Question

Explain the concept of nitrogen equilibrium/balance including positive and negative nitrogen balance. Then provide an example of each of these three situations.

Briefly describe how your body digests protein (including the secretions and enzymes involved), which part of the GI tract proteins are absorbed from, and how absorbed proteins are utilized.

Answer 1

Healthy adults usually maintain a constant lean body mass and neither accumulate protein nor lose protein mass. .Since , their combined nitrogen intake mainly as protein more or less equals their nitrogen losses, this is said to be nitrogen balance .

Nitrogen balance is a measure of nitrogen input minus nitrogen output .

Nitrogen Balance = Nitrogen input - Nitrogen loss

sources of nitrogen intake include meat , dairy , eggs ,nuts and legumes and grain and cereals . Example of nitrogen losses include urine , feces , sweat , skin ,and hair .

Blood urea nitrogen can be used in estimating nitrogen balance that is as the urea concentration in urine .

Nitrogen is a fundamental component of amino acids , which are the molecular building blocks of protein .Therefore measures nitrogen input and losses that can be used to study the protein metabolism .

Positive nitrogen balance is associated with periods of growth , hypothyroidism , tissue repair and pregnancy .This means that the intake of nitrogen into the body is greater than the loss of nitrogen from the body So there is an increase in the total body pool of protein .

Negative nitrogen balance is associated with burns , serious tissue injuries , fevers , hypthroidism , wasting disease and during periods of fasting .This means that the amount of nitrogen excreted from the body is greater than the amount of nitrogen ingested .A negative nitrogen balance can be used as part of a clinical evaluation of malnutrition .

Dietary nitrogen from metabolising proteins and other nitrogen - containing compunds that has been linked to changes in genome evolution . Species which primarily obtain energy from metabolising nitrogen - rich compunda use more nitrogen in their DNA than species which primarily break down carbohydrates for their energy . DIetary nitrogen alters codon bias and genome composition in parasitic microorganisms.

PROTEIN DIGESTION

Dietary protein are very large complex molecules that cannot be absorbed from the intestine ..To be absorbed , dietary protein must be digested to small simple molecules ( amino acid) which are easily absorbed from the intestine .

Digestion in the stomach

Protein digestion begins in the stomach by the gastric juice .

Role of gastric HCL ;- it cause denaturation of protein , it activates pepsinogen to pepsin, It makes pH in the stomach suitable for the action of pepsin .

Pepsin ;- It is an endopeptidases acting on central peptide bond in which amino group belongs to armatic amino acids e,g ; phenyl alanine , tyrosine and tryptophan , Pepsin is secreated inan inactive form called pepsinogen .It is activated by HCL then by autoactivation.

Renin;- It is a milk - clotting enzyme ,It is present in stomach of infants and young animals .It act on casein converting it to soluble paracasein , which in turns binds calcium ions forming insoluble calcium paracaseinate . calcium paracaseinate is then digested by pepsin.

Gelatinase ;- It is an enzyme that liquefies gelatin .The end product of protein digestion in the stomach are peptones , proteoses and large polypeptides .

Digestion in the small intestine ;-

digestion of protein is completed in the small intestine by proteolytic enzymes present in pancreatic and intestinal juices.

Pancreatic juices ;-

Trypsin ;- It is an endopeptidases that hydrolyzes central peptide bond in which the carboxyl group belong s to basic amino acids eg. arginine , lysine and histidine ,.It is secreted in an inactive form called trypsinogen . and It is activated by enterokinases enzyme then by auto activation.

Chymotrypsin;- It is an endopeptidases that hydrolyses central peptide bond in which the carboxyl group belongs to aromatic amino acids . IT is secreated inan inactive form called chymotrypsinogen , It is activated by trypsin.

Elastase enzyme incative form- proelastase and carboxypeptidase inactive form - procarboxypeptidase , they both are activated by trypsin.

Intestinal juices ;-

• Aminopeptidases ;- it release a single amino acid
• Tripeptidase ;- It release a single amino acid and dipeptide
• Dipeptidase :- It release two amino acids

The end product of protein digestion in the small intestine are amino acids

PROTEIN ABSORBTION

• It is an active process that needs energy
• Energy needed is derived from hydrolysis of ATP
• It occurs in small intestine
• Absorption of amino acids is rapid in the duodenum and jejunum but slow in the ileum.

There are two mechanism for amino acid absorption

(1) carrier protein transport system

It is the main system for amino acid absorption , , Here absorption of one amino acid molecule needs onwe ATP molecule .There are 7 carrier protein , 1 for each group of amino acid .Each carrier protein has to sites one for amino acid and ane for sodium  ion. It co-transport amino acid and sodium ion from intestinal lumen to cytocol of intestinal mucosa cells . The absorbed amino acid passes to the portal circulation while sodium ion is extruded out of thecell in exchange with potassium ion by sodium pump .

(2) Glutathione transport system

Glutamthione is used to transport amino acid from intestinal lumen to cytosol of intestinal mucosa cells .It is an active process that needs energy . Absorption of 1 amino acid molecules needs 3 ATP molecules .Glutathione reacts with amino acids in the presence of glutamyl transpeptidase to form glutamyl amino acid . Glutamyl amino acid release amino acid in the cytosol intestinal mucosa cells with formation of 5 - oxoproline that is used for regeneration of glutathione to begin another turn of the cycle .