3. Glucose and Carbohydrate Metabolism (Part II) Flashcards
Which enzyme allows the conversion of glycogen to glucose?
Glucose-6-phosphatase
Where is glucose-6-phosphatase present?
Solely in the liver, kidney, and gut
Why can’t muscle glycogen export glucose?
- Because muscle does not possess glucose-6-phosphatase
- Glucose is the only substance within the pathway that may cross the plasma membrane
Which cells may synthesize glucose from gluconeogenic precursors?
Hepatocytes (liver)
Describe the Cori Cycle.
- Glycogen is converted to lactate in muscle tissue.
- Lactate is transported to the liver.
- Lactate is converted to glucose.
- Glucose is transported to muscle.
Describe the Glucose-Alanine Cycle.
- Pyruvate in peripheral cells accepts an amino group, synthesizing alanine.
- Alanine travels to the liver, where it becomes pyruvate.
- Pyruvate becomes glucose through gluconeogenesis.
- Glucose is transported to peripheral tissues.
What is the brain’s requirement for glucose per day?
120 grams (480 kilocalories)
How much glucose does hepatic glycogen availability store per day?
190 grams
How much water does the liver store?
- 4 grams of water for every gram of glycogen (190 grams per day)
- Kilogram of both water and glycogen
What explains rapid weight loss during a fasting state (or a very low-calorie diet)?
- Glycogen stores are broken down to maintain the brain’s glucose requirement
- Promotes a rapid loss of water through the loss of glycogen
Why would some people call gluconeogenesis, glucopaleogenesis?
Because the glucose that we’re synthesizing during gluconeogenesis is not new, as we are regenerating glucose from precursors
What are gluconeogenic precursors?
Amino acids, pyruvate, and glycerol
How does glycerol relate to glucose?
- Three-carbon unit from TG
- Originally synthesized from glucose
How does pyruvate relate to glucose?
- Product of glycolysis
- Generated from glucose
How do amino acids relate to glucose?
The carbon skeleton is synthesized from glucose
Where does gluconeogenesis occur? What is it stimulated by?
- In the liver
- Stimulated by glucagon and gluocorticoids when plasma glucose is low
Why would stress hormones encourage gluconeogenesis? Give examples of these hormones.
- To increase the availability of glucose in order to respond to the crisis at hand (fight-or-flight)
- Cortisol, epinephrine
What occurs to muscle during a fast?
- Gluconeogenesis breaks down protein to allow for the utilization of glucogenic amino acid precursors
- While protein catabolism is not ideal, survival instincts prevail over muscle mass
How long does it take for glycogen stores to become exhausted?
Around 24 hours
How is glucose homeostasis maintained at first during a fast?
- Glycogenolysis
- Gluconeogenesis
What does the oxidation of fatty acids create? When does this occur?
- During a long-term fast
- Creates Acetyl-CoA
What occurs to Acetyl-CoA when there is no oxidation of glucose?
- Acetyl-CoA accumulates
- Joins together to form ketone bodies
What are the three ketone bodies?
- Acetoacetate
- Acetone
- 3-hydroxybutyrate
What is the function of ketone bodies?
- May be transported in circulation and utilized in the brain for energy purposes
- Good survival mechanism if fasting, as it provides a protein-sparing effect, while feeding the brain with energy
Why does Acetyl-CoA accumulate when there is no glucose oxidation?
- Anaplerosis is required to regenerate TCA cycle intermediates
- If there is no glucose, the precursors from glycolysis (pyruvate) may not be utilized for anaplerosis
- Acetyl-CoA is produced faster than it is removed in the TCA cycle
- Also, fatty acid B-oxidation occurs in the absence of glucose, which causes Acetyl-CoA to accumulate
What occurs when ketone bodies are overproduced?
- Cause metabolic ketoacidosis
- Decreases pH of the blood
- Impacts the brain
- Potentially fatal
What is the life-threatening problem of untreated diabetes?
Ketoacidosis
What occurs first during a long-term fast: protein breakdown or fat breakdown?
Protein breakdown, then fat breakdown (producing ketone bodies)
How is substrate utilization measured?
- CO2 that we are producing and the O2 that we consume in relation to normal room air
- Indirect calorimetry
What is the equation for the respiratory quotient?
RQ = CO2 produced/O2 consumed
What is the RQ if an individual is solely burning carbohydrates?
1.0
What is the RQ if an individual is solely burning fat?
0.7
Why is the RQ lower if an individual is burning fat?
- Fatty acid molecule has more carbons than oxygens
- More oxygen must be consumed from room air than glucose
What is the basal metabolic rate for a 70kg man?
- 1.0 kcal/kg/h
- 1700 kcal/day
What is the basal metabolic rate for a 55kg man?
- 0.9 kcal/kg/h
- 1200 kcal/day
Why is the basal metabolic rate lower in women than in men?
Because women possess less muscle mass
What percentage does the basal metabolic rate make up in total energy expenditure?
67%
What percentage does diet-induced thermogenesis make up in total energy expenditure?
10%
What is diet-induced thermogenesis?
The amount of energy expenditure above the resting metabolic rate due to the cost of processing food for use and storage
What is adaptive thermogenesis?
Energy needed to warm the body in a cool environment, and cool the body in a warm environment
What percentage does activity make up in total energy expenditure?
23% (highly variable)
Which component of energy expenditure may be modulated?
Activity
Where is GLUT1 present?
- RBCs
- Brain-blood barrier
- Fetus
Why is GLUT1 important?
- Because RBCs have no organelles (no TCA cycle) to produce energy
- Their only source of ATP is glycolysis, so they need glucose