Protein metabolism Flashcards
Explain the digestion and absorption of protein
> Protein digestion starts in the stomach – involves denaturation and hydrolysis of peptide bonds
> Pepsin (enzyme) hydrolyzes about 10% peptide bonds
> Small batches of acidic chyme containing large polypeptides enter the small intestine
> Bicarbonate ions help neutralise the acidic chyme and allow activation of enzymes that break peptide bonds liberating amino acids
> The “free” amino acids are absorbed via the intestinal wall into the bloodstream
What are the 3 sources of Amino Acids in the AA pool in the body?
Dietary protein
Protein turnover
Biosynthesis of nonessential AAs in the liver
What is nitrogen balance?
When the amount of nitrogen taken into the body as protein = the amount of nitrogen excreted from the body in waste materials
HCl functions in stomach?
Denatures proteins, exposing peptide bonds
Kills most bacteria (pH = 1.5-2.0)
Activates pepsinogen (inactive zymogen) to active pepsin enzyme
Pancreatic enzymes activated in small intestines for peptide bond cleavage?
- Trypsin
- Chymotrypsin
- Carboxypeptidase
Name the 3 steps needed to remove a =NH2 group?
- Transamination
- Oxidative Deamination
- The Urea Cycle
Where does amino acid degradation take place and what are the two steps involved?
> Liver
- Removal of the –NH2 group (transamination, oxidative deamination and urea cycle)
- Degradation of the remaining carbon skeleton
What is transamination?
The transfer of the –NH2 group of an α-AA to an α-keto acid
What is Oxidative Deamination?
> amino acid converted into a keto acid with the release of an ammonium ion
i.e. glutamate loses its amino group (NH3+), with the amino group accepting additional H+ to form the ammonium ion (NH4+). Once glutamate has lost its amino group, it is now a keto acid (alpha- ketoglutarate).
> occurs in liver and kidney mitochondria
> catalyzed by the enzyme Glutamate dehydrogenase
> requires NAD+ as coenzyme, which produces NADH that in turn enters the ETC and forms ATP.
Urea cycle purpose
Urea is produced from NH4+ and aspartate
Gets rid of toxic nitrogen from the body
Where does the urea cycle occur?
The first step of the urea cycle occurs in the mitochondria, where the required enzyme is located.
Steps 2, 3 and 4 occur in the cytosol, where the required enzymes are located.
Where does the input required for the urea cycle come from?
The oxidative deamination removes the amino group from glutamate
What must happen to the ammonium ion before the urea cycle can begin?
Used to create a compound called carbamoyl phosphate
What is the purpose of performing an oxidative deamination reaction (shown below), as part of amino acid metabolism?
Oxidative deamination produces the ammonium ion (NH4+), which is incorporated into carbamoyl phosphate that is required to begin the urea cycle
