CARBOHYDRATE METABOLISM Flashcards
1st stage of processing food products
Biochemical process by which food molecules, through hydrolysis, are broken down into simpler chemical units that can be used by cells for their metabolic needs
Digestion
enzyme that hydrolyses some a-glycosidic linkage of starch and glycogen to produce smaller polysaccharide and disaccharide
Salivary a-amylase
A if only the first statement is correct
B if only the second statement is correct
C if both of the statements are correct
D if neither of the statements is correct
- Only small amounts of carbohydrates are digested in the mouth, because food is swallowed quickly to the stomach
- In the stomach, digestion has no effect against the gastric juice; carbohydrate digestion enzyme is not present
C
A if only the first statement is correct
B if only the second statement is correct
C if both of the statements are correct
D if neither of the statements is correct
- Small intestine is not the primary site for carbohydrate digestion
- Final step of digestion happens in the outer membrane of intestinal mucosal cell
B
Pancreatic alpha amylase digestive enzyme is also found in the mouth
Could hydrolyzed the polysaccharides to disaccharides (maltose, sucrose, lactose)
Has different enzymes that could breakdown the disaccharides:
Maltase: produce 2 monomer units of glucose
Sucrase: glucose and fructose units
Lactase: glucose and galactose
A if only the first statement is correct
B if only the second statement is correct
C if both of the statements are correct
D if neither of the statements is correct
- Monosaccharide units are absorbed in the bloodstream via capillaries in the intestinal lining (villi)
- Galactose and fructose are converted in the heart to be glucose
A
Metabolic pathway by which glucose (a C6 molecule) is converted into two molecules of pyruvate (a C3 molecule)
Can happen in active skeletal muscle
Chemical energy in the form of ATP is produced, and NADH-reduced coenzymes are produced
Oxidation process; no molecular oxygen is utilized (anaerobic pathway)
Oxidizing agent: NAD
Glycolysis
Products: pyruvate and NADH
Glucose to pyruvate
10 step process; every step is known to be enzyme catalyzed
Has 2 stages in its overall process based on the number of carbon of the molecule involved in the process
____ pathways: metabolic pathways in which molecular oxygen is not a participant
____ pathways: pathways that require molecular oxygen
Anaerobic
Aerobic
6 Carbon Stage of Glycolysis:
Formation of Glucose 6-Phosphate.
Glycolysis begins with the phosphorylation of glucose to yield glucose 6-phosphate.
Enzyme: Hexokinase; Reacts with glucose to produce glucose 6-phosphate
Needs magnesium ion for its activity
- Phosphorylation
1 phosphate from ATP will be attached to the C-6 of glucose: H in the hydroxyl group will be replaced
Energy needed is derived from ATP hydrolysis; endothermic reaction is involved
Product: glucose 6-phosphate
6 Carbon Stage of Glycolysis:
Formation of Fructose 6-Phosphate
Glucose 6-phosphate is isomerized into fructose 6-phosphate though the enzyme
Net result: C1 is not part of the ring structure; C-6 is still in the structure
Enzyme: phosphoglucose isomerase
- Isomerization
Functional group found in the structure is different
Glucose: aldehyde; Fructose: ketone
Product: fructose 6-phosphate
6 Carbon Stage of Glycolysis:
Formation of Fructose 1,6-Bisphosphate.
Like Step 1, is a phosphorylation reaction and therefore requires the expenditure of energy (ATP)
Enzyme: phosphofructokinase; Requires magnesium ion for its activity
Additional phosphate group is bonded to the phosphate derivative of fructose
- Phosphorylation
Additional phosphate group from the ATP is bonded in the C-1 of fructose
Product: Fructose 1,6-Bisphosphate
Fructose 1,6-Bisphosphate can enter only in glycolysis, while other products of step 1 and 2 can enter another metabolic pathway
Bisphosphate: when 2 Phosphate group are bonded in different C atom; they are not connected to each other; found on 2 different C atom
3 Carbon Stage of Glycolysis:
Formation of Two Triose Phosphates.
Reacting C6 species is split into two C3 (triose) species.
Enzyme: aldolase
6-C molecule (Fructose 1,6-Bisphosphate) is cleaved forming 2 different types of triose species
- Cleavage
Fructose 1,6-bisphosphate, the molecule being split, is unsymmetrical, the two trioses produced are not identical.
