BIOL 2420 Lecture 5: Unit 10 – Lecture 5
26 Apr 2018
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Unit 10 – Lecture 5
Fasted-State Metabolism
- Once all nutrients from a meal have been digested, absorbed, and distributed to various cells,
plasma concentrations of glucose begin to fall
o Acts as a signal for the body to shift from fed state to fasted state metabolism
- Metabolism is under the control of hormones
o The goal is to maintain blood glucose homeostasis so that the brain and neurons have
adequate fuel
- Glucose homeostasis
o Achieved through catabolic pathways that convert glycogen, proteins, and fats into
intermediates that can be used to make either glucose or ATP
o Using protein or fat for ATP production spares plasma glucose for use by the brain
Glycogen Converts to Glucose
- The glycogen store is the most readily available glucose for plasma glucose homeostasis
o Most of the glycogen is stored in the liver
o Live glycogen can provide enough glucose to meet 4-5 hours of the ody’s energy needs
- Glycogenolysis
o Glycogen is broken down to glucose
o Most glycogen is converted directly to glucose-6-phosphate in a reaction that splits a
glucose molecule from the polymer with the help of an inorganic phosphate from the
cytosol
▪ Only ~10% of stored glycogen is hydrolyzed to plain glucose metabolism
- Skeletal muscle glycogen can be metabolized to glucose, but not directly
o Muscle cells lack the enzyme that makes glucose from glucose 6-phosphate
o Furthermore glucose 6-phosphate is metabolized into pyruvate (aerobic conditions) or
lactate (anaerobic)
o Pyruvate/lactate is then transported to the liver
o The liver uses pyruvate/lactate to make glucose (gluconeogenesis)
Proteins Can Be Used to Make ATP
- Free amino acid pool of the body normally provides substrate for ATP production during the
fasted state
o If the fasted state goes on for an extended time muscle proteins can be broken down to
provide energy
- The first step in protein catabolism – digestion of a protein into smaller polypeptides by
enzymes (proteases, endopeptides)
o Exopeptides break the peptide bonds at the ends of the smaller polypeptide, freeing
individual amino acids
- Amino acids can be converted into intermediates that enter either glycolysis or the citric acid
cycle
o First – conversion is deamination – removal of the amino group from the amino acid
▪ Creates ammonia molecule and an organic acid
▪ Come of the organic acids created this way are pyruvate, acetyl CoA, and several
intermediates of the citric acid cycle
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