyl CoA.
An enzymatic anaerobic reduction of pyruvate to lactate occurs where?
Mainly Muscles
What is the purpose of lactate fermentation?
Conversion of NADH to NAD+ for increased rate of glycolysis
When is lactate converted back to pyruvate?
Lactate is converted back to pyruvate when aerobic conditions are reestablished in the cell.
Muscle fatigue associated with strenuous physical activity is attributed to what
increased build-up of lactate
This is the enzymatic anaerobic conversion of pyruvate to ethanol and carbon dioxide
Ethanol Fermentation
What are the two (2) Reactions under ethanol fermentation?
(1) Pyruvate decarboxylation by pyruvate decarboxylase
(2) Acetaldehyde reduction to ethanol by alcohol dehydrogenase
How is ethanol fermentation applied?
Ethanol fermentation involving yeast causes bread and related products to rise as a result of CO2 bubbles being released during baking along with producing beer, wine and alcoholic drinks
This shuttles electrons from NADH, but not NADH itself, across the membrane
Glycerol 3-phosphate-dihydroxyacetone phosphate transport system
Why can’t NADH produced from step 6 of glycolysis participate in electron transport chain?
because mitochondria are impermeable to NADH and NAD+
These freely cross the mitochondrial membrane
Dihydroxyacetone phosphate and glycerol phosphate
This shuttles the electrons from NADH to FADH2
interconversion
How many ATP molecules are produced in muscle and nerve cells
30 ATP Molecules
(1) 26 from oxidative phosphorylation of electron transport chain
(2) 2 from oxidation of glucose to pyruvate
(3) 2 from conversion of GTP to ATP
This is 15 times more efficient in the ATP production as compared to anaerobic lactate and ethanol processes
Aerobic oxidation of glucose
In other cells such as heart and liver cells a more complex shuttle system is used and _______ molecules are produced instead of 30 per glucose molecule
32
This is a branched polymer form of glucose is the storage form of carbohydrates in humans and animals (animal starch):
Glycogen
What is glycogen in muscles?
source of glucose for glycolysis
What is glycogen in liver?
source of glucose to maintain normal blood glucose levels
What are the three (3) steps of glycogenesis?
(1) Formation of Glucose 1-phosphate
(2) Formation of UDP Glucose
(3) Glucose transfer to a Glycogen Chain
This is the metabolic pathway by which glycogen is synthesized from glucose
Glycogenesis
What happens in step 1 of glycogenesis?
Glucose 6 phosphate is converted to glucose 1 phosphate
What enzyme is responsible for the formation of glucose 1 phosphate?
phosphoglucomutase
What happens in step 2 of glycogenesis?
High energy compound UTP (uridine triphosphate) activates glucose 1-phosphate to uridine diphosphate glucose (UDP-glucose)
Is glycogenolysis a reverse of glycogenesis?
It is not just reverse of glycogenesis because it does not require UTP or UDP molecules
What happens in step 3?
The glucose unit of UDP-glucose is attached to the end of a glycogen chain and UDP is produced. UDP reacts with ATP to form UTP and ADP
Adding one glucose unit to a glycogen chain requires the investment of two ATP molecules. One in the formation of glucose 6-phosphate and one in the regeneration of UTP
What are the two (2) steps of glycogenolysis?
Step 1: Phosphorylation of a glucose residue
Step 2: Glucose 1-phosphate isomerization
what happens in step 1 of glycogenolysis?
Glycogen phosphorylase catalyzes the removal of an end glucose residue from a glycogen molecule as glucose 1-phosphate.
What happens in step 2 of glycogenolysis?
Phosphoglucomutase isomerizes glucose 1-phosphate is to glucose 6-phosphate (reverse of the first step of glycogenesis)
What stimulates glycogenolysis?
Low glucose levels stimulates glycogenolysis in liver cells
What happens to glucose 6 phosphate?
