D- and L-amino acids are a type of stereoisomer called enantiomers. They are mirror images of one another—they have the same molecular formula and connectivity, but the spatial arrangement of their atoms is different. Both L- and D-amino acids are naturally occurring amino acids, but the L-form plays an integral role in protein synthesis, whereas the D-form does not.
Quick Look
- Amino acids are categorized as essential, non-essential, and conditionally essential.
- Every amino acid contains four groups: a single bonded hydrogen, an amino group (NH2), a carboxyl group (COOH), and a unique R-group (side chain).
- Isomers are molecules with the same molecular formulas divided into structural isomers and stereoisomers.
- Stereoisomers have the same molecular formula and connectivity, but the atoms have different spatial arrangements—they are mirror images of one another and are broken down into L-amino acids and D-amino acids.
What Are Amino Acids?
Amino acids are organic compounds that serve as the building blocks for proteins; amino acids and proteins are the building blocks of life. Both animal and plant proteins are produced by combinations of 20 amino acids—nine essential and eleven non-essential or conditionally essential.
Amino acids are essential for synthesizing body proteins and other important nitrogen-containing compounds, including creatine, peptide hormones, and some neurotransmitters.
Protein and nitrogenous compounds are constantly in a state of turnover—they are degraded and resynthesized continuously. However, the protein turnover rate is several times higher than what’s consumed, suggesting that the reutilization of amino acids is an important feature of protein metabolism. But, this process isn’t completely efficient, and some amino acids are lost via oxidative catabolism. Other metabolic products of amino acids–urea, creatinine, uric acid, and other nitrogenous products–are excreted through urine; nitrogen is also eliminated from the body through feces and sweat, along with sloughed hair, skin, and nails. A continuous supply of dietary amino acids is required to replace these losses, even after growth has stopped.
If you consume amino acids in excess of what’s needed for the synthesis of nitrogenous tissue constituents, they’re not stored but degraded:
- Nitrogen is excreted as urea.
- Keto acids (minus the amino group) are utilized directly for energy or converted to carbohydrates or fat.
Here’s a breakdown of the 20 amino acids and their respective categories:
| Essential | Non-essential | Conditionally essential |
|---|---|---|
| Phenylalanine | Alanine | Arginine |
| Valine | Aspartic acid (aspartate) | Cysteine |
| Tryptophan | Asparagine | Glutamine |
| Threonine | Glutamic acid (glutamate) | Glycine |
| Isoleucine | Selenocysteine | Proline |
| Methionine | Serine | |
| Histidine | Tyrosine | |
| Leucine | ||
| Lysine |
- Essential amino acids (EAAs): Amino acids that cannot be synthesized in the body and are dietarily essential (or through supplementation).
- Non-essential amino acids: Amino acids that can be synthesized from other essential amino acids in the body.
- Conditionally essential amino acids: Amino acids that cannot be synthesized in the body in sufficient quantities during specific physiological periods of growth, such as pregnancy, adolescent development, or trauma recovery. In these situations, they are dietarily essential.
Key takeaway: Amino acids are the building blocks of proteins. The body requires the nine essential amino acids to come from diet or supplementation, but the non-essential and conditionally essential amino acids can be synthesized in the body.
What Do Amino Acids Do?
Most amino acids are proteinogenic, meaning they’re used to synthesize functional proteins. As we mentioned, they’re also used to synthesize other important nitrogen-containing compounds, including creatine, peptide hormones, and neurotransmitters.
Here’s a breakdown of what each of the nine EAAs does:
- Phenylalanine: Phenylalanine is a precursor for synthesizing catecholamines (dopamine, epinephrine, norepinephrine) and melanin. It’s also a precursor for the synthesis of the amino acid L-tyrosine.
- Valine: Valine is one of the branched-chain amino acids (BCAAs) involved in muscle growth and tissue regeneration. It is also essential for energy production.
- Threonine: Threonine is an integral component of structural proteins, such as collagen and elastin, which provide structure to the skin and connective tissues. It also serves as a substrate for protein synthesis, particularly mucin.
- Tryptophan: Tryptophan is the sole precursor to synthesizing serotonin, which regulates mood, appetite, behavior, cognition, and sleep.
- Methionine: Methionine is a sulfur-containing amino acid essential for tissue growth, metabolism, and detoxification, along with the absorption of essential minerals, such as zinc and selenium. It also serves as a precursor to several important compounds, including succinyl-CoA, homocysteine, cysteine, creatine, and carnitine.
- Leucine: Along with valine, leucine is heavily involved in protein synthesis, muscle growth, and repair. It’s also needed for wound healing, blood sugar regulation, and synthesis of growth hormones.
- Isoleucine: Isoleucine is a BCAA involved in muscle metabolism, immunity, hemoglobin synthesis, and energy regulation.
- Lysine: Lysine is important for synthesizing protein, enzymes, hormones (collagen and elastin), calcium absorption, energy production, and proper immune function.
- Histidine: Histidine is required for protein synthesis and plays especially important roles in the active sites of various enzymes. It’s also required for maintaining the myelin sheath of the nervous system and the synthesis of histamine, which is involved in immune function, digestion, sleep, and sexual function.
