85 Questions
What is the major fuel for most tissues?
Glucose
Which process converts glucose to pyruvate?
Glycolysis
What is the end product of anaerobic glycolysis?
Lactate
Which pathway does glucose participate in for the synthesis of glycogen?
Pentose phosphate pathway
What is the precursor of fatty acids and cholesterol in metabolism?
Acetyl-CoA
How does skeletal muscle utilize glucose as a fuel?
Forming Pyruvate
What is the role of insulin in controlling glucose availability for glycolysis in skeletal muscle?
Regulating transport of glucose into the cell
Which enzyme in the liver has a high affinity for glucose and acts at a constant rate to provide glucose-6-phosphate under normal conditions?
Hexokinase
What is the function of glucokinase in the liver?
Regulate glucose concentration in hepatic portal blood
Which cells contain glucokinase to detect high concentrations of glucose?
Pancreatic β-islet cells
What happens as more glucose is phosphorylated by glucokinase in the liver?
Increased glycolysis and ATP formation
How does increased ATP formation due to glucokinase activity affect membrane potential in pancreatic β-islet cells?
Causes membrane depolarization
What is the main end product of glycolysis in fast-growing cancer cells?
Lactate
Which enzyme catalyzes the phosphorylation of glucose to glucose-6-phosphate in glycolysis?
Hexokinase
What causes lactic acidosis in some cases?
Impaired activity of pyruvate dehydrogenase
Which molecule is used for gluconeogenesis in the liver, contributing to hypermetabolism in cancer cachexia?
Lactate
What inhibits hexokinase allosterically in the process of glycolysis?
Glucose-6-phosphate
What happens when there is a shortage of oxygen during glycolysis?
Impairment of mitochondrial re-oxidation of NADH
What leads to the fusion of insulin secretory granules with the cell membrane?
Influx of calcium ions
Which compound is important at the junction of several metabolic pathways including glycolysis, gluconeogenesis, and glycogenolysis?
Glucose 6-phosphate
How many molecules of ATP are formed in glycolysis per molecule of glucose undergoing the process?
2 molecules
Which enzyme catalyzes the conversion of fructose to fructose 1-phosphate in glycolysis?
Hexokinase
What is a consequence of fructose bypassing the main regulatory steps in glycolysis?
Increased lipogenesis in the liver and adipose tissue
Which step in glycolysis involves a dehydration process, forming phosphoenolpyruvate?
Enolase
What inhibits enolase, an enzyme involved in glycolysis?
Fluoride
Why is the reaction catalyzed by pyruvate kinase considered irreversible under physiological conditions?
Large free energy change involved
What occurs to pyruvate under anaerobic conditions?
Reduced to lactate
Which enzyme catalyzes the conversion of pyruvate to acetyl-CoA under aerobic conditions?
Not specified in the text
What happens to the reducing equivalents from NADH formed in glycolysis?
Taken up into mitochondria for oxidation
Why is glycolysis inhibited when blood samples are taken for glucose measurement?
Fluoride inhibits enolase
What is the main function of the liver in maintaining blood glucose concentration between meals?
Breaking down glycogen to release glucose
In gluconeogenesis, substrates like lactate and pyruvate from the cytosol enter the mitochondrion to produce which precursor for glucose synthesis?
Oxaloacetate
Which tissue has a substantial requirement for glucose, even in prolonged fasting?
Brain
Which pathway occurs in the cytosol of skeletal muscle for synthesizing fatty acids?
Fatty acid synthesis
Where does glycolysis primarily take place within the cell?
Cytosol
What effect does phosphorylation of fructose to fructose 1-phosphate have on glycolysis?
Bypasses main regulatory steps
Which enzyme catalyzes the formation of fructose 6-phosphate from glucose 6-phosphate?
Phosphofructokinase
Why is phosphoenolpyruvate formed during glycolysis a critical compound?
It is involved in gluconeogenesis
Which enzyme catalyzes the conversion of fructose to fructose 1-phosphate in glycolysis?
Phosphohexose isomerase
What is the primary outcome of the fusion of insulin secretory granules with the cell membrane?
Release of insulin
What is the main function of glucokinase in the liver?
To regulate the concentration of glucose available to peripheral tissues
Why is glucokinase present in pancreatic β-islet cells?
To detect high concentrations of glucose
What is the result of increased ATP formation due to glucokinase activity in the liver?
Membrane depolarization and opening of a calcium channel
What distinguishes glucokinase from hexokinase in terms of affinity for glucose?