One product is dihydroxyacetone phosphate (acetone; ketone)
The other is glyceraldehyde 3-phosphate (glycerol; aldehyde)
3 Carbon Stage of Glycolysis:
Formation of Glyceraldehyde 3-Phosphate
Changing the location of the functional group
Enzyme: triosephosphate isomerase
- Isomerization
Dihydroxyacetone phosphate is isomerized using the enzyme forming another glyceraldehyde 3-phosphate
Double bond in C-2 will be broken down to form glyceraldehyde 3-phosphate
3 Carbon Stage of Glycolysis:
Formation of 1,3 Bisphosphoglycerate.
Oxidizing agent: NAD (also reduced, forming NADH)
Enzyme: glyceraldehyde 3-phosphate dehydrogenase (oxidation)
H of glyceraldehyde will be replaced by the free phosphate group forming 1,3-bisphosphoglycerate
- Oxidation and Phosphorylation
Product: 2 1,3-bisphosphoglycerate, 2 NADH, and 2 H
Phosphate group is added to C-1 of glyceraldehyde 3-phosphate to produce 1,3-bisphosphoglycerate
The newly added phosphate group in 1,3-bisphosphoglycerate is a high-energy phosphate group.
A high-energy phosphate group is produced when a phosphate group is attached to a carbon atom that is also participating in a double bond: carbon–carbon or carbon–oxygen
3 Carbon Stage of Glycolysis:
Formation of 3-Phosphoglycerate.
Energy (ATP) generating; utilizes 2 ADP to produce 2 ATP molecules are produced aside from the product
Products: 2 ATP and 2 3-Phosphoglycerate
Enzyme: phosphoglycerokinase
Diphosphate species (1,3-bisphosphoglycerate) is converted into ATP (3-Phosphoglycerate)
- Phosphorylation of ADP
Diphosphate species just formed are converted back to a monophosphate species.
ATP production in this step involves substrate-level phosphorylation.
Substrate-level phosphorylation is the biochemical process by which a high energy phosphate group from an intermediate compound (substrate) is directly transferred to ADP to produce ATP.
3 Carbon Stage of Glycolysis:
Formation of 2-Phosphoglycerate.
Enzyme: phosphoglyceromutase
Mutase: it transfers the position of one functional group to another functional group within a molecule
- Isomerization
It moves phosphate from C-3 to C-2 forming the 2-Phosphoglycerate
Phosphate group of 3-phosphoglycerate is moved from carbon 3 to carbon 2.
3 Carbon Stage of Glycolysis:
Formation of Phosphoenolpyruvate (Alcohol dehydration)
Enzyme: enolase; Requires magnesium ion for its activity
Product: phosphoenolpyruvate, 2 water molecule (H2O)
C-2 and C-3 are dehydrated to form double bond
- Dehydration
Phosphoenolpyruvate: there is a carbon double bond in the structure; energy-rich compound
Result is another compound containing a high-energy phosphate group
The phosphate group is attached to a carbon atom that is involved in a carbon–carbon double bond
3 Carbon Stage of Glycolysis:
Formation of Pyruvate.
2nd step that produces ATP; similar to step 7
Products: 2 ATP and 2 pyruvate molecules after glycolysis (acetone; ketose; C2 is double bonded to O2)
- Phosphorylation of ADP
Enzyme: pyruvate kinase; requires magnesium and potassium ions for its activity
Phosphoenolpyruvate transfers its high-energy phosphate group to an ADP molecule to produce ATP and pyruvate
FILL IN THE BLANKS:
Step 1: Glucose —> glucose 6-phosphate
__ (ATP consumed)
Step 3: Fructose 6-phosphate —> fructose 1,6-bisphosphate
__ (ATP consumed)
Step 7: 2 (1,3-Bisphosphoglycerate —> 3-phosphoglycerate)
__ (ATP produced)
Step 10: 2 (Phosphoenolpyruvate —> pyruvate)
__ (ATP produced)
Product after Glycolysis:
Net: __ (ATP)
-1
-1
2
2
2
Entry of Galactose and Fructose into Glycolysis:
Galactose and fructose are not directly absorbed in the bloodstream, they are converted in the liver to glucose
_____: undergoes isomerization and phosphorylation to produce glucose 1-phosphate and enter step 2 of glycolysis where it is isomerized to glucose 6-phosphate and proceed to other steps
_____: phosphorylated forming fructose 1-phosphate and enter step 4 where it cleaved into 2 triose specie
Glucose
Fructose
Fates of Pyruvate:
Aerobic Conditions in humans, animals, and microorganisms
Produces:_____
Anaerobic Conditions in humans, animals, and microorganisms
Produces: _____
Anaerobic Conditions in some microorganisms
Produces: _____
Acetyl CoA
Lactate Fermentation
Ethanol Fermentation
Under aerobic (oxygen-rich) conditions, pyruvate is oxidized to acetyl CoA.
