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Significance and Classification of Amino Acids

For medical students2 min readUpdated 2026-10-10

Amino acids are the fundamental structural blocks used to build all human body proteins and serve as precursors for crucial neurotransmitters. Our body proteins contain 20 $\alpha$-amino acids that constantly participate in synthesis and degradation processes, maintaining a dynamic metabolic equilibrium.

Daily Dietary IntakeAn average person needs to consume about 100 g of dietary protein daily.

Turnover RateApproximately 400 g of protein is degraded and resynthesized in the body every day.

Neurotransmitter RoleCertain free amino acids (e.g., glycine, glutamate) function directly as neurotransmitters.

Total Body MassThe total protein content in an adult human body is about 15 kg.

General Significance and Protein Turnover

Dietary proteins are the primary source of building blocks for tissues. The human body contains approximately 15 kilograms of protein structures. These are not static: cells maintain a constant dynamic equilibrium. About 400 grams of protein are degraded and resynthesized daily. The turnover rate varies significantly depending on the tissue type. For instance, collagen, the backbone of connective tissue, is completely renewed over 300 days. Conversely, blood clotting proteins may be replaced within minutes to a couple of days.

In addition to bound proteins, the body maintains a free amino acid pool with a mass of about 35 grams. Most of this pool is directed toward synthesizing new protein molecules. Furthermore, free molecules serve as raw materials for producing biologically active compounds, such as biogenic amines (neurotransmitters).

Classification by Synthesis Capability (Essentiality)

Human proteins contain 20 $\alpha$-amino acids. Based on whether the body can synthesize them independently, they are divided into three main groups:

Non-essential. Synthesized by human enzyme systems in adequate 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 (semi-essential). Can be synthesized by the body, but the rate of synthesis is extremely low. During periods of active growth in children, this amount is insufficient to cover physiological needs. These include arginine and histidine. Additionally, tyrosine (derived from phenylalanine) and cysteine (synthesized from methionine) are considered conditionally essential.

Nutritional Value and the Limiting Amino Acid

The biological value of any protein source 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 the cells, normal synthesis of endogenous proteins is impaired and halted.

Classification by Carbon Skeleton Fate

After the removal of the amino group, a carbon skeleton (nitrogen-free residue) remains. The further metabolic fate of this skeleton depends on the molecular structure. There are three classes:

Group	Characteristics	Representatives
Glucogenic	Converted into pyruvate or citric acid cycle intermediates (fumarate, succinyl-CoA, $\alpha$-ketoglutarate, oxaloacetate). Serve as substrates for gluconeogenesis.	Alanine, arginine, aspartate, asparagine, valine, histidine, glycine, glutamate, glutamine, proline, serine, methionine, threonine, cysteine
Glucoketogenic (Mixed)	Catabolized to yield both gluconeogenesis metabolites and ketone bodies (acetoacetate, acetyl-CoA).	Tyrosine, tryptophan, phenylalanine, isoleucine
Ketogenic	Catabolized exclusively into acetyl-CoA or acetoacetyl-CoA (ketone body precursors).	Lysine, leucine

Frequently asked questions

Which free amino acids can function as neurotransmitters?

Several free amino acids 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) — the most abundant inhibitory neurotransmitter in the CNS;
Glycine — an inhibitory neurotransmitter for neurons in the brainstem and spinal cord.

Which enzymes and coenzymes are involved in the biosynthesis of non-essential amino acids?

Biosynthesis of non-essential amino acids via transamination involves aminotransferases (e.g., alanine aminotransferase and aspartate aminotransferase). Their coenzyme is pyridoxal phosphate (the active form of vitamin B6).

For specific amino acids:

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 general catabolic pathway intermediates via transamination reactions involving glutamate.

Alanine is synthesized from pyruvate via alanine aminotransferase (ALT).
Aspartate is synthesized from oxaloacetate via aspartate aminotransferase (AST).

What happens if a diet lacks a single essential amino acid?

The limiting amino acid rule applies. The absence of even one essential component completely disrupts endogenous protein synthesis in the body.

Why does the body need a free amino acid pool?

These molecules (about 35 g) are primarily used to assemble new proteins. A portion of the pool is also utilized to synthesize biogenic amines and act directly as neurotransmitters.

Why are arginine and histidine classified as conditionally essential?

Their endogenous synthesis is very slow. While adults may produce enough to meet their needs, growing children require additional dietary intake of these substances.

How do ketogenic amino acids differ from glucogenic ones?

The carbon skeleton of ketogenic amino acids is converted solely into ketone body precursors (acetyl-CoA), whereas the carbon skeleton of glucogenic amino acids is used for gluconeogenesis via pyruvate and TCA cycle intermediates.

Go deeper

Mechanisms of biogenic amine formation and their functions in the nervous system.
Negative and positive nitrogen balance.
The citric acid cycle (TCA) and pathways of carbon skeleton incorporation.
Stages of gluconeogenesis from glucogenic amino acids.
Synthesis pathways of conditionally essential amino acids (tyrosine and cysteine).