Gluconeogenesis and Urea Cycle Notes

Gluconeogenesis & Metabolic Toxins

Learning Objectives
  • Understand the toxicity of nitrogen produced from amino acid breakdown.
  • Describe the urea cycle and the transportation of nitrogen in the bloodstream.
  • Predict effects of dietary changes or mutations affecting urea cycle enzymes on nitrogen levels in the body.
  • Explain gluconeogenesis and differentiate it from glycolysis.
Case Study: OTC Deficiency
  • Ornithine transcarbamoylase (OTC) deficiency is a urea cycle disorder.
  • Symptoms include ataxia, lethargy, and potential death if untreated.
  • Important to contextualize within nitrogen metabolism discussions.
Nitrogen Balance
  • Body cannot store protein; excess is broken down and excreted.
  • This phenomenon is referred to as nitrogen balance.
Breaking Down Amino Acids
  • Amino acids consist of carbon (C), hydrogen (H), oxygen (O), and nitrogen (N).
  • Carbon, hydrogen, and oxygen are metabolized for energy, while nitrogen becomes a concern.
  • Breakdown yields ammonia ( ext{NH}_3), which is toxic (especially to neurons) and must be eliminated.
Moving Nitrogen
Transamination
  • Transamination involves the transfer of nitrogen between molecules.
  • This process allows tissues to utilize the remaining carbon skeleton for energy or to generate non-essential amino acids.
  • ext{α-ketoglutarate/glutamate} are the most common nitrogen acceptors.
  • Requires pyridoxal phosphate (PLP), the active form of vitamin B6.
Deamination
  • Removes nitrogen, generating nitrogenous waste ( ext{NH}3/ ext{NH}4^+).
  • The process generates useful carbon skeletons for energy metabolism or storage.
  • ext{NH}3/ ext{NH}4^+ is toxic and must be excreted; excess can prevent the TCA cycle from functioning, causing energy shortages.
Nitrogen Excretion
  • Ammonium ( ext{NH}_4^+) is generated from amino acid metabolism.
  • It is toxic if it accumulates in the blood; thus, it’s converted to safer forms for transport, mainly glutamine and alanine, or as urea.
Alanine: Transport of Nitrogen
  • Alanine is polar and non-toxic, suitable for transport in the blood.
  • Tissues convert excess nitrogen into alanine, which travels to the liver for conversion into pyruvate and nitrogen transfer to glutamate.
  • Pyruvate can either provide energy or be converted back into glucose; glutamate can then be deaminated for nitrogen excretion.
Nitrogen Balance in Adults
  • Healthy adults maintain nitrogen balance, with nitrogen intake roughly equal to excretion, mainly as urea.
  • Excess protein consumption leads to its breakdown and subsequent nitrogen release.
  • During fasting, muscle protein may be broken down to sustain blood glucose levels, leading to nitrogen waste production.
Urea Cycle
  • The urea cycle occurs in the liver; problems in any steps can disrupt homeostasis and result in nitrogen buildup in the blood.
  • Understanding the cycle's significance is crucial, though specific details of all steps are not required.
Impacts of Diet on Nitrogen Levels
  • Low carbohydrate, high protein diets can lead to ammonia accumulation in the blood if urea synthesis cannot keep up.
Case Study: Gene Therapy for OTC Deficiency
  • Gene therapy was explored to correct OTC deficiency but faced complications, including severe immune responses in trial subjects.
  • History includes the tragic case of Jesse Gelsinger, who died after treatment in 1999.
Gluconeogenesis Overview
  • Gluconeogenesis occurs in the liver, producing glucose during fasting to maintain blood glucose levels.
  • It is the synthesis of glucose from non-carbohydrate sources and is essentially the reverse of glycolysis.
Differences Between Gluconeogenesis and Glycolysis
  • While gluconeogenesis closely mirrors glycolysis, four specific reactions employ different enzymes:
    • ext{Pyruvate} to ext{Oxaloacetate} (catalyzed by pyruvate carboxylase).
    • ext{Oxaloacetate} to ext{PEP} (catalyzed by PEP carboxykinase).
    • ext{Fructose-1,6-bisphosphate} to ext{Fructose-6-bisphosphate} (catalyzed by fructose-1,6-bisphosphatase).
    • ext{Glucose-6-phosphate} to ext{Glucose} (catalyzed by glucose-6-phosphatase).
Gluconeogenic Precursors
  • Primary precursors for gluconeogenesis include:
    • Lactate
    • Glycerol
    • Amino acids (notably alanine).
References
  • Lieberman, M. & Peet, A. (2023). MARKS’ Basic Medical Biochemistry: A Clinical Approach. 6th Edition, Wolters Kluwer Health.
  • Smolin, L. A., Grosvenor, M. B., & Gurfinkel, D. (2020). Nutrition: Science and Application. 3rd Canadian Edition, John Wiley & Sons Canada.