Integration of Met

• What are the four mechanisms that control metabolic pathways?

  Availability of substrates, allosteric regulation, covalent modification, induction/repression
• What does the absorptive state refer to?

  The 2–4 hours after eating a meal
• How does fructose 2,6-bisphosphate affect glycolysis in the liver?

  It stimulates glycolysis as an allosteric activator
• What is the effect of fructose 2,6-bisphosphate on gluconeogenesis?

  It inhibits gluconeogenesis as an allosteric inhibitor
• What role does phosphorylation/dephosphorylation play in enzyme activity?

  It modifies specific residues to control activity
• In the absorptive state, what form are most enzymes regulated by covalent modifications?

  Dephosphorylated form and active
• Which enzymes are exceptions and inactive in their dephosphorylated form?

  Glycogen phosphorylase kinase, glycogen phosphorylase, hormone-sensitive lipase
• What does induction and repression of enzyme synthesis lead to?

  Changes in the total population of active sites
• How does enzyme synthesis change during embryonic development?

  Enzymes are synthesized at high levels for growth
• What is the key site for processing absorbed nutrients after a meal?

  The liver
• What does the liver do with nutrient-rich blood from the hepatic portal vein?

  Processes absorbed nutrients and responds to insulin
• What happens to carbohydrates, lipids, and amino acids during the absorptive period?

  The liver takes them up for metabolism or storage
• How does the liver smooth out fluctuations in nutrient availability?

  By metabolizing, storing, or routing nutrients
• What is the liver's primary role in carbohydrate metabolism?

  It is a glucose-producing tissue
• What happens to glucose metabolism in the liver after a carbohydrate meal?

  The liver becomes a net consumer of glucose
• What increases hepatic glucose metabolism after a meal?

  Increased phosphorylation, glycogen synthesis, HMP activity, glycolysis, decreased gluconeogenesis
• What is the primary tissue for de novo synthesis of fatty acids?

  The liver
• When does fatty acid synthesis occur in the liver?

  During the absorptive period with excess caloric intake
• What substrates favor fatty acid synthesis in the liver?

  Acetyl CoA and NADPH from glucose metabolism
• What is the rate-limiting reaction in fatty acid synthesis catalyzed by acetyl CoA carboxylase?

  Formation of malonyl CoA from acetyl CoA
• What is favored during TAG synthesis in the liver?

  Availability of fatty acyl CoA and glycerol 3-phosphate
• How does the liver package TAG for use by other tissues?

  Into very-low-density lipoprotein (VLDL) particles
• What happens to surplus amino acids in the absorptive period?

  They are released into the blood or deaminated
• What happens to the carbon skeletons of deaminated amino acids?

  They are degraded to pyruvate, acetyl CoA, or TCA cycle intermediates
• How does the liver handle branched-chain amino acids?

  They pass through unchanged and are metabolized in muscle
• How does protein synthesis change in the absorptive state?

  There is a transient increase in hepatic protein synthesis
• What is the role of adipose tissue in energy storage?

  It distributes fuel molecules second to the liver
• How much does adipose tissue weigh in a 70-kg man?

  Approximately 14 kg
• What can adipose tissue constitute in obese individuals?

  Up to 70% of body weight
• What occupies the volume of each adipocyte?

  A droplet of triacylglycerol (TAG)
• How does glucose transport into adipocytes occur?

  Through GLUT-4, sensitive to insulin concentration
• What happens to glucose transport during the absorptive state?

  Insulin levels elevate, increasing glucose influx
• What is the effect of increased intracellular glucose availability in adipose tissue?

  Enhanced rate of glycolysis for TAG synthesis
• How does adipose tissue metabolize glucose via the HMP?

  It produces NADPH essential for fat synthesis
• Is de novo synthesis a major source of fatty acids in adipose tissue?

  No, it is not a major source
• When does fasting begin?

  After the absorptive period ends
• What can cause fasting?

  Inability to obtain food or desire to lose weight
• What happens to plasma levels of glucose, amino acids, and TAG during fasting?

  They fall, triggering insulin decline and glucagon increase
• What characterizes the catabolic period during fasting?

  Degradation of TAG, glycogen, and protein
• What are the metabolic fuels available in a normal 70-kg man at the beginning of a fast?

  • Fat stores (sufficient for about 3 months)
  • Glycogen stores (limited compared to fat)