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L-amino acids and D-amino acids are stereoisomers of amino acids, which have their own roles in nature, but are related to one another. L-amino acids are the precursors of proteins, indispensable to life; D-amino acids are a rare but special sort of biological entity. In this article, we will talk about the nature, role and uses of these two forms – what they do in biological systems, and how they could be useful in medicine and industry.
What are L Amino Acids?
L-amino acids are a group of L-stereo chemically related organic molecules which consist of an amino group (-NH2) and a carboxyl group (-COOH). They are the constituent amino acids of proteins, and they play many roles in biology, from the production of enzymes, hormones and antibodies.
There are two enantiomer classes of amino acids, L-form and D-form. Each protein in nature consists of L-amino acids, but rarely D-amino acids are present in life. L-amino acids are named "L" because their side chains or R groups lie to the left of carbon atoms and due to this stereo-chemistry they have a specific purpose in biochemical reactions.
In living things, L-amino acids are essential. They're the constituent proteins, but also play roles in cell organization, hormone release and growth factor regulation. Moreover, L-amino acids are very important for plants growth too, glutamic acid is one such amino acid of plants. In the human body, amino acids are divided into essential amino acids and non-essential amino acids. Essential amino acids are amino acids that cannot be synthesized by the body on its own and must be consumed through the diet. Non-essential amino acids are amino acids that the body can synthesize on its own. For example, L-lysine, L-tryptophan, and L-phenylalanine are all essential amino acids.
L-amino acids are indispensable molecules in living organisms, and they play an important role in maintaining the normal physiological function and health of living organisms.
L-Amino Acid Example
L-amino acids are a class of organic compounds containing amino and carboxyl groups with L-stereochemical properties. They are the basic building blocks of proteins and play a variety of key roles in living organisms. Some common L-amino acids include:
Glycine: It is one of the 20 naturally occurring non-polar side chain amino acids found in proteins.
Alanine: Also one of the 20 proteins, with polar side chains.
Lysine: is an alkaline amino acid commonly used to treat shingles caused by the herpes virus.
Phenylalanine: It is one of the essential amino acids involved in the synthesis of neurotransmitters and pigments.
Tryptophan: It has sedative and hypnotic effects and is a precursor of serotonin.
What are D Amino Acids?
D-amino acids are an enantiomer of amino acids, as opposed to L-amino acids. In nature, amino acids are usually found in the form of L-, while D-amino acids are less common, but they still play an important role in living organisms.
D-amino acids can be obtained in the human body through diet, bacterial metabolism, and endogenous synthesis. For example, D-serine and D-aspartate are two important D-amino acids that play a key role in neurotransmission, learning and memory, and regulating the immune system. D-amino acids can act as co-agonists or agonists of N-methyl-D-aspartate receptors (NMDA receptors) and are involved in the transmission of nerve signals and synaptic plasticity.
In microorganisms, D-amino acids such as D-serine and D-aspartic acid are important components of cell wall peptidoglycan and play a key role in biofilm formation and cell wall remodeling in bacteria. In addition, some D-amino acids have also been found to enhance the activity of antibiotics to fight bacterial infections.
Although D-amino acids are relatively undistributed in living organisms, recent studies have shown that they have potential applications in disease diagnosis and treatment. For example, abnormalities in D-amino acid levels have been linked to a variety of diseases such as Alzheimer's disease, Parkinson's disease, etc., so they can be used as biomarkers for early diagnosis of diseases.
Although D-amino acids are not as common as L-amino acids, they have shown unique value in physiological functions, disease mechanisms, and drug development.
D-Amino Acid Example
D-amino acids are a special type of amino acid in living organisms, and unlike common L-amino acids, they play an important role in many biological processes. Here are some examples of D-amino acids:
D-Serine: D-Serine is an important neurotransmitter that is synthesized and metabolized primarily in the mammalian brain. It regulates nerve signaling by activating the glycine-binding site of the NMDA receptor.
D-Aspartate: D-aspartate is present in the nervous and endocrine tissues of mammals and is involved in the synthesis and metabolism of neurotransmitters.
D-Alanine: This amino acid plays a key role in the synthesis of bacterial cell walls and is an important component of the outer membrane of bacteria.
D-Glutamate: Although less common, D-glutamate is also found in some organisms, especially in certain fermented foods.
D-Proline: This amino acid is also present in some organisms and has a specific biological function.
These D-amino acids have a variety of functions in different biological systems, including nerve signaling, maintenance of cell wall structure, and as biomarkers. Their presence and role reveal the importance and diversity of atypical amino acids in living organisms.