Structure of Amino Acids
Understanding the difference between D- and L-amino acids hinges on first understanding the structure of amino acids.
First off, it’s important to note that all amino acids have the same foundational structure. Every amino acid is composed of three functional groups:
- Amino group (NH3)
- Carboxylic acid group (-COOH)
- R-group (side chain)
When you add an amino group together with a carboxyl (acid) group, you form an amino acid.
Here’s a visual breakdown of the structure of amino acids:
The R-group or side chain varies based on the amino acid. For example, glycine is the simplest and smallest amino acid, having a lone hydrogen atom as the R-group. The R-group on methionine, however, is C3H7S, making a larger and more complex amino acid.
Check out the image below to see the difference:
These R-groups are what give each individual amino acid their unique characteristics. It’s why glycine supports sleep, mood, and memory, while methionine has antioxidant properties and supports detoxification.
Key takeaway: All 20 amino acids have the same basic structure—they contain an amino group (NH3), a carboxylic acid group (-COOH), and a unique R-group (side chain). The R-group is what differentiates amino acids and gives them their unique properties.
Isomers and Stereoisomers
Now that you understand the basic structure of amino acids, it’s a bit easier to understand what we’re talking about when we say every amino acid (except glycine) has a stereoisomer. But before understanding the actual difference between D- and L-amino acids, it’s time for a quick chemistry lesson about isomers.
There are two types of isomers:
- Structural (constitutional) isomers: Molecules that have the same molecular formula but different bonding configurations among atoms.
- Stereoisomers: They have the same molecular formulas and atom arrangements. They only differ in the spatial orientation of groups within the molecule.
Here’s a visual representation:
As you can see, cis-2-butene and trans-2-butene have the same molecular formula and atom arrangements, but the circled groups differ in spatial arrangement, making them stereoisomers. On the other hand, 1-butene has the same molecular formula but different atom arrangements, making it a structural or constitutional isomer of cis-2-butene and trans-2-butene.
The chart below breaks down the types of stereoisomers and how they relate.
D- vs L-Amino Acids: Stereoisomers
Now that you understand what isomers are, it’s important to know that all amino acids except glycine are stereoisomers. What does this mean? Amino acids have mirror images of their structure. Just like we have a left and right hand that are, in most cases, mirror images of each other, amino acids have the same, but they are called L (left-handed) and D (right-handed) to differentiate between the mirror images. These are a branch of stereoisomers called enantiomers.
Let’s look at an example of D-tryptophan vs L-tryptophan.
The L and the D refer to the order of the side chain structures attached to an amino acid’s central carbon atom. As previously mentioned, those groups are:
- Single hydrogen atom (H)
- Carboxyl group (COOH)
- Amine group (NH2)
- R-group
Let’s break down the L- and D-amino acids further.
- “L” Stands for “Levorotation”: For your eyes to read the amino acid correctly (in alphabetical color order), the molecule itself has to rotate counterclockwise or to the left. Levorotation refers explicitly to that counterclockwise rotation.
- “D” Stands for “Dextrorotation”: For your eyes to read the amino acid (in alphabetical color order), the molecule has to spin clockwise or to the right. The prefix of dextrorotation is derived from the Latin word dextro, meaning “to the right.”
So, how do you remember which is which?
L stands for left-moving—the molecule rotates to the left so you can read the structure forward, to the right. D stands for your ‘dextrous’ right hand—the molecule spins to the right, so you are reading its structure counterclockwise, backward to the left.
Key takeaway: Most amino acids exist in two forms, D- and L-amino acids. These are considered stereoisomers because they have the same molecular formula and connectivity but different spatial arrangements. The D- and L-amino acids are mirror images of each other, but their functions differ.
D- or L-Amino Acids: What Do They Do?
To give you an idea of how D and L conformations function in nature, here’s a short comparison chart:
| Similarities | Differences |
|---|---|
| They are one of two forms amino acids can take in nature | L stands for left-moving—the molecule spins to the left; D stands for your dextrous right hand—the molecule rotates to the right |
| They are mirror images of each other | L-amino acids are used in protein synthesis; D-amino acids are not used in protein synthesis |
| They each contain a central carbon connected to a single hydrogen, an amino group, a carboxyl group, and an R-group (side chain) | L-amino acids play a variety of roles in the body; D-amino acids are found in bacterial cell walls |
When it comes to functionality, there’s a big difference between D- and L-amino acids. L-amino acids comprise the 20 essential and non-essential amino acids involved in protein synthesis, hormone and enzyme production, and regulating various bodily functions. D-amino acids, however, are not incorporated into proteins. That said, D-serine does act as a neurotransmitter in the brain.
Wrap Up
All 20 amino acids (except glycine) are stereoisomers, meaning the compounds have a mirror image. They have the same molecular formula and group connectivity, but the spatial arrangement of the atoms within the molecule differs—we call these the D- and L-forms. Because of this, they serve different functions in the body. But remember, whether you’re chowing down on a beef burger or stirring a scoop of essential amino acid powder into your water, it’s likely to be the L-form.