Pyruvate formed in the cytosol through glycolysis crosses the two mitochondrial membranes and enters the mitochondrial matrix, where the oxidation takes place.
COO of pyruvate is removed and replaced by CoA; removed COO will become CO2
Involves oxidation and decarboxylation (because CO2 is produced)
Oxidation to Acetyl CoA
Enzyme: pyruvate dehydrogenase complex (oxidation)
Product: Acetyl CoA
Most acetyl CoA molecules produced from pyruvate enter the citric acid cycle (TCA); changes NAD into reduced form
All NADH can enter ETC directly or indirectly and is regenerated
Most Pyruvate are converted into acetyl CoA
The overall reaction process involves four separate steps and requires NAD, CoA-SH, FAD, and two other coenzymes (lipoic acid and thiamine pyrophosphate, the latter derived from the B vitamin thiamine)
anaerobic
biochemical process by which NADH is oxidized to NAD without the need for oxygen.
Activated during strenuous exercise or working muscles: increase build up in lactate
Enzyme: Lactate dehydrogenase
Lactate fermentation is the enzymatic anaerobic reduction of pyruvate to lactate
The sole purpose of this process is the conversion of NADH to NAD
Lactate Fermentation
The lactate formed is converted back to pyruvate when aerobic conditions are again established in a cell.
Conversion of pyruvate to lactate requires NADH and H ion
RBCs can also form lactate; because there is no mitochondria
Lactate can be from RBCs and muscles
When the reaction for conversion of pyruvate to lactate is added to the net glycolysis reaction
Enzymatic anaerobic conversion of pyruvate to ethanol and carbon dioxide
Enzyme: Pyruvate decarboxylase
NAD and NADH doesn’t appear in the equation but are both generated and consumed during the process
Ethanol Fermentation
TRUE OR FALSE:
The first step in conversion of pyruvate to ethanol is a decarboxylation reaction to produce acetaldehyde.
TRUE
COO is removed and H is replaced in the pyruvate
Products: acetaldehyde and CO2
Enzyme: pyruvate decarboxylase
Involves yeast
CO2 bubbles causes the rise of bread and other product and also fermentation
Acetaldehyde reduction to produce ethanol
Redox reaction
Enzyme: alcohol dehydrogenase (oxidation)
An overall reaction for the production of ethanol from glucose is obtained by combining the reaction for the conversion of pyruvate with the net reaction for glycolysis
Ethanol Fermentation
NADH produced during Step 6 of Glycolysis cannot directly participate in the electron transport chain because mitochondria are impermeable to NADH and NAD+
Glycerol 3-phosphate-dihydroxyacetone phosphate transport system shuttles electrons from NADH, but not NADH itself, across the membrane
Regeneration of NAD+ from NADH
Dihydroxyacetone phosphate and glycerol phosphate freely cross the mitochondrial membrane
The interconversion shuttles the electrons from NADH to FADH2
30 ATP molecules are produced by the muscle and nerve cells
26 ATP: oxidative phosphorylation of ETC
2 ATP: oxidation of glucose to pyruvate
2 ATP: conversion of GTP to ATP
Glycogenesis
Metabolic pathway by which glycogen is synthesized from glucose 6-phosphate
Glycogen (animal starch): storage form of carbohydrate in humans and animals, found in muscle and liver tissue
Muscle: source of glucose in glycolysis
Liver: source of glucose to maintain normal blood glucose levels
Glycogen Synthesis and Degradation:
3 Steps of Glycogenesis:
The starting material for this step is not glucose itself but rather glucose 6-phosphate
Glucose 6-phosphate formed during the first step of glycolysis
Glucose —> Glucose 6-phosphate (with Hexokinase)
Glucose 6-phosphate —> Glucose 1-phosphate
- Formation of Glucose 1-phosphate
Enzyme: phosphoglucomutase; isomerize glucose 6-phosphate to glucose 1-phosphate (C6 —> C1)
Glucose 6-phosphate enters glycogenesis when the body needs to store glucose
Reaction: isomerization
Product: glucose 1-phosphate
3 Steps of Glycogenesis:
Glucose 1-phosphate from Step 1 must be activated before it can be added to a growing glycogen chain.
