Protein Catabolism and Urea Cycle

Protein Catabolism

• When does it occur

• Process

Occurs when:

• Dietary protein intake exceeds the body’s needs

• During increased energy needs (gluconeogenesis)

Process:

• Proteins broken down into amino acids

• Product of keto acid is used for energy

• Product of nitrogen is excreted as urea

How can keto acids be used for energy

AA that is converted to keto acids can enter these metabolic pathways (but it is an expensive energy course):

• TCA Cycle: Broken down to CO2 and H2O with release of energy

• Gluconeogenesis: Makes glucose

• Lipogenesis: Fat production

• Ketogenesis: Formation of ketone bodies for energy

Urea Cycle #ffd800

• What is a toxic byproduct produced from protein catabolism

• Why is it toxic

• Excreted as

• Which organ facilitates this

• Process

  • Requires

  • Urea transported to which organ for excretion

• What does it also synthesise

Toxic byproduct produced from protein catabolism: Ammonia

Why it’s toxic: Toxic to the nervous system

Excreted as: Urea which is the less toxic form as it is water soluble

Organ that facilitates this: Liver

Process: Converts NH3 (ammonia) + CO2 —> urea + H2O

• Requires: Energy

• Urea transported to: Kidney for excretion through urine

Also synthesises: Arginine

How are the aa ornithine (#008bff), cirtulline (#ff9500), arginine (#00c454) play a role in the urea cycle

• Where in the cycle

• Location

• Function

• Forms

Ornithine:

• Where in cycle: Starts the cycle

• Location: Mitochondria

• Function: Combines with carbamoyl phosphate to form citrulline

  • Ornithine + Carbamoyl Phosphate —> Citrulline

• Forms: Citrulline that is transported into cytoplasm

Citrulline:

• Where in cycle: Intermediate in nitrogen transport

• Location: Cytoplasm

• Function: Combines with aspartate using ATP

• Forms: Argininosuccinate

  • Broken down to form arginine (now contains both nitrogen atoms from NH3 and aspartate) and fumarate (by-product used in TCA cycle)

Arginine:

• Where in cycle: Intermediate as urea precursor

• Location: Cytoplasm

• Function: Broken down by arginase

• Forms: Urea and ornithine (recycled back into mitochondria)