Biological functions of proteins
Protein molecules perform diverse tasks, ensuring the proper functioning of tissues and organs:
- Transport: delivery of oxygen, lipids, carbohydrates, and nutrients by erythrocyte and plasma proteins.
- Contractile: movement of muscular structures.
- Structural (Supportive): the structural basis of bone and cartilage tissue.
- Catalytic: acceleration of biochemical reactions by enzymes.
- Protective: immune response mediated by antibodies.
- Antitoxic: detoxification of toxins in the liver.
- Blood clotting: hemostasis involving plasma proteins and platelets.
Amino acid classification and protein value
Dietary proteins are divided into non-essential (synthesized by the body) and essential (must be obtained from the diet). There are also conditionally essential amino acids whose synthesis is limited during stress and pregnancy (arginine, histidine, tyrosine, cysteine).
Biological value is assessed by the amino acid score—the percentage ratio of an essential amino acid in a food product to its content in the reference "ideal protein". The limiting factor is the amino acid with the lowest score; plant proteins are most commonly limited by lysine, threonine, and isoleucine.
Metabolism and the role of the liver
The central link of metabolism is the blood amino acid pool. Tissue proteins are constantly renewed through ATP-dependent mechanisms involving ubiquitin and 26S proteasomes. Released amino acids are partially reutilized.
The liver plays a key role in protein metabolism:
- Synthesizes albumins (95%), fibrinogen (100%), and globulins (85%).
- Performs deamination and transamination of amino acids.
- Neutralizes ammonia via the synthesis of urea.
- Produces vitamin K-dependent blood clotting factors.
Mnemonic phrases are used to memorize essential amino acids, but it is more reliable to remember the reference composition of the "ideal protein," where leucine (70 mg) and aromatic compounds (60 mg) predominate.
Class and exam questions
Why are there no reserve protein depots in the body?
Unlike carbohydrates and lipids, proteins perform structural, catalytic, and regulatory functions within tissues and lack specialized fat or glycogen depots. Therefore, during dietary deficiency, the body begins to catabolize functional protein structures.
What is an amino acid score and why is it needed?
This is an indicator of the biological value of a protein, reflecting the percentage of a specific essential amino acid in a product relative to the reference "ideal protein." The limiting amino acid is determined by the lowest score.
How does the liver participate in ammonia detoxification?
The liver converts toxic ammonia, formed during the deamination of amino acids, into safe urea via the urea cycle (ornithine cycle), after which the end product is excreted by the kidneys.
What else to study on this topic
- Ubiquitin-proteasome protein degradation system
- The ornithine cycle of urea synthesis in the liver
- Classification of simple proteins: protamines, histones, albumins, and globulins
- Hormonal regulation of protein metabolism (hyperthyroidism, hypercorticism, estrogens)
- Impact of protein metabolism on acid-base balance (ABB)
Master the entire topic in the "Biochemistry" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Biochemistry of Proteins and Amino Acids" and 3,616 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 115 video lessons on the subject
Free video lessons: biochemistryShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.