Difference Between D and L Amino Acids
L-amino acids and D-amino acids are stereoisomers of amino acids, differing in the spatial arrangement of their amino and carboxyl groups. L-amino acids are predominant in nature and essential for protein synthesis in all living organisms, serving as the building blocks of enzymes, hormones, and structural proteins. In contrast, D-amino acids are less common, mainly found in bacterial cell walls or specific physiological roles such as neural signaling. Despite their structural similarity, their biological functions and metabolic pathways are distinct.
| Aspect | L-Amino Acids | D-Amino Acids |
|---|---|---|
| Prevalence in Nature | Commonly found in all living organisms (animals, plants, fungi, bacteria). | Relatively rare, mainly found in bacterial cell walls or as components of some pathogens. |
| Role in Protein Synthesis | Essential for protein synthesis in living organisms. | Not involved in protein synthesis but plays roles in other biological processes. |
| Biological Functions | Functions as enzymes, hormones, and regulators of physiological processes. | Involved in processes such as NMDA receptor co-activation, immune regulation, and antibacterial activity. |
| Metabolic Pathways | Stable and efficient metabolic pathways, are widely utilized in biological systems. | Metabolized through specialized enzymes, with less stable pathways compared to L-amino acids. |
| Unique Examples | All proteinogenic amino acids (e.g., L-lysine, L-alanine). | D-serine acts as an NMDA receptor co-agonist in neural signaling. |
| Biological Significance | Core components of life are essential for cellular functions. | Play unique roles in specific physiological and pathological processes. |
D and L Amino Acid Structure
The main difference between D-type and L-type amino acids in chemical structure is the difference in the configuration of their chiral central carbon atoms. This difference is reflected in their spatial structure and their biological functions. To better understand this, we can analyze the chirality of molecules, their configurations, and their role in biological systems.
L-Amino Acid Structure
L-amino acids are the basic units for the synthesis of proteins in living organisms. Each L-type amino acid molecule contains an α-carbon atom, and this α-carbon atom is attached to four different groups, namely an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a side chain (R group). Among them, the α-carbon atom is the chiral center, meaning it is structurally different from other amino acids and compounds. Thus, L-amino acids have a specific spatial structure.
The side chain (R group) of L-amino acids is an important factor in distinguishing different amino acids. Different side chains determine the nature and function of amino acids. For example, phenylalanine has a benzene ring structure, while glutamic acid carries a carboxyl side chain, which makes their role in proteins different.
Fig.1 The conversion between different L-amino acids. (Lu, Jianguang, et al., 2020)
Table.1 Related amino acid products at Creative Peptides.
| Product Name | M.W | Molecular Formula | Price |
|---|---|---|---|
| L-Serine | 105.09 | C3H7NO3 | Inquiry |
| D-Serine | 105.09 | C3H7NO3 | Inquiry |
| L-Phenylalanine | 165.19 | C9H11NO2 | Inquiry |
| L-Glutamic acid | 147.13 | C5H9NO4 | Inquiry |
| D-Aspartic acid | 133.1 | C4H7NO4 | Inquiry |
| D-Serine | 105.09 | C3H7NO3 | Inquiry |
D-Amino Acid Structure
In contrast to L-amino acids, D-amino acids are its mirror isomers. The amino and carboxyl groups on the α-carbon atom of D-type amino acids are in opposite positions, i.e., in D-type amino acids, the amino group (-NH2) is on the right and the carboxyl group (-COOH) is on the left. Although L-type amino acids are much more common than D-type amino acids in nature, D-type amino acids still have important biological significance in some special environments and organisms.
Fig.2 Structural characteristics of D- and L-Alaine. (Marchetti, Louis., 2015)
L and D Stereochemistry of Amino Acids
The difference between D-type and L-type amino acids is essentially their stereochemical configuration. With the Fischer projection method, we can clearly represent the chirality of amino acid molecules. For example, the amino group of L-amino acids is usually located to the left of the α-carbon atom, while the opposite is true for D-amino acids. In the Fischer projection method, this difference in configuration can be described by the configuration of chiral centers.
The Fischer projection method allows us to visualize the three-dimensional structure of the molecule by unfolding the molecule and representing the chemical bonds in the molecule with a straight line. In this way, the spatial configurations of D-type and L-type amino acids are clearly distinguished.
D and L Amino Acids Fischer Projection
In the Fischer projection method, the difference between D-type and L-type amino acids is mainly reflected in the position of their chiral centers (α-carbon). The Fischer projection method represents each chiral center in the molecule as a crossover structure, where the vertical lines represent the bonds in the molecule pointing to the paper surface, while the horizontal lines represent the bonds pointing outside the paper surface. For amino acids, α-carbon is linked to an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a side chain (R group).
In the Fischer projection method, the amino group of the L-type amino acid (-NH2) is located to the left of the α-carbon, while the amino group of the D-type amino acid is located on the right side. For most amino acids, α-carbon is a chiral center, so they have two mirror isomers, the L and D forms. It is important to note that with the exception of glycine, it does not have a chiral center, so there is no distinction between L and D forms. Through the Fischer projection method, researchers can visually represent the chiral configuration of amino acids, which is essential for analyzing their biological activity and structural function.
