Amino Acid Metabolism & Urea Cycle

Amino Acid Metabolism & Urea Cycle

• Amino acids are small biomolecules with unique chemical properties.
• They consist of a carbon skeleton and an amine group (-NH₂).
• They play various roles, including building blocks for proteins, precursors for important molecules like hormones, and fuel molecules.
• Amino acids are obtained from the diet and synthesized by the body (essential and non-essential).
• Excessive dietary protein is not stored but broken down to amino acids, then to intermediates such as pyruvate, oxaloacetate, or acetyl CoA.
• Excess amino acids can be converted to fat.
• At physiological pH, amino groups are positively charged (NH₃⁺) and carboxyl groups are negatively charged (COO⁻).
• Amino acids are considered as a carbon skeleton (alpha keto acid) plus an amino group.
• Careful handling of the amino groups is essential as ammonia is toxic.

Classification of Amino Acids

• Amino acids can be classified based on their physical-chemical characteristics, metabolic fate (glucogenic/ketogenic), and dietary requirements (essential/non-essential).
• Most amino acids are glucogenic (their carbon skeletons can be converted to glucose).
• Some (e.g., phenylalanine, tyrosine, and tryptophan) are ketogenic (their carbon skeletons are broken down to produce ketone bodies).
• Essential amino acids cannot be synthesized by the body and must be obtained from the diet.
• Non-essential amino acids can be synthesized by the body.

Amino Acid Biosynthesis

• The body requires essential amino acids from the diet.
• To synthesize non-essential amino acids, the body needs a source of carbon (a ketoacid) and an amine group donor (e.g., glutamate).
• Amino acid carbon skeletons are derived from various metabolic precursors, such as glycolysis, the TCA cycle, and the pentose phosphate pathway.
• Once formed, amino acids can be used to synthesize proteins and other important molecules.

Metabolic Fate of Dietary & Intracellular Protein

• All cells can modify amino acids, but mostly, amino acid metabolism occurs in the liver.
• Remodelling involves removing the amino group and recycling the carbon skeleton.
• Toxic ammonia is converted to urea in the liver, which is then excreted in urine.

Catabolic Fate of Amino Acids

• Following deamination, the carbon chains of amino acids enter central metabolic pathways.
• These pathways converge to seven key intermediates.
• Amino acid carbon skeletons can be used for glucose synthesis, fatty acid synthesis, or for energy production.

Metabolic Link Between Major Biomolecules

• Dietary protein contributes to tissue protein.
• Excess amines are converted to urea via the urea cycle.
• Carbon skeletons can be converted to pyruvate and enter the central metabolic pathways (e.g., TCA cycle or fatty acid synthesis).

Catabolism of Amino Acids: Deamination

• Most amino acids are deaminated in the liver.
• Deamination occurs through enzymatic reactions involving aminotransferases and glutamate dehydrogenase.
• Amino groups are safely transported to the liver via alanine.

Glucose-Alanine Cycle

• The glucose-alanine cycle involves transporting excess nitrogen from muscle to liver, and converting it into glucose via gluconeogenesis.
• This glucose is then exported back to the muscle for energy.

Four Key Amino Acids in Nitrogen Metabolism

• Glutamate, aspartate, glutamate, and glutamine play crucial roles in nitrogen metabolism, acting as acceptors or donors of amino groups.

Some Detail of Deamination

• Two key mechanisms, transamination and oxidative deamination, are involved in deamination.
• Transamination transfers an amino group to a keto acid, and involves no free ammonia.
• Oxidative deamination removes an amino group and forms ammonia, which also requires a cofactor (pyridoxal phosphate).

Catabolism of Amino Acids: Deamination - oxidative deamination

• Glutamate is the central amino acid for the removal of nitrogen from other amino acids and undergoes oxidative deamination to yield ammonia.

Overview of the Urea Cycle

• High levels of ammonia are toxic. The liver and kidneys work together to ensure that the level of ammonia does not accumulate.
• The urea cycle combines two amino groups into a urea molecule that is subsequently transported via the blood to the kidneys for excretion.

Formation of Urea

• One amine is derived from NH₄⁺ (obtained primarily via glutamate deamination).
• The second amine is derived from aspartate, which is formed from oxaloacetate in a transamination reaction.

Sources of Nitrogen for Urea Cycle

• The urea cycle uses nitrogen from various amino acid sources, including glutamate and aspartate.

Summary of the Urea Cycle

• Urea synthesis predominantly occurs in the liver through a series of reactions.
• Citrulline moves from the mitochondria to the cytosol to complete the remainder of the cycle.
• Urea is excreted via the kidneys.

Urea and TCA Cycles Overlap

• The urea cycle and TCA cycle overlap in their use of fumarate and oxaloacetate.
• Fumarate is converted to malate then to oxaloacetate in the TCA cycle which can then be a source of the second amino group used to make urea.

Connection Between Urea Cycle and TCA Cycle

• Urea Cycle and TCA cycle overlap through the intermediate fumarate.

Regulation of the Urea Cycle

• The urea cycle is largely regulated by a feed-forward mechanism that responds to increased ammonia levels.
• Dietary intake (especially protein) can modulate the rate of the cycle by impacting ammonia production.
• There are metabolic/hormonal factors that increase the cycle's rate, such as during fasting or increased amino acid metabolism following intake of higher protein.

Urea Cycle Disorders

• Defects in urea cycle enzymes can lead to dangerous ammonia buildup in the body.
• Symptoms (such as lethargy, coma, and even death) often appear in infancy.

Clinical Measurement of Urea

• Urea is a key metabolic waste product.
• Blood urea levels can be used to assess kidney or liver function.
• Increases or decreases from the normal urea concentration can indicate underlying issues in the metabolic pathways that are involved with the urea cycle.

This quiz explores the metabolism of amino acids and the urea cycle, highlighting their importance in protein synthesis and energy production. Understand the roles of different amino acids, their classification, and the implications of amino group handling in the body. Test your knowledge on essential and non-essential amino acids and their metabolic pathways.