Nitrogen Metabolism Flashcards
What are proteases?
They cleave specific sites on peptides:
- internal cleavage
- cleavage at termini (amino and carboxypeptidase)
What does pepsin digest
Proteins to form large polypeptides
Where is pepsin synthesised and where does it act
Stomach chief cells; stomach
What do trypsin, chymotrypsin, carboxypeptidase and elastase digest?
Large polypeptides to form small polypeptides
Where are trypsin, chymotrypsin, carboxypeptidase and elastase synthesised?
Pancreas
Where do trypsin, chymotrypsin, carboxypeptidase and elastase act?
Small intestine
What does aminopeptidase digest?
small polypeptides to form amino acids
Where is aminopeptidase synthesised?
Brush border cells of small intestine
Where does aminopeptidase act?
small intestine
What is a proenzyme
A larger inactive enzyme precursor, requires proteolytic cleavage to be activated
Why is pepsinogen formed/ why is pepsin not synthesized in active form?
It will degrade intercellular protein
Where is pepsin activated?
stomach
How is pepsinogen activated to form pepsin?
Catalytic site of pepsinogen blocked by inhibitory domain.
At low pH in stomach, inhibitory domain unfolds, catalytic site activated and cleaves its own inhibitory domain (autolytic activation)-> forms pepsin
Pepsin activates another pepsinogen via cleavage of inhibitory domain (catalytic activation)
Positive feedback activation mechanism
What pH is optimal for pancreatic enzymes
pH 9, bicarbonate of pancreaic juic neutralises stomach acid from 2.5 to 9
What cleaves and activates trypsinogen to form trypsin?
Enteropeptidase
Which enzymes do trypsin cleave and activate
Proelastase-> elastase
Chymotrypsinogen=> chymotrypsin
Procarboxypeptidase-> carboxypeptidase
What are the sources for amino acids?
Biosynthesis, breakdown of dietary and body’s proteins
What can amino acids be used for?
metabolism for energy, protein synthesis, non-protein body constituents, small loss in urine and sweat
What is nitrogen balance?
Dietary N intake vs urinary N output as urea.
Positive nitrogen balance, intake> output
(growth, pregnancy, refeeding, muscle building)
Negative balance (starvation, amino acid/protein deficiency)
What is transamination?
The transfer of amino group to keto acid to form a non-essential amino acid
name 2 transaminases
Aspartate aminotransferase (AST) Alanine aminotransferase (ALT)
Describe the action of ALT
transfers amino group from alanine to alpha kg to form glutamate (nonessential amino acid) and pyruvate which can enter TCA cycle
What are pyruvate, OAA and alpha-kg
ketoacids
Describe the action of AST
Transfers amino group from aspartate to alpha kg to form glutamate (non-essential amino acid) and oxaloacetate which can enter TCA cycle