Fig.3 D and L amino acids in Fischer projection.
How to Determine D and L Configuration of Amino Acids?
Determining the configuration of D- and L-form amino acids usually requires analysis of their spatial structure. In practice, we can determine the chiral configuration of amino acid solutions by measuring their optical rotation with a polarimeter. L-form amino acids are usually left-handed, while D-type amino acids are right-handed.
In addition to experimental measurements, the three-dimensional structure of amino acids can also be determined by chemical methods such as X-ray crystallography and nuclear magnetic resonance (NMR) techniques. These methods are able to provide precise molecular structure information that can help us distinguish between D-type and L-type amino acids.
L or D Amino Acids in Humans
In humans, both L-amino acids and D-amino acids are present, but their sources, functions, and metabolic pathways differ.
L-Amino Acids in Humans
L-amino acids play an important role in the human body and are the basic building blocks of proteins. The human body needs 20 amino acids, 9 of which are essential amino acids, which must be consumed through the diet because the body cannot synthesize them on its own.
L-amino acids are not only involved in protein synthesis but also play a role in a variety of physiological processes. For example, L-tryptophan helps with sleep and mood regulation, while L-lysine is associated with immune function and wound healing. In addition, L-amino acids also have an effect on glucose metabolism, such as certain amino acids such as L-arginine and L-leucine can affect blood sugar levels by regulating the insulin response.
In the diet, the intake of L-amino acids should be adjusted according to the needs of the individual, especially for athletes or people with special health needs, who may need additional supplementation to meet their higher needs. In addition, the absorption and utilization of amino acids are also affected by their stereoisomers (D-type), but the body mainly utilizes L-form amino acids.
L-amino acids play a key role in maintaining human health, supporting growth and development, and regulating metabolism, so they need to be paid enough attention in daily diet and nutritional supplementation.
Fig.4 Metabolism of D-amino acids in the human body. (Murtas, Giulia, and Loredano Pollegioni., 2023)
Table.2 Peptide modification services at Creative Peptides.
| Service Name | Price |
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| PNA Backbone Modification | Inquiry |
| PNA Nucleobase Modification | Inquiry |
| Custom Conjugation Service | Inquiry |
| Peptide N-Terminal Modification | Inquiry |
| Peptide C-Terminal Modification | Inquiry |
| Biotinylated Peptides | Inquiry |
| Peptide PEGylation | Inquiry |
| Fluorescence and Dye Labeling Peptides | Inquiry |
| Post-translational Modification | Inquiry |
| Custom Labeling | Inquiry |
| Peptide Tags | Inquiry |
| Unusual and Non-natural Amino Acids Modification | Inquiry |
| Peptide Lipidation | Inquiry |
| Cyclic Peptides Synthesis | Inquiry |
D-Amino Acids in Humans
D-amino acids have a variety of important physiological functions and potential clinical applications in the human body. The following is a detailed description of the role and research progress of D-amino acids in humans:
Sources and metabolism: D-amino acids are mainly derived from diet, intestinal bacteria, and synthesis in the body (e.g., D-serine and D-aspartic acid). They enter the body through dietary ingestion or microbial metabolism and are metabolized in the body by specific enzymes such as D-amino acid oxidase.
Physiological function: In the central nervous system, D-amino acids such as D-serine (D-Ser) and D-aspartic acid (D-Asp) play an important role. D-Ser is a potent co-agonist of the N-methyl-D-aspartate receptor (NMDA receptor) and is involved in neurotransmission and synaptic plasticity. In addition, D-amino acids also play a role in the endocrine system, such as regulating testosterone synthesis and pituitary hormone release.
Disease-related changes: D-amino acid levels are associated with a variety of diseases. For example, decreased D-serine levels have been linked to neuropsychiatric disorders such as schizophrenia, Alzheimer's disease, and amyotrophic lateral sclerosis. In addition, the levels of D-amino acids in patients with prediabetes and type 2 diabetes mellitus also changed, suggesting that it may have potential diagnostic value in metabolic diseases.
As biomarkers: D-amino acids are considered potential biomarkers due to their concentration changes in different pathological states. For example, D-serine has been studied as a biomarker for schizophrenia, while D-aspartic acid may be used to predict early cognitive decline.
Other physiologic effects: In kidney disease, abnormal metabolism of D-amino acids is associated with disease progression. In addition, D-amino acids also play a role in antioxidants, reproductive health, hypertension, etc., showing a wide range of physiological functions.
Research challenges and future directions: Although the importance of D-amino acids is gaining increasing attention, the specific mechanism of D-amino acids in humans still needs further research. Current research methods need to be standardized and automated to improve the accuracy and effectiveness of detection.
