Nitrogen Balance / Protein Metabolism - Lecture 155

Nitrogen Balance - Protein Metabolism - Lecture 155

1. Define the terms glucogenic and ketogenic when applied to amino acids.

Glucogenic:
an amino acid that can be turned into glucose through glycoloneogenesis
---> all amino acids except leucine and lysine

Ketogenic:
an amino acid that can be degraded directly into AcetylCoA through ketogenesis
---> leucine, lysine - only ketogenic
---> others that are glucogenic AND ketogenic: isoleucene, threonine, phenylalanine, tyrosine, tryptophan

Overall Protein metabolism

2. List the seven amphibolic end points of amino acid catabolism and indicate which are glucogenic and which are ketogenic.

Glucogenic: 

• Pyruvate
• Oxaloacetate
• Fumarate
• Succinyl CoA
• α-ketoglutarate

Ketogenic:

• AcetylCoA
• Acetoacetyl CoA

Carbon-Skeleton breakdown - amino acid catabolisation:

3. Know the important classes of non-protein nitrogen-containing compounds synthesised from amino acids in mammals

1. Purines (A, G) and pyrimidines (C, T, U) - nucleotides and nucleic acid
2. Porphyrins - haem (oxygen transport) and cytochromes (respiratory chain)
3. Hormones (adrenaline, noradrenaline) and neurotransmitters (acetylcholine, catecholamines)
4. Creatine - as creatine phosphate - store more energy in muscle
5. Others - carnitine (fat metabolism), glutathione (anti-oxidant), melanins (skin pigment)

4. Explain what is meant by the term ‘nitrogen balance’

N_in = N_out

Positive Nitrogen balance ---> more N in than out
                                    - growing/pregnant animal to make tissues
Negative Nitrogen balance ---> less N in than out
                                    - starvation - not enough protein
                                    - tissues broken down
Nitrogen balance:

• Most animals need 1g protein per kg per day of average quality protein - containing nutritional essentials
• Disturbances can build up NH₃ in body and result in symptoms of ammonia poisoning

5. Describe the processes by which nitrogen from proteins is excreted from the body, including transamination, oxidative deamination and the synthesis of urea

Overall process: 

Transamination:

• change of C=O from α-ketoglutarate with NH2 from any amino acid forms glutamate and α-keto acid
• glutamate travels to liver but first undergoes glutamate synthesis

Glutamate Synthesis:

 - For transport to liver, glutamate has to be transferred to glutamine by combining with NH₄⁺

- this uses ATP

- Once in liver, an enzyme converts back to glutamate and NH₄⁺

- Glutamate then enters Oxidative deamination

Oxidative Deamination:

• Glutamate is the only amino acid undergoing significant oxidative deamination to form NH₄⁺
• Glutamate combines with NAD+ and water
• Enzyme used is glutamate dehydrogenase
• forms α-ketoglutarate, NH₄⁺, and NADH
• If reaction goes to much in reverse direction, NH₃ builds up and becomes toxic
• NH₄⁺ enters Urea cycle

Urea Cycle: 

• uses 4 high energy Phosphate bonds
• 2NH₄⁺ + CO₂ + 3ATP ----> Urea + 2ADP + AMP + 4Pi
• Animal needs a lot of energy to excrete urea
• a high protein diet means you need even more energy to retain Nitrogen balance
• 5% urea is excreted in faeces
• Aspartate donates NH₂ group for urea formation
• Arginase enzyme is essential for urea formation
• Carbamoyl Phosphate is used up. All others regenerated in cycle

Ammonia toxicity:

• N balance not maintained
• Reverses Oxidative Deamination, using up α-ketoglutarate - a key intermediate in the CAC
• Promotes conversion of glutamate, a neurotransmitter, to glutamine
• Clinical signs
• intolerance of high protein foods
• vomiting
• mental retardation
• coma
• death
• Causes:
• genetic defect in urea cycle enzyme
• liver disease 
• liver damage
• Treatment:
• lower NH₃ levels
• low protein diet
• only feed small amounts of protein at a time
• feed several times throughout the day