Glucose 6-phosphate is ionic and cannot cross the membrane: Enzyme glucose 6-phosphatase found in liver, kidneys and intestine convert glucose 6-phosphate to glucose. This enzyme is not present in muscle and brain tissues. The free glucose is then transported to muscle and brain via blood
This is the metabolic pathway by which glucose is synthesized from non-carbohydrate sources
gluconeogenesis
Glycogen stores in muscle and liver tissue are depleted within ___________from fasting or in even less time from heavy work or strenuous physical activity
12-18 hours
Why is gluconeogenesis important?
Gluconeogenesis helps to maintain normal blood-glucose levels in times of inadequate dietary carbohydrate intake
About 90% of gluconeogenesis takes place in the ______.
liver
Pyruvate to glucose conversion requires the expenditures of how many ATP and GTP?
4 ATP and 2 GTP
What are the non carbohydrate starting materials for gluconeogenesis?
(1) Pyruvate
(2) Lactate (from muscles and from red blood cells)
(3) Glycerol (from triacylglycerol hydrolysis)
(4) Certain amino acids (from dietary protein hydrolysis or from muscle protein during starvation)
Explain the Cori Cycle.
(1) Lactate produced diffuses from muscle cells into the bloodstream and transported to liver
(2) Enzyme lactate dehydrogenase converts lactate to pyruvate in the liver
(3) Pyruvate is then converted to glucose via gluconeogenesis
(4) The glucose thus produced enters the bloodstream and transported to the muscles
This is an 11-step process in which pyruvate is converted to glucose
Gluconeogenesis
This is a 2-Step process in which glycogen is synthesized from glucose 6-phosphate
Glycogenesis
This is a 10 step process in which glucose is converted to pyruvate
Glycolysis
This is the process in which glycogen is converted to glucose 6-phosphate
Glycogenolysis
This is a metabolic pathway in which glucose is used to produce NADPH, ribose 5-phosphate (a pentose phosphate) and numerous other sugar phosphates
pentose phosphate pathway
This is the reduced form of NADP+ (nicotinamide adenine dinucleotide phosphate)
NADPH
These are the phosphorylated version of NAD+/NADH
NADP+/NADPH
Like ATP, this is essential for biosynthetic reactions/pathways.
NADPH
What happens in the oxidative stage of pentose phosphate pathway?
Involves three steps through which glucose 6-phosphate is converted to ribulose 5-phosphate and CO2
Why is pentose-phosphate pathway important?
(1) When ATP demand is high, the pathway continues to its end products which enter glycolysis
(2) When NADPH demand high, intermediates are recycled to glucose 6-phosphate (the start of the pathway), and further NADPH is produced
(3) Helps generate ribose 5-phosphate for nucleic acid and coenzyme production
What happens in the non oxidative stage of pentose phosphate pathway?
In the first step of the nonoxidative stage of the pentose phosphate pathway, ribulose 5-phosphate (ketose) is isomerized to ribose 5-phosphate (aldose
Three major hormones control carbohydrate metabolism
Insulin
Glucagon
Epinephrine
This promotes utilization of glucose by cells. Its function is to lower blood glucose levels. It is also involved in lipid metabolism. It also produces a rate of glycogen synthesis.
Insulin
What triggers the release of insulin?
High glucose levels
What does insulin action include?
The mechanism for insulin action involves insulin binding to proteins receptors on the outer surfaces of cells which facilitates entry of the glucose into the cells
How many amino acid polypeptide is in insulin?
51 amino acid polypeptide
This is produced in the pancreas by alpha cells. It is released when blood glucose levels are low. The principal function is to increase blood-glucose concentration by speeding up the conversion of glycogen to glucose (glycogenolysis) in the liver
Glucagon
This is released by the adrenal glands in response to anger, fear, or excitement. The function is similar to glucagon, i.e., stimulates glycogenolysis. The primary target are the muscles because it promotes energy generation for quick action. It also functions in lipid metabolism
Epinephrine
How many amino acid polypeptide hormone is in glucagon?
29 amino acid peptide hormone
