General significance and protein metabolism
Dietary proteins serve as the primary source of building materials for tissues. The human body contains approximately 15 kilograms of protein structures. However, these are not static: cells maintain a constant dynamic equilibrium. About 400 grams of protein are degraded and resynthesized every day. The rate of this turnover varies greatly depending on the tissue type. For example, collagen, the main component of connective tissue, is completely renewed in 300 days. At the same time, blood clotting proteins can be replaced within a timeframe ranging from a few minutes to a couple of days.
In addition to the bound state, the body maintains a pool of free amino acids with a mass of about 35 grams. Most of this pool is directed toward creating new protein molecules. Furthermore, free molecules serve as raw materials for producing biologically active compounds—biogenic amines (neurotransmitters).
Classification by synthesis capacity (essentiality)
Human proteins contain 20 $\alpha$-amino acids. Depending on whether the body can synthesize them independently, they are divided into three main groups:
- Non-essential. Synthesized by human enzyme systems in the required amounts. These include: alanine, asparagine, aspartic acid, glutamine, glutamic acid, glycine, proline, and serine.
- Essential. Cannot be synthesized by our cells at all and must be obtained from the diet. This group includes: valine, isoleucine, leucine, lysine, methionine, threonine, tryptophan, and phenylalanine.
- Conditionally essential. Can be produced in the body, but the rate of this process is extremely low. During periods of active growth in children, this amount is insufficient to cover physiological needs. This group includes arginine and histidine. Tyrosine (formed from phenylalanine) and cysteine (synthesized from methionine) are also classified as conditionally essential.
Nutritional value and the limiting amino acid
The biological value of any protein food is determined by its composition, specifically the presence of essential components. Biochemistry strictly follows the limiting amino acid rule: if even a single essential amino acid is missing in cells, the normal synthesis of the body's own proteins is disrupted and halts.
Classification by carbon skeleton utilization
After the removal of the amino group, a carbon skeleton (alpha-keto acid) remains. The further metabolic fate of this skeleton depends on the molecular structure. There are three classes:
| Group | Characteristic | Representatives |
|---|---|---|
| Glucogenic | Converted into pyruvate or tricarboxylic acid cycle intermediates (fumarate, succinyl-CoA, $\alpha$-ketoglutarate, oxaloacetate). They serve as substrates for gluconeogenesis. | Alanine, arginine, aspartate, asparagine, valine, histidine, glycine, glutamate, glutamine, proline, serine, methionine, threonine, cysteine |
| Glucogenic-ketogenic (mixed) | Degraded to yield both gluconeogenesis metabolites and ketone bodies (acetoacetate, acetyl-CoA). | Tyrosine, tryptophan, phenylalanine, isoleucine |
| Ketogenic | Catabolized exclusively to acetyl-CoA or acetoacetyl-CoA (precursors of ketone bodies). | Lysine, leucine |
Briefly: answers to topical questions
Which free amino acids can function as neurotransmitters?
Several free amino acids can function as neurotransmitters in the central nervous system. Excitatory neurotransmitters include:
- Glutamate (glutamic acid) — acts on specific receptors (e.g., NMDA);
- Aspartate (aspartic acid).
Inhibitory neurotransmitters include:
- GABA (gamma-aminobutyric acid) is the most abundant inhibitory neurotransmitter in the central nervous system;
- Glycine — an inhibitory neurotransmitter for neurons in the medulla oblongata and spinal cord.
Which enzymes and coenzymes are involved in the biosynthesis of non-essential amino acids?
The biosynthesis of non-essential amino acids via transamination involves aminotransferase enzymes (e.g., alanine aminotransferase and aspartate aminotransferase). Their coenzyme is pyridoxal phosphate (the active form of vitamin B6).
For specific amino acids, sources indicate:
- For glycine: the enzyme serine hydroxymethyltransferase, with tetrahydrofolate as the coenzyme.
- For conditionally essential tyrosine: the enzyme phenylalanine hydroxylase.
From which carbohydrate metabolism intermediates are alanine and aspartate synthesized?
Alanine and aspartate are synthesized from intermediates of the general catabolic pathways via transamination reactions involving glutamate.
- Alanine is synthesized from pyruvate with the participation of the enzyme alanine aminotransferase (ALT).
- Aspartate is synthesized from oxaloacetate with the participation of the enzyme aspartate aminotransferase (AST).
Class and exam questions
What happens if an essential amino acid is missing from the diet?
The limiting amino acid rule will apply. The absence of even a single essential component will completely disrupt the synthesis of the body's own proteins.
Why does the body need a pool of free amino acids?
These molecules (about 35 g) are used primarily for assembling new proteins. Part of the pool also goes toward synthesizing biogenic amines and acts independently as neurotransmitters.
Why are arginine and histidine classified into a separate group of conditionally essential amino acids?
Their internal synthesis is very slow. This volume may be sufficient for an adult, but a growing child's body requires an additional dietary intake of these substances.
How do ketogenic amino acids differ from glucogenic ones?
The carbon skeleton of ketogenic amino acids is converted exclusively into ketone body precursors (acetyl-CoA), whereas the carbon skeleton of glucogenic amino acids is used for gluconeogenesis via pyruvate and TCA cycle intermediates.
What else to study on this topic
- Mechanisms of biogenic amine formation and their functions in the nervous system.
- Negative and positive nitrogen balance.
- The tricarboxylic acid cycle (TCA) and pathways of carbon skeleton incorporation.
- Stages of gluconeogenesis from glucogenic amino acids.
- Processes of synthesis of conditionally essential amino acids (tyrosine and cysteine).
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 "Significance and Classification of Amino Acids" and 3,616 more subject pages
- 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
Other topics in the "Amino acid metabolism" section
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.