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Carbohydrate Metabolism (Metabolic Pathways)
BIOCHEMISTRY (CHM3)
Our Lady of Fatima University
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WEEK 17
CARBOHYDRATE METABOLISM
DIGESTION AND ABSORPTION OF
CARBOHYDRATES
Digestion is the 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.
*First stage in processing food products
*Only small amount of CHO is digested in the mouth because the food is quickly swallowed to the stomach
*There’s no carbohydrate digestive enzyme present on the stomach so there is really no changes/effect. The salivary amylase is inactivated in the stomach because of the stomach acidity.
*The primary site for carbohydrate digestion is the small intestine. Pancreatic a-amylase is the digestive enzyme present and can also be found in the mouth.
*Final step happens on the outer intestinal mucosal cells that has different enzymes that could break down disaccharides; maltase, sucrase, lactase.
*The monosaccharides are being absorbed in the intestinal lining or intestinal villi. Transported monosaccharides will go to the bloodstream
8There are protein carriers and STP hydrolysis that can mediate the passage of monosaccharide units in the cell membrane.
*Glucose can be utilized by the cells once transported through the bloodstream
*Galactose and fructose are converted in the liver to become glucose GLYCOLYSIS Glycolysis is the metabolic pathway by which glucose (a C6 molecule) is converted into two molecules of pyruvate (a C3 molecule), chemical energy in the form of ATP is produced, and NADH- reduced coenzymes are produced
*First metabolic pathway
*Oxidation process, but there is no molecular oxygen present; NAD as oxidizing agent
*It is a 10-step process where each step is enzyme-catalyzed. It has two stages based on the number of carbon in molecules involved in the process.
• Anaerobic pathways- metabolic pathways in
which molecular oxygen is not a participant
• Aerobic pathways- pathways that require
molecular oxygen Six-Carbon Stage of Glycolysis (Steps 1 – 3)
- *The six-carbon stage of glycolysis is known to be energy-consuming, which means that it utilizes energy. The energy used is the ATP.
- *This is where the phosphate derivatives glucose and fructose, which means that glucose and fructose are coupled by an ATP. The phosphate that will bond the two will be from the ATP.
-
WEEK 17
Step 1: Phosphorylation: Formation of Glucose 6 -
Phosphate. Glycolysis begins with the
phosphorylation of glucose to yield glucose 6- phosphate.
- *Enzyme used is the hexokinase; it will react to the glucose to form glucose 6-phosphate. It needs magnesium ion for its activity
- *Enzymes used in phosphorylation end with
kinase
- *Energy needed is derived from ATP hydrolysis. Endothermic reaction is involved.
Step 2: Isomerization: Formation of Fructose 6-
Phosphate
- *The glucose 6-phosphate is isomerized to form fructose 6-phosphate
- *The net result is that the C1 is no longer part of the ring structure
- *Phosphoglucoisomerase is the enzyme used; responsible for isomerization
- *The phosphate is still in the C6, only the C1 is changed
- *The functional groups found on these two are different (aldehyde & ketone)
Step 3. Phosphorylation: Formation of Fructose 1, 6 -
Bisphosphate. This step, like Step 1, is a
phosphorylation reaction and therefore requires the expenditure of energy. - *ATP is consumed - *Enzyme involved is phosphofructokinase that needs magnesium ion for its activity - *From fructose 6-phosphate, there is an additional phosphate group bonded to the phosphate derivative of fructose. The additional phosphate group will be bonded to the C1 of fructose - *Biphosphate is used to specify that the two phosphate group are bonded on different C atom; they are not connected to each other - *The product from this step can only enter glycolysis - *The products from step 1 and 2 can enter other metabolic pathways. Three-Carbon Stage of Glycolysis (Steps 4 – 10) - *Known to be energy-generating - *For the three-carbon intermediates, they are all phosphorylated derivatives of either glycerol or acetone bonded to phosphate group.
Step 4: Cleavage: Formation of Two Triose
Phosphates. In this step, the reacting C6 species is
split into two C3 (triose) species. Because fructose 1,6-bisphosphate, the molecule being split, is unsymmetrical, the two trioses produced are not identical. One product is dihydroxyacetone phosphate, and the other is glyceraldehyde 3-phosphate.