Glucokinase has a high affinity (low Km) for glucose
How does hexokinase function in the liver under normal conditions?
Acts at a constant rate to provide glucose-6-phosphate for glycogen synthesis
Why is pyruvate kinase reaction irreversible under physiological conditions?
The large free energy change involved
What happens to NADH formed in glycolysis under anaerobic conditions?
It is reduced to lactate by lactate dehydrogenase
What permits the oxidization of NADH and allows another molecule of glucose to undergo glycolysis?
Formation of lactate from pyruvate
What happens to pyruvate under aerobic conditions?
It is transported into mitochondria for oxidation to acetyl-CoA
Why does enolase require the presence of Mg2+ or Mn2+ ions?
To act as cofactors for its enzymatic activity
What is the main fuel used by most tissues?
Glucose
In glycolysis, what is the end product when it occurs anaerobically?
Lactate
Which pathway involves glucose in the synthesis of glycogen?
Glycolysis
What is the source of reducing equivalents (NADPH) for fatty acid synthesis?
Glucose
Which compound provides the carbon skeletons for the synthesis of nonessential amino acids?
Acetyl-CoA
What does skeletal muscle form aerobically when utilizing glucose as a fuel?
$CO_2$
In skeletal muscle, what regulates the availability of glucose for glycolysis?
Glucose transport into the cell
What is the function of glucokinase in the liver following a meal?
Regulate glucose concentration in peripheral tissues
Which enzyme in the liver has a Km much higher than the normal intracellular concentration of glucose?
Glucokinase
What leads to closure of an ATP-potassium channel in pancreatic β-islet cells?
Increased glycolysis
Which enzyme provides more glucose 6-phosphate than needed for glycolysis in the liver?
Glucokinase
What happens as a result of increased ATP formation due to glucokinase activity in the liver?
Membrane depolarization
What is the end product of glycolysis in fast-growing cancer cells?
Lactate
Which enzyme catalyzes the conversion of glucose to glucose-6-phosphate in glycolysis?
Hexokinase
What inhibits hexokinase allosterically during glycolysis?
Glucose-6-phosphate
What is the main fuel for most tissues during energy production?
Glucose
Why does lactic acidosis occur under certain conditions?
Deficiency of vitamin B1 (thiamin)
Where does gluconeogenesis take place for converting lactate to glucose in the body?
Liver
Why is enolase inhibited by fluoride in blood samples taken for glucose measurement?
To prevent glycolysis from occurring and altering glucose levels.
What makes the reaction catalyzed by pyruvate kinase irreversible under physiological conditions?
The formation of enol-pyruvate as an immediate product.
What happens to NADH under anaerobic conditions during glycolysis?
It is used to reduce lactate catalyzed by lactate dehydrogenase.
Why does pyruvate undergo oxidative decarboxylation to acetyl-CoA under aerobic conditions?
To permit the oxidization of NADH.
What is the primary role of lactate dehydrogenase in glycolysis under anaerobic conditions?
To reduce NADH to lactate.
Why is the reaction catalyzed by pyruvate kinase considered irreversible?
Since its immediate product undergoes spontaneous isomerization.
What is the primary function of skeletal muscle in terms of carbohydrates metabolism?
Synthesizes muscle protein
During gluconeogenesis, where do substrates such as lactate and pyruvate enter to yield oxaloacetate as a precursor for glucose synthesis?
Mitochondrion
What is the significant role of the liver between meals in terms of blood glucose concentration maintenance?
Maintains blood glucose concentration by breaking down glycogen
Which tissue has a substantial requirement for glucose even during prolonged fasting?
Brain tissue
What is the significance of glycolysis, the pentose phosphate pathway, and fatty acid synthesis occurring in the cytosol?
Facilitates various metabolic processes within the cell
What is essential for tissues like the brain and erythrocytes due to their reliance on glucose for fuel?
Adequate blood concentration of glucose
What is the role of the pyruvate dehydrogenase complex in glucose metabolism beyond glycolysis?
Facilitating the conversion of pyruvate to acetyl-CoA for entry into the citric acid cycle
Why are erythrocytes completely reliant on glucose as their metabolic fuel?
Due to the absence of mitochondria in erythrocytes
What effect does anoxic conditions have on glycolysis in skeletal muscle?
Promotes the rapid production of ATP via glycolysis
What molecular mechanism is responsible for the inhibition of GAPDH by methylglyoxal in cancer cells?