Enzyme: UDP-glucose pyrophosphorylase
The activator is the high energy compound UTP (uridine triphosphate)
- Formation of UDP-glucose
UMP is bonded in the phosphate of glucose 1-Phosphate
Product: 2 phosphate groups, uridine diphosphate glucose
3 Steps of Glycogenesis:
The glucose unit of UDP-glucose is then attached to the end of a glycogen chain
Enzyme: glycogen synthase
- Glucose Transfer to a Glycogen Chain
UDP formed can be converted to UTP: UDP + ATP —> UTP + ADP
UTP formed can again be used in step 2 of glycogenesis
Metabolic pathway by which glucose 6-phosphate is produced from glycogen
Glycogen —> glucose 6-phosphate (Breakdown of glycogen )
Not simply reversed of glycogenesis; doesn’t require high energy compound (UTP or UDP)
In muscle and brain cells an immediate need for energy is the stimulus that initiates glycogenolysis.
Glucose 6-phosphate that is produced directly enters the glycolysis pathway at Step 1 and its multi step conversion to pyruvate begins.
Glycogen Synthesis and Degradation: Glycogenolysis
Low level of glucose is the stimulus that initiates glycogenolysis in liver cells
Glucose 6-phosphate produced must be converted to free glucose before it can enter the bloodstream, as glucose 6-phosphate cannot cross cell membranes
This change is affected by the enzyme glucose 6-phosphatase, an enzyme found in liver cells but not in muscle cells or brain cells.
Converts glucose 6-phosphate to glucose; needs water
Removes phosphate in the molecule
2 Steps of Glycogenolysis:
Enzyme: glycogen phosphorylase; catalyzes the cleavage of bond by phosphate group
Phosphate group is needed to react with glycogen; in the glycogen 1 glucose unit is removed and is phosphorylated to form glucose 1-phosphate
- Phosphorylation of a Glucose Residue
2 Steps of Glycogenolysis:
Reverse of step 1 of glycogenesis
Enzyme: phosphoglucomutase
Product: glucose 6-phosphate
Glucose 1-phosphate Isomerization
Metabolic pathway by which glucose is synthesized from noncarbohydrate materials; energy consuming
Noncarbohydrate starting materials for gluconeogenesis: lactate from muscles, glycerol from triacylglycerol hydrolysis, amino acid from protein hydrolysis or muscle protein during starvation
Glycogen Synthesis and Degradation: Gluconeogenesis
helps to maintain normal blood-glucose levels in times of inadequate dietary carbohydrate intake
Not exact opposite of glycolysis (10 steps)
11 steps; which in step 1 is where pyruvate is converted to Oxaloacetate
Enzyme: pyruvate carboxylase (pyruvate converted to Oxaloacetate)
COO is added in pyruvate (COO is added in the C-4 of pyruvate)
COO is from CO2
Oxaloacetate is converted to phosphoenolpyruvate if it proceeds to gluconeogenesis
Enzyme: phosphoenolpyruvate carboxykinase (requires GTP)
Phosphate group of GTP and COO of Oxaloacetate is removed and yields CO2 and GDP
Kinase: high-energy compound is required during the process
Phosphorylation reaction is involved; in the C-2 of phosphoenolpyruvate phosphate group is bonded
Phosphoenolpyruvate: there is phosphate group and pyruvate as the sugar and enol
Glycogen Synthesis and Degradation: Gluconeogenesis
Step 9 of gluconeogenesis enzyme: fructose 1,6- bisphosphatase
Converting fructose 1,6-bisphosphate to fructose fructose 6- phosphate; doesn’t require ATP along with step 11
Glycogen Synthesis and Degradation: Gluconeogenesis
Step 11 of gluconeogenesis enzyme: glucose 6-phosphatase
Coverts glucose 6-phosphate to glucose; no ATP is required
Step 1 and 3 needs ATP
Pyruvate to glucose requires expenditure of 4ATP and 2GTP
Glycogen Synthesis and Degradation: Gluconeogenesis
Cyclic biochemical process in which glucose is converted to lactate in muscle tissue
Lactate is reconverted to glucose in the liver
Lactate is formed from glucose under anaerobic conditions in muscle cells
When transferred to liver that is when it is converted into glucose and transferred back to the muscle
Cori Cycle
Glucose is returned to the muscle tissue
Enzyme: lactate dehydrogenase (LDH)
Uses NAD as substrate and oxidizing agent and is reduced to NADH and Lactate is converted into pyruvate
Formed pyruvate can be converted via gluconeogenesis then is transported into the bloodstream into the muscle
Metabolic pathway by which glucose is used to produce NADPH, ribose 5-phosphate, and numerous other sugar phosphates.