WEEK 17
- *Phosphoglyceromutase is the enzyme used; Mutase is an enzyme that will transfer the position of a functional group to another functional group
Step 9: Dehydration: Formation of
Phosphoenolpyruvate. The 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
- *The reaction is alcohol dehydration
- *The enzymes used is enolase which also needs magnesium for its activity
- *C2 and C3 are dehydrated to form double bond
- *2 water molecules will be released
Step 10: Phosphorylation of ADP: Formation of
Pyruvate. Phosphoenolpyruvate transfers its high-
energy phosphate group to an ADP molecule to produce 2 ATP and 2 pyruvate
- 2nd step that produces ATP
*Pyruvic kinase as the enzyme used
- Pyruvic kinase requires two ions for it to be activated; it needs magnesium and potassium for its activity
*Pyruvate is a ketose (derivative of acetone)
*In steps 1 and 3, the ATP is consumed, while in steps 7 and 10, ATP is produces
*Net overall equation for hydrolysis: glucose + 2NAD + 2ADP + 2Pi → 2Pyruvate + 2NADH + 2ATP + 2H + 2H2O
Entry of Galactose and Fructose into Glycolysis
- *Galactose undergo isomerization and phosphorylation to produce glucose 1- phosphate
- *Fructose will be phosphorylated, forming fructose 1-phosphate → step 4
WEEK 17 FATES OF PYRUVATE
Oxidation to Acetyl CoA
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.
- *Pyruvate has carboxyl group that is being removed, which will then be replaced by CoA to produce acetyl CoA
- *The removed carboxyl group will become CO
- *Utilizes CoA-SH and NAD
- *Enzyme involved is pyruvate dehydrogenase complex
- *Acetyl CoA can enter Krebs Cycle (TCA or citric acid cycle)
- *Most pyruvates are converted to acetyl CoA
Lactate Fermentation
Fermentation is a biochemical process by which NADH is oxidized to NAD without the need for oxygen.
Lactate fermentation is the enzymatic anaerobic reduction of pyruvate to lactate
The sole purpose of this process is the conversion of NADH to NAD. The lactate so formed is converted back to pyruvate when aerobic conditions are again established in a cell. - *Lactate dehydrogenase (LDH) is used - Lactate fermentation is triggered or stimulated by strenuous exercise (oxygen deficient)
Oxidation to Acetyl CoA The reaction which involves both oxidation and decarboxylation (because CO2 is produced)
- *Enzyme involved is pyruvate dehydrogenase complex The overall reaction process involves four separate steps and requires NAD, CoA-SH, FAD, and two other coenzymes (lipoic and thiamine pyrophosphate, the latter derived from the B vitamin thiamine) Most acetyl CoA molecules produced from pyruvate enter the citric acid cycle
- *The electrons of NADH are transferred to the oxygen (NAD → NADH)
- Net overall reaction: Glucose + 2 ADP + 2 Pi + 4NAD + 2 CoA-SH → 2 acetyl CoA +2 CO2 + 2 ATP + 4NADH + 4 H + 2H2O Lactate Fermentation When the reaction for conversion of pyruvate to lactate is added to the net glycolysis reaction, an overall reaction for the conversion of glucose to lactate is obtained:
WEEK 17 GLYCOGEN SYNTHESIS AND DEGRADATION: Glycogenesis
Glycogenesis is the metabolic pathway by which glycogen is synthesized from glucose 6-phosphate
- *Glycogen is the storage form of CHO on animals and humans; mostly found on muscles and liver tissue; also known as animal starch
- *Glycogen in muscles is used as the source of glucose when needed for glycolysis
- *Glycogen in liver is used for maintaining normal blood glucose levels
Step 1 : Formation of Glucose 1 - phosphate. The
starting material for this step is not glucose itself but rather glucose 6-phosphate (available from the first step of glycolysis).
- *Phosphoglucomutase is the enzyme used
- *Isomerization is the reaction
Step 2: Formation of UDP-glucose. Glucose 1-
phosphate from Step 1 must be activated before it can be added to a growing glycogen chain. The activator is the high energy compound UTP (uridine triphosphate).
- UMP will be attached to the phosphate group of glucose 1-phosphate
- UDP-glucose pyrophosphorylase is the enzyme used
- *Products: UDP, glucose, 2 phosphate groups
Step 3: Glucose Transfer to a Glycogen Chain. The
glucose unit of UDP-glucose is then attached to the end of a glycogen chain. - *Glycogen synthase is the enzyme involved - *UDP is converted back to UTP through the help of ATP (UDP + ATP → UTP +ADP)
GLYCOGEN SYNTHESIS AND DEGRADATION:
Glycogenolysis Glycogenolysis is the metabolic pathway by which glucose 6-phosphate is produced from glycogen.
