Protein Metabolism | Biochemistry

The below mentioned article provides a note on protein metabolism.

Notion of Nitrogen Balance:

If one determines, on the one hand, the quantity of nitrogen ingested by an animal and on the other hand the quantity excreted in urine and faeces, one may observe three situations:

i. The nitrogen excreted is less than the nitrogen ingested, i.e. there is retention of nitrogen by the animal; this is the case with growing animals or other conditions requiring considerable protein synthesis (lactation for ex­ample);

ii. The nitrogen excreted exceeds the nitrogen ingested, which means that the animal loses nitrogen; this happens when dietary protein is insufficient or during some diseases;

iii. The nitrogen excreted is equal to the nitrogen ingested. The animal is said to be in nitrogen balance-, one can define the quantity of nitrogen to be supplied through diet to cover exactly the metabolic requirements of the organism and maintain the (adult) animal in nitrogen balance; this is called the “protein minimum” (in man, about 1 g proteins/kg body weight/24 h).

But this protein minimum is not sufficient by itself: the diet must of course also include, on the one hand, the essential amino acids, and on the other hand, carbohydrates and lipids to meet the energy requirements (moreover, some lipids have a vitamin activity and are also essential).

If it is desired to maintain the nitrogen balance in an adult, or the growth of a young individual, by means of a single protein, one must make sure that the latter is suitably digestible (the percentage of nitrogen absorbed through the intestinal mucosa varies from one protein to another) and that it contains essential amino acids in sufficient quantities (e.g., some seed storage proteins are very poor in lysine).

Dynamic State of Proteins:

It would be wrong to believe that an animal in nitrogen balance will excrete any additional nitrogen ingested (like rice grains spilling out when added to an already filled vessel).

In reality, if an amino acid labeled with (¹⁵N) nitrogen is administered, it is observed that, on the one hand, the major part of this ¹⁵N is incorporated into the proteins while only a small part is excreted, and on the other hand, the major part of the nitrogen excreted is not labeled and belongs to the proteins of the animal.

In other words — contrary to what was believed for a long time — the tissue proteins of an adult animal are not stable during the entire life of the animal, but are on the contrary, in a perpetual process of renewal (without any change in their concentration).

The terms “dynamic state of proteins” denote this permanent protein metabolism, characterized on one hand by a continuous degradation and on the other, by a permanent biosyn­thesis of proteins from the “metabolic pool of amino acids” (consisting of dietary amino acids and those liberated by the hydrolysis of tissue proteins).

If the amino acid — labeled with the ¹⁵N isotope — administered to the animal is leucine for example, it is observed that in liver proteins, it is leucine which has the highest percentage of ¹⁵N (a part of the labeled leucine administered was therefore incorporated as such), but other amino acids are also labeled. Next to leucine, glutamic acid has the highest percentage of isotope, which is not surprising as we have seen the predominant role played by this amino acid in transamination processes.

This labelling technique has enabled to study the protein turnover rate in various tissues. Thus, in rats, half the liver proteins turns over in 5 days, while the half-life of proteins in muscle and conjunctive tissues is only 21 days; however since the latter represent more than 25 times the weight of liver proteins, if one considers protein biosynthesis in the whole animal, it is clear that much more proteins is formed per unit time in the muscle and conjunctive tissues than in the liver.

This permanent turnover of proteins (from dietary amino acids or amino acids produced by the hydrolysis of tissue proteins), the fact that mammals cannot build large reserves of proteins or amino acids (contrary to our observation concerning carbohydrates and lipids) and par­ticularly the fact that the ordered incorporation of amino acids into proteins is controlled by the genetic information, constitute some remarkable characteristics of protein metabolism. In the case of bacteria, especially — the mechanisms which enable the cell to control the synthesis of some enzymatic proteins and adapt it to its requirements.