L or D Amino Acids in Nature
In nature, amino acids can exist in the form of D-isomers or L-isomers, whose chirality is determined by the arrangement of α carbon atoms in the molecule. L-amino acids are the most commonly found form of protein synthesis, while D-amino acids, although less abundant in living organisms, still have specific biological functions in microorganisms, plants, mammals, and humans.
L-Amino Acids in Nature
L-amino acids exist widely in nature and are the basic units of protein synthesis. According to the evidence, L-amino acids are organic compounds containing amino (-NH2) and carboxylic acid groups (-COOH), and have L-stereochemical properties. In nature, most amino acids exist in the L- form because their alpha carbon is chiral, and all amino acids except glycine can exist as either L or D enantiomers.
In living organisms, L-amino acids are the main building blocks of proteins, and there are 20 naturally occurring L-amino acids. These amino acids play a crucial role in the synthesis of proteins, enzymes and hormones. For example, L-amino acids such as glycine, alanine, lysine, and phenylalanine are common components of proteins.
Although L-amino acids are commonly found in nature, it has also been suggested that D-amino acids can be found in certain environments, such as a defense mechanism in some bacteria. Overall, however, L-amino acids remain the most dominant form found in nature.
L-amino acids not only play an important role in living organisms, but also have a wide range of applications in industry and medicine. For example, L-amino acids can be produced on a large scale through fermentation methods for the production of various bioactive molecules. In addition, L-amino acids also play an important role in plant growth, such as improving plant resistance to temperature and pressure and improving water and nutrient absorption efficiency.
L-amino acids are widely distributed in nature and are an integral part of living organisms. They not only participate in a variety of biochemical processes in living organisms, but also have important applications in industry and agriculture.
D-Amino Acids in Nature
D-amino acids exist widely in nature and play important biological functions in a variety of organisms. Although L-amino acids are the most common form found in nature, D-amino acids also have significant physiological effects in specific environments.
Distribution and source: D-amino acids exist not only in bacteria, but also widely in plant, animal and human tissues. For example, in the brain and neuroendocrine tissues of mammals, D-serine (D-Ser) and D-aspartate (D-Asp) are present in higher concentrations. In addition, D-amino acids are also found in plants, such as ginkgo biloba, maple and redwood trees, D-amino acids have been detected in the leaves.
Biological function: In mammals, D-amino acids such as D-serine and D-aspartate are involved in the regulation of neurotransmitters, possibly acting as endogenous ligands for N-methyl-D-aspartate receptors (NMDA receptors). The high concentration of these amino acids in the brain suggests that they may play an important role in nerve signaling. In plants, D-amino acids are involved in various physiological processes, including plant growth and development. For example, D-amino acids are found in plant root secretions and may influence interactions between plants and rhizosphere microbes.
Medical and nutritional applications: D-amino acids have potential applications in the medical field. For example, D-glutamic acid (D-Glu) and D-aspartate (D-Asp) have shown some potential in cancer treatment and renal function protection. In the food industry, D-amino acids are also used to improve the flavor and nutritional value of foods. For example, during fermentation, lactic acid bacteria can produce D-amino acids, which can affect the flavor of food.
Ecological and environmental effects: In complex ecosystems, some microorganisms regulate their living environment by secreting D-amino acids. For example, certain bacteria use D-amino acids as a defense mechanism to inhibit the growth of other microorganisms.
Research Challenges and future directions: Although the importance of D-amino acids has received increasing attention, further research on their specific functions and metabolic pathways is still needed. For example, the specific mechanism of action of D-amino acids in plants is not fully understood at present, and more experimental studies are needed to reveal its role in plant physiological processes.
D-amino acids play a variety of important roles in nature, from nerve signaling to plant growth regulation to applications in medicine and the food industry, and their research provides new perspectives for understanding life processes. However, due to its relatively low abundance and complex metabolic pathways, related research remains challenging.
Summary
D-type and L-type amino acids have different functions in living organisms, with L-form being the main building block of proteins, while D-type amino acids have unique roles in neurotransmission, microbial cell walls, and certain diseases such as Alzheimer's disease. In-depth study of D-type amino acids can help to understand their role in physiology and pathology, and provide new research directions and treatment strategies for disease diagnosis, treatment, and new drug development.
References
- Marchetti, Louis. Regioselective carbon-nitrogen bond formation via oxidative transition metal catalysis. (2015).
- Lu, Jianguang, et al., D-and unnatural amino acid substituted antimicrobial peptides with improved proteolytic resistance and their proteolytic degradation characteristics. Frontiers in microbiology 11 (2020): 563030.
- Murtas, Giulia, and Loredano Pollegioni. D-Amino Acids and Cancer: Friends or Foes? International Journal of Molecular Sciences 24.4 (2023): 3274.