- *It is not a complete opposite or reverse of glycogenesis because it does not require UTP/UDP (high energy compounds) to the processes
Step 1: Phosphorylation of a Glucose Residue.
- You need phosphate group that will react to the glycogen. One glucose unit will be removed from the glycogen and will be phosphorylated
WEEK 17
- *Glycogen phosphorylase (enzyme) catalyze the cleavage of bond of by phosphate
Step 2: Glucose 1-phosphate Isomerization
- Enzyme is phosphoglucomutase
- *It is the reverse of the step 1 of glycogenesis
- *Glucose 6-phosphate is the product formed
In muscle and brain cells an immediate need for energy is the stimulus that initiates glycogenolysis
The glucose 6-phosphate that is produced directly enters the glycolysis pathway at Step 1 and its multistep conversion to pyruvate begins
A low level of glucose is the stimulus that initiates glycogenolysis in liver cells. Here, the glucose 6- phosphate produced must be converted to free glucose before it can enter the bloodstream, as glucose 6-phosphate cannot cross cell membrane
This change is affected by the enzyme glucose 6 - phosphatase, an enzyme found in liver cells but not in muscle cells or brain cells.
- *Water is needed in converting glucose 6- phosphate to glucose GLYCOGEN SYNTHESIS AND DEGRADATION: GLUCONEOGENESIS
Gluconeogenesis is the metabolic pathway by which glucose is synthesized from noncarbohydrate materials.
- *Opposite, but not exact opposite of glycolysis, which has 10 steps because it has 11 steps
- *energy-consuming
The noncarbohydrate starting materials for gluconeogenesis are lactate, glycerol and certain amino acids.
About 90% of gluconeogenesis takes place in the liver. Hence gluconeogenesis helps to maintain normal blood-glucose levels in times of inadequate dietary carbohydrate intake - *Utilized during starvation
- *Oxaloacetate can proceed to TCA not on gluconeogenesis when the body needs energy not glucose
- *The phosphate group will bond on the C2 of phosphoenolpyruvate (CO2 +GDP)
- *Steps 9 and 11 of gluconeogenesis don’t require ATP
- *Overall net reaction: 2 Pyruvate + 4 ATP + 2 GTP + 2NADH + 2 H2O → Glucose + 4 ADP + 2 GDP + 6 Pi + 2 NAD
- *gluconeogenesis: liver; glycolysis: active skeletal muscle The Cori Cycle Cori cycle is a cyclic biochemical process in which glucose is converted to lactate in muscle tissue, the lactate is reconverted to glucose in the liver, and the glucose is returned to the muscle tissue
WEEK 17
- glyceraldehyde 3-phosphate + dihydroxyacetone phosphate + transaldolase → fructose 6-phosphate
The pentose phosphate pathway, with its many intermediates, helps meet cellular needs in numerous ways:
*The nonoxidative stage has the provision for the conversion of ribose 5-phosphate
*The overall net reaction:
3 Glucose-6-Phosphate + 6NADP + 3H2O → 2 fructose-6-phosphate + 3CO2 + glyceraldehyde-3-phosphate +6NADPH + 6H+
- When ATP demand is high, the pathway continues to its end products, which enter glycolysis.
- When NADPH demand is high, intermediates are recycled to glucose 6-phosphate (the start of the pathway), and further NADPH is produced.
- When ribose 5 - phosphate demand is high, for nucleic acid and coenzyme production, most of the nonoxidative stage is nonfunctional, leaving ribose 5-phosphate as a major product. HORMONAL CONTROL OF CARBOHYDRATE METABOLISM
- Insulin- is a hormone produced by the beta cells of the pancreas. Insulin 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.
- *51 AA protein
- *It is the only hormone in the body that could lower the blood glucose level
- *The release of insulin in the body is triggered by high blood glucose levels
- *Also produces an increase in the rates of glycogen synthesis and fatty acid synthesis
- Glucagon- polypeptide hormone produced in the pancreas by alpha cells. It is released when
blood-glucose levels are low. Its principal
function is to increase blood-glucose concentrations by speeding up the conversion of glycogen to glucose (glycogenolysis) and gluconeogenesis in the liver.
- Has 29 AA. It is not considered as protein because it has less than 40 AA.
Transketolase: is a 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.
Transaldolase: is a 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.
WEEK 17
- 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.
- *Aka adrenaline, and it is released by the adrenal glands in response to excitement, fear, and anger
- *Has similar effects with glucagon that is to increase blood sugar level
Carbohydrate Metabolism (Metabolic Pathways)
Course: BIOCHEMISTRY (CHM3)
University: Our Lady of Fatima University
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