Nicotinamide Adenine Dinucleotide Phosphate (NADPH): needed in lipid synthesis; phosphorylated version of NADH
Pentose Phosphate Pathway
Similar to NADH difference is that is has phosphate group
Involved in reactions in lipids and nucleic acid reactions
Ribose 5-phosphate: pentose derivative that it needed in nucleic acid
Pentose ribose is a component of ATP, GTP, UTP, CoA, NAD, FAD, and RNA
2 stages within the Pentose Phosphate Pathway:
Involves three steps through which glucose 6-phosphate is converted to ribulose 5-phosphate (undergoes non oxidative stage) and CO2.
Step 1 enzyme: glucose 6-phosphate dehydrogenase; Utilizes NADP; redox
Glucose 6-phosphate to 6-phospho glucono-1,5-lactone
Oxidative stage
Step 2 enzyme: gluconolactonase
6-phospho glucono-1,5-lactone to 6-phosphogluconate
Step 3 enzyme: 6-phosphogluconate dehydrogenase; Utilizes NADP; redox
6-phosphogluconate to Ribulose 5-phosphate
Product: Ribulose 5-phosphate; Undergoes non oxidative stage
2 stages within the Pentose Phosphate Pathway:
First step involves isomerization of ribulose 5-phosphate (a ketose) to ribose 5-phosphate (an aldose) specially when used for nucleic acid synthesis
Then further conversion of ribose 5-phosphate to numerous other sugar phosphates.
Non Oxidative stage
Ultimately, glyceraldehyde 3-phosphate and fructose 6-phosphate (both glycolysis intermediates) are formed.
Product: ribulose 5-phosphate
Contains provision for the conversion of ribose 5-phosphate
Key enzyme in the non-oxidative branch of the pentose phosphate pathway that transfers a two-carbon aldehyde unit from ketose-donor to aldose-acceptor sugars.
Transketolase
Reactants: xylulose 5-phosphate + ribose 5-phosphate —> glyceraldehyde 3-phosphate + sedoheptulose 7-phosphate
Key enzyme in the non-oxidative branch of the pentose phosphate pathway that transfers a three-carbon aldehyde unit from ketose-donor to aldose-acceptor sugars.
Transaldolase
Reactants: sedoheptulose 7-phosphate + glyceraldehyde 3-phosphate —> erythrose 4-phosphate + fructose 6- phosphate
insulin, glucagon, epinephrine (major hormones for regulation of carbohydrate metabolism)
Second major method for regulating carbohydrate metabolism is hormonal control
Hormonal Control of Carbohydrate Metabolism
51 amino acid protein
Hormone produced by the beta cells of the pancreas.
promotes the uptake and utilization of glucose by cells.
Thus its function is to lower blood glucose levels
It is also involved in lipid metabolism
Insulin
Only hormone in the body that could lower blood glucose level
Triggered when there is a high blood glucose level
Increase in the rates of glycogen and fatty acid synthesis
Mechanism: it will bind in the protein receptors in the outer surface of the cells, which facilitates the entrance of glucose in the cells
29 amino acid
Polypeptide hormone produced in the pancreas by alpha cells.
It is released when blood-glucose levels are low
Glucagon
Its principal function is to increase blood-glucose concentrations by speeding up the conversion of glycogen to glucose (glycogenolysis) and gluconeogenesis in the liver
Polypeptide, not protein for it is has less than 40 amino acid
Effect is opposite of the insulin
Adrenaline; from adrenal glands
Similar to glucagon: increase blood glucose levels
Binding to the receptor site outside cell membrane
Epinephrine
Stimulation of glycogenolysis, the release of glucose from glycogen
Its primary target is muscle cells, where energy is needed for quick action
It also functions in lipid metabolism
