1. بايو كمستري / د . احمد ثاني اسنان موصل 19 / 4 / 2016
Protein Metabolism

2. Protein turnover Catabolism Synthesis Amino acids Protein Gut
Deposition
Catabolism
Gut
Synthesis
Degradation

3. Protein synthesis
On-going, semicontinuous activity in all cells but rate varies greatly between tissues
Rate is regulated by hormones and supply of amino acids and energy
Energetically expensive
requires about 5 ATP per one peptide bond
Accounts for about 20% of whole-body energy expenditure

4. Protein catabolism
Some net catabolism of body proteins occurs at all times
Expressed as urinary nitrogen excretion
yields urea
Minimal nitrogen excretion is termed endogenous urinary nitrogen (EUN)

5. Urinary nitrogen excretion
LIVER
Amino acids
keto acids
NH3
CO2
Urea
Blood
KIDNEY
Urea
Urine

6. Protein catabolism Amino acid metabolism Occurs when
dietary protein exceeds requirements
composition of absorbed amino acids is unbalanced
gluconeogenesis is increased
Amino acid metabolism
Biosynthesis of nonessential amino acids
Catabolism
Conversion to glucose or fat
Urea cycle

7. Biosynthesis of nonessential amino acids
Transamination reactions
allow extensive interconversion between nonessential amino acids
requires vitamin B6 as a coenzyme
Biosynthesis of nonessential amino acids
Tyrosine
From phenylalanine (PKU; 1 in 15,000)
hydroxylation of phenylalanine
important in adrenaline, noradrenaline, thyroxine and melanin synthesis

8. 2

13. Heme Catabolism
Heme to Bilirubin in liver to gall bladder to small intestine
Converted to urobilinogen reabsorbed to blood, liver, kidney

14. Bilirubin
Heme to bilirubin to bilirubin diglucuronide (soluble)

16. Urea cycle 3
Definition
The urea cycle (also known as the ornithine cycle) is a cycle of biochemical reactions occurring in many animals that produces urea (NH2)2CO from ammonia (NH3)

17. Function
Organisms that cannot easily and quickly remove ammonia usually have to convert it to some other substance, like urea or uric acid, which are much less toxic.
Insufficiency of the urea cycle occurs in some genetic disorders (inborn errors of metabolism), and in liver failure.

18. Urea is less toxic than ammonia.
The Urea Cycle occurs mainly in liver.
The 2 nitrogen atoms of urea enter the Urea Cycle as NH3 (produced mainly via Glutamate Dehydrogenase) and as the amino N of aspartate.
The NH3 and HCO3- (carbonyl C) that will be part of urea are incorporated first into carbamoyl phosphate.

20. Urea Cycle Enzymes in mitochondria: 1. Ornithine Trans- carbamylase
Enzymes in cytosol:
2. Arginino-
Succinate
Synthase
3. Arginino-
succinase
4. Arginase.