Protein Metabolism
بايو كمستري / د . احمد ثاني اسنان موصل 19 / 4 / 2016 * 1Amino acids
ProteinDeposition
Catabolism
Gut
Synthesis
Degradation
Protein turnover
*
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*
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)*
Urinary nitrogen excretion
Urine
KIDNEY
LIVER
Urea
Urea
CO2
Amino acids
keto acids
NH3
Blood
*
Protein catabolism
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
* Biosynthesis of nonessential amino acids
Transamination reactions allow extensive interconversion between nonessential amino acids requires vitamin B6 as a coenzymeBiosynthesis of nonessential amino acids
Tyrosine From phenylalanine (PKU; 1 in 15,000) hydroxylation of phenylalanine important in adrenaline, noradrenaline, thyroxine and melanin synthesis
* 2
*
*
*
*
Heme Catabolism
Heme to Bilirubin in liver to gall bladder to small intestine Converted to urobilinogen reabsorbed to blood, liver, kidney*
Bilirubin
Heme to bilirubin to bilirubin diglucuronide (soluble)*
*
Urea cycle
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)
* 3
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.*
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.
*
*
Urea Cycle Enzymes in mitochondria: 1. Ornithine Trans- carbamylase Enzymes in cytosol: 2. Arginino- Succinate Synthase 3. Arginino- succinase 4. Arginase.
*
*
*
*
*