Formation of covalent adducts with enzymes
How does the absence of oxygen influence the function of glycolysis in cancer cells?
Promotes high rates of glycolysis for ATP production
Study Notes
Carbohydrate Metabolism
- Glucose is the major fuel for most tissues, metabolized to pyruvate through glycolysis
- Aerobic tissues metabolize pyruvate to acetyl-CoA, which enters the citric acid cycle for complete oxidation to CO2 and H2O, linked to ATP formation
- Glycolysis can also occur anaerobically, with the end product being lactate
Glucose Metabolic Role
- Glucose and its metabolites participate in other processes, such as:
- Synthesis of glycogen and the pentose phosphate pathway
- Providing reducing equivalents (NADPH) for fatty acid synthesis
- Triose phosphate intermediates giving rise to the glycerol moiety of triacylglycerols
- Pyruvate and citric acid cycle intermediates providing carbon skeletons for nonessential amino acid synthesis
- Acetyl-CoA being the precursor of fatty acids and cholesterol
Skeletal Muscle
- Utilizes glucose as a fuel, both aerobically and anaerobically
- Stores glycogen as a fuel for muscle contraction and synthesizes muscle protein
- Accounts for approximately 50% of body mass, storing protein that can be drawn upon to supply amino acids for gluconeogenesis in starvation
Liver
- Hexokinase has a high affinity for glucose, providing glucose-6-phosphate to meet the liver's needs
- Glucokinase has a higher Km than hexokinase and is involved in:
- Removing glucose from the hepatic portal blood following a meal
- Regulating the concentration of glucose available to peripheral tissues
- Providing more glucose 6-phosphate than required for glycolysis, which is used for glycogen synthesis and lipogenesis
Glycolysis
- Occurs in the cytosol, with all enzymes being cytosolic
- Glucose enters glycolysis by phosphorylation to glucose-6-phosphate, catalyzed by hexokinase
- Hexokinase is inhibited allosterically by its product, glucose-6-phosphate
- In tissues other than the liver and pancreatic β-islet cells, glucose availability for glycolysis is controlled by transport into the cell, regulated by insulin
- Glucokinase is also found in pancreatic β-islet cells to detect high concentrations of glucose
- As more glucose is phosphorylated by glucokinase, there is increased glycolysis, leading to increased ATP formation, which leads to:
- Closure of an ATP-potassium channel
- Membrane depolarization
- Opening of a voltage-gated calcium channel
Glycolysis Regulation
- Major sites of regulation are:
- Hexokinase (and glucokinase)
- Phosphofructokinase
- Pyruvate kinase
- Reactions catalyzed by these enzymes are irreversible
- Most reactions of glycolysis are freely reversible
Fructose Metabolism
-
Fructose enters glycolysis by phosphorylation to fructose 1-phosphate, bypassing the main regulatory steps
-
This leads to increased formation of pyruvate and acetyl-CoA, and can contribute to the development of obesity
-
In the liver and adipose tissue, this leads to increased lipogenesis### Enolase Inhibition
-
Enolase is inhibited by fluoride
-
Fluoride is used in blood sample tubes to inhibit glycolysis when measuring glucose levels
Enolase Dependence
- Enolase requires the presence of either Mg2+ or Mn2+ ions
Pyruvate Kinase Reaction
- Pyruvate kinase catalyzes the transfer of the phosphate from phosphoenolpyruvate to ADP, producing two molecules of ATP per glucose molecule
- The reaction is irreversible under physiological conditions due to:
- Large free energy change
- Spontaneous isomerization of enol-pyruvate to pyruvate, making the product unavailable for the reverse reaction
Anaerobic Conditions
- Under anaerobic conditions, NADH cannot be reoxidized through the respiratory chain
- Pyruvate is reduced to lactate by lactate dehydrogenase, permitting the oxidation of NADH
- This allows another glucose molecule to undergo glycolysis
Aerobic Conditions
- Under aerobic conditions, pyruvate is transported into mitochondria
- Pyruvate undergoes oxidative decarboxylation to acetyl-CoA and then oxidation to CO2 in the citric acid cycle
- Reducing equivalents from NADH formed in glycolysis are taken up into mitochondria for oxidation
This quiz covers the metabolism of carbohydrates in tissues such as skeletal muscle and liver. Topics include glucose transport into cells, regulation by insulin, glycogen synthesis, and the role of enzymes like hexokinase in the liver.
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