Protein Metabolism - Amino Acid Pool, Nitrogen Balance

Proteins perform a great variety of specialized and essential functions in the living cells. These functions may be broadly grouped as static (structural) and dynamic.

Structural functions: Certain proteins perform the brick and mortar roles and are primarily responsible for the structure and strength of the body. These include collagen and elastin found in the bone matrix, vascular system and other organs and D-keratin present in epidermal tissues.

Dynamic functions: The dynamic functions of proteins are more diversified in nature. These include proteins acting as enzymes, hormones, blood clotting factors, immunoglobulins, membrane receptors, storage proteins, besides their function in genetic control, muscle contraction, respiration etc. Proteins performing dynamic functions are appropriately regarded as the working horses of cells.

An adult has about 100 g of free amino acids which represent the amino acid pool of the body. The amino acid pool may be an oversimplification of the facts since there is no single compartment—rather, several compartments exist.

An amino acid pool is a collection of amino acids available in an organism's cells at a given time, based on the food recently consumed by the organism.

Amino acids are not stored in cells over long period periods of time, so the makeup of an amino acid pool shifts regularly.

Glutamate and glutamine together constitute about 50% and essential amino acids about 10% of the body pool (100 g).

The concentration of intracellular amino acids is always higher than the extracellular amino acids. Amino acids enter the cells against a concentration gradient by active transport.

The amino acid pool of the body is maintained by the sources that contribute (input) and the metabolic pathways that utilize (output) the amino acids.

SOURCES OF AMINO ACID POOL

(a) Protein turnover: The protein present in the body is in a dynamic state. It is estimated that about 300-400 g of protein per day is constantly degraded and synthesized which represents body protein turnover.

There is a wide variation in the turnover of individual proteins. For instance, the plasma proteins and digestive enzymes are rapidly degraded, their half-lives being in hours or days. The structural proteins (e.g. collagen, actin, myosin) have long half-lives, often in months and years.

(b) Dietary protein: There is no storage form of amino acids as is the case for carbohydrates (glycogen) and lipids (triacylglycerols). The excess intake of amino acids is metabolized—oxidized to provide energy, converted to glucose or fat. The amino groups are lost as urea and excreted. Protein is digested by proteolytic enzymes to amino acids which are absorbed in the intestine and enter the body pool of amino acids

c) Synthesis of non-essential amino acids: Ten out of the 20 naturally occurring amino acids can be synthesized by the body which contributes to the amino acid pool.

UTILIZATION OF AMINO ACID POOL

(a) Synthesis on New Protein: Most of the body proteins (300-400 g/day) degraded are synthesized from the amino acid pool. These include enzymes, hormones, immunoproteins, contractile proteins etc.

b) Synthesis of non-protein nitrogenous compounds: Many important nitrogenous compounds (porphyrins, purines, pyrimidines, etc.) are produced from the amino acids. About 30 g of protein is daily utilized for this purpose

c) Production of energy: Generally, about 10-15% of body energy requirements are met from the amino acids, which can vary based on when you are eating protein.

d) Synthesis of carbs, fats: The amino acids are converted to carbohydrates and fats. This becomes predominant when the protein consumption is in excess of the body requirements. Different amino acids enter different stages of the glycolysis pathway to produce energy, carbs or fats.

e)Synthesis of non-essential amino acids: All the non-essential amino acids are also synthesised through keto acids after deamination/transamination of the amino acids.

Now to check whether we are consuming an adequate amount of proteins we have a term called NITROGEN BALANCE.

NITROGEN INPUT = dietary protein(nitrogen input)

NITROGEN OUTPUT = protein lost by our body through urine, faeces, sweat etc. (nitrogen output)

NITROGEN BALANCE = NITROGEN INPUT- NITROGEN OUTPUT

If Nitrogen Balance is positive: you will have the potential to gain muscles. (ANABOLISM)
If Nitrogen Balance is negative: there is the probability of muscle loss. (CATABOLISM)
If Nitrogen balance is in equilibrium: You won't lose nor gain muscles. (EQUILIBRIUM)

Nitrogen balance will depend on the protein quantity, quality and overall calorie intake.

If you seriously want to know about your nitrogen balance then you can use this method :

Steps to Calculate Nitrogen Balance :

Determine nitrogen lost in urine by a 24-hour urinary urea nitrogen test.
Add 4 ( 4 g is used to estimate the nitrogen losses from non-urine sources (sweat faeces, etc.) to the UUN to account for non-urinary losses of nitrogen.
Determine nitrogen intake by dividing the daily protein intake by 6.25 ( since every protein contains approx 16% nitrogen)
N-bal = value from Step 3 - value from Step 2

If the nitrogen balance is 0, the person is consuming the correct amount of protein for the maintenance of muscle tissues.

If the nitrogen balance is negative, increase the protein intake by a factor determined by multiplying the nitrogen balance figure by 6.25.

If the goal is to gain muscles, increase protein intake above what would result in nitrogen equilibrium.

Example :

Protein intake: 90 g

24 hour UUN: 20 g N

20 + 4 = 24 g N (total output) N intake = 90/6.25 = 14.4 g N

N-balance = 14 - 24 = -10 g N

This person is losing 10 x 6.25 = 62.5 g protein per day.

To maintain he/she has to add 62.5 grams of protein to his current intake.

To gain muscle mass he/she has to add >62.5 grams of protein.

These calculations can give you the best approximations on how much protein your body actually requires but not the exact values(since we have done some approximations with urea output values)

But using these equations you will get to know more about how your body is actually functioning and you can always try making changes in the protein intake to see whether you are improving or not.

Advisable for :

a. those who want to explore more about their body functioning.

b. if you are facing chronic fatigue or improper recovery.

For more detailed information on Protein Metabolism make sure to watch this video: