Role and classification of proteins
Proteins consist of amino acids and perform many vital tasks. Depending on chain length, oligopeptides (from 2 to 10 amino acid residues) and polypeptides (from 10 to 50–100 residues) are distinguished. Complex proteins such as glycoproteins, lipoproteins, metalloproteins, nucleoproteins, and chromoproteins are present in the body.
The main functions of proteins include:
- Catalytic: enzymes that accelerate biochemical reactions.
- Motor: contractile proteins (actin, myosin).
- Plastic (structural): material for building new cellular structures.
- Energy: provision of anabolism and catabolism.
- Maintenance of colloid-osmotic pressure: plasma proteins, primarily albumins, are responsible for this.
- Immune: globulins participate in defense reactions.
- Regulatory: serve as a substrate for the formation of hormones and neurotransmitters.
Amino acid pool and its dynamics
A key concept in protein metabolism is the amino acid pool, which is located in the vascular bed. This pool is constantly replenished and depleted.
Sources of intake (anabolism):
- Dietary proteins (exogenous pathway) — digested in the gastrointestinal tract and enter the blood.
- Catabolism of own tissues — reversible breakdown of structural proteins.
- De novo synthesis — formation of non-essential amino acids from other metabolic products via transamination and amination.
Utilization pathways (catabolism):
- Biosynthesis of hormones, enzymes, and neurotransmitters.
- Formation of nitrogen-containing molecules (purines, pyrimidines, creatinine).
- Synthesis of inert structures such as hair and nails.
- Excretion of excess: via the kidneys (directly or after deamination with ammonia formation) and via the intestines.
Nitrogen balance and protein requirement
Since proteins are not stored in reserve, the intake of complete proteins containing essential amino acids is critical. Protein metabolism is often assessed by excreted nitrogen.
Three states of nitrogen balance are distinguished:
- Equilibrium: nitrogen intake equals excretion (normal for a healthy adult).
- Positive balance: intake exceeds excretion (typical for growth, pregnancy, recovery from illnesses).
- Negative balance: excretion exceeds intake (observed in starvation, aging, severe diseases).
There is a concept of the wear-and-tear coefficient, which reflects inevitable protein loss and amounts to 0.025–0.075 g of nitrogen per 1 kg of body weight per day. To maintain balance, a person needs a minimum of 60–75 g of protein per day; systematic protein starvation is fatal.
Regulation of protein metabolism
Maintaining a stable level of amino acids and plasma proteins is ensured by self-regulation mechanisms, while higher control is carried out by the hypothalamic-pituitary system via the autonomic nervous system and endocrine glands.
Effects of main hormones:
- Somatotropic hormone (STH, growth hormone): stimulates protein synthesis.
- Insulin: facilitates the entry of amino acids into cells.
- Sex hormones (testosterone, estrogens): exert an anabolic effect, enhancing protein accumulation.
- Glucocorticoids: stimulate protein catabolism, increasing blood amino acid levels.
- Thyroxine (T4): activates general metabolism and accelerates protein turnover in cells.
Briefly: answers to topical questions
Which hormones exert an anabolic effect on protein metabolism?
An anabolic effect on protein metabolism is exerted by:
- Growth hormone (STH) — an anabolic hormone; increases amino acid entry into cells and enhances protein synthesis.
- Testosterone and estrogens — increase protein accumulation, exerting an anabolic effect.
Also, by source: insulin accelerates the transport of amino acids inside the cell, while thyroxine (T4) increases the intensity of protein metabolism.
Which proteolytic enzymes carry out protein digestion in the gastrointestinal tract?
Protein digestion in the gastrointestinal tract is carried out by endopeptidases and exopeptidases of the stomach and pancreas.
- Pepsins (Pepsin) — the main gastric enzymes (optimum pH 1.5–2.0) that cleave internal peptide bonds.
- Gastricsin — a gastric enzyme (optimum pH 3.2–3.5).
- Renin (chymosin) — curdles milk.
- Trypsin (Trypsin), chymotrypsin (Chymotrypsin), and elastase — pancreatic endopeptidases acting in the duodenum.
- Carboxypeptidases A and B — exopeptidases that cleave amino acids from the carboxyl end of the protein molecule.
Through which pathways does ammonia detoxification occur in the body?
Detoxification of toxic ammonia in the body occurs predominantly through its conversion into urea, as well as via the formation of ammonium salts.
- Urea synthesis — occurs exclusively in the liver via the ornithine cycle (Krebs-Henseleit cycle) involving mitochondrial and cytosolic enzymes.
- Formation of ammonium salts (ammonium ions) — carried out in the kidneys during amino acid deamination, after which they are excreted in the urine.
What nitrogen-containing molecules are formed during amino acid catabolism?
During amino acid catabolism, the following nitrogen-containing compounds are formed and excreted according to their sources:
- Ammonia ($NH_3$) and ammonium ions ($NH_4^+$) are formed during amino acid deamination in renal tissue and excreted in the urine.
- Urea is synthesized exclusively in the liver, serves as the end product of nitrogen metabolism, and excretes excess ammonia.
- Ammonium salts are formed in the kidneys and represent one of the forms of ammonia excretion.
Other nitrogen metabolism products excreted by the kidneys include creatinine and uric acid.
Class and exam questions
What is nitrogen balance and how to calculate it?
Nitrogen balance is the ratio between nitrogen intake from food and nitrogen excretion from the body. Since proteins contain about 16% nitrogen, the calculation is based on the formula: 1 g of excreted protein corresponds to the breakdown of 6.25 g of protein.
Are proteins stored in the human body?
No, proteins are not stored as reserves in the body. The only source for their replenishment is dietary intake or the synthesis of non-essential amino acids de novo.
What is the difference between essential and non-essential amino acids?
Non-essential amino acids can be synthesized by the body independently when they are deficient. Essential amino acids are not formed in tissues and must be obtained from food.
How do glucocorticoids affect protein metabolism?
Glucocorticoids enhance protein breakdown (catabolism) in tissues, leading to an increased concentration of free amino acids in blood plasma.
What else to study on this topic
- Mechanisms of amino acid transamination and amination
- The process of renal deamination and ammonia formation
- Features of protein metabolism in muscles and liver
- Pathophysiology of protein deficiency
- Detailed structure of complex proteins (lipoproteins, glycoproteins)
Master the entire topic in the "Physiology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Protein Metabolism" and 4,128 other subject pages
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 114 video lessons on the subject
Other topics in the "Metabolism and Energy" section
Free video lessons: PhysiologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.