Serine is an amino acid that is one of the twenty natural amino acids and is non-essential. The D form of serine acts as a co-agonist in neuronal signal transduction and may play a role in various mental disorders.
What is serine?
Serine is an amino acid with the structural formula H2C(OH)-CH(NH2)-COOH. It occurs in the L-form and is one of the non-essential amino acids, as the human body can produce it itself. Serine owes its name to the Latin word “sericum”, which means “silk”. Silk can serve as a raw material for serine by technically processing the silk glue sericin. Like all amino acids, serine has a characteristic structure. The carboxyl group consists of the atomic sequence carbon, oxygen, oxygen, hydrogen (COOH); the carboxyl group reacts acidically when an H+ ion is split off. The second atomic group is the amino group. It is composed of one nitrogen atom and two hydrogen atoms (NH2). In contrast to the carboxyl group, the amino group reacts alkaline by adding a proton to the free electron pair of the nitrogen. Both the carboxyl group and the amino group are the same in all amino acids. The third atomic group is the side chain, to which amino acids owe their various properties.
Function, effect and tasks
Serine has two important functions for the human body. As an amino acid, serine is a building block for proteins. Proteins are macromolecules and form enzymes and hormones as well as basic materials such as actin and myosin, which make up muscles. The antibodies of the immune system and hemoglobin, the red blood pigment, are also proteins. In addition to serine, nineteen other amino acids exist in natural proteins. The specific arrangement of the amino acids results in long protein chains. Due to their physical properties, these chains fold and form a spatial, three-dimensional structure. The genetic code determines the order of amino acids within such a chain. In most human cells, serine is present in its L-form. In the cells of the nervous system – the neurons and glial cells – however, D-serine is formed. In this variant, serine acts as a co-agonist: it binds to the receptors of nerve cells and thereby triggers a signal in the neuron, which it transmits as an electrical impulse to its axon and passes on to the next nerve cell. In this way, information transmission takes place within the nervous system. However, a messenger substance cannot bind to any receptor at will: According to the lock-and-key principle, neurotransmitter and receptor must have matching properties. D-serine occurs, among other things, as a co-agonist at the NMDA receptors. Although serine is not the main messenger there, it has an amplifying effect on signal transmission.
Formation, occurrence, properties, and optimal levels
Serine is essential for the body’s function. Human cells form serine by oxidizing and aminating 3-phosphoglycerate, that is, by adding an amino group. Serine belongs to the neutral amino acids: its amino group has a balanced pH value and is therefore neither acidic nor basic. In addition, serine is a polar amino acid. Since it is one of the building blocks of all human proteins, it is very abundant. L-series forms the natural variant of serine and occurs primarily at a neutral pH of about seven. This pH value prevails inside the human body cells where serine is processed. L-serine is a zwitterion. A zwitterion is formed when the carboxyl group and the amino group react with each other: The proton of the carboxyl group migrates to the amino group and binds to the free electron pair there. As a result, the zwitterion has both a positive and a negative charge and is uncharged in total. The body often degrades serine to glycine, which is also an amino acid that, like serine, is neutral but nonpolar. In addition, pyruvate can be formed from serine, which is also called acetyl formic acid or pyruvic acid. This is a ketocarboxylic acid.
Diseases and disorders
In its L form, serine is found in neurons and glial cells, where it is thought to play a role in various mental disorders. L-serine binds as a co-agonist to N-methyl-D-aspartate receptors, or NMDA receptors.It enhances the action of the neurotransmitter glutamate, which binds to the NMDA receptors, causing the activation of the nerve cell. Learning and memory processes depend on NMDA receptors; it indexes the remodeling of synaptic connections, thereby changing the structure of the nervous system. This plasticity is expressed at the macro level as learning. Science considers this connection relevant to mental illness. Mental illnesses lead to numerous functional impairments, which often include memory problems. Faulty learning processes can also contribute to the development of mental illness. One example of this is depression. Particularly when severe, depression leads to poorer cognitive performance. However, learning ability and memory performance improve again when the depression recedes. A current theory is that frequent activation of certain neural pathways increases the likelihood that these pathways will be activated more quickly in response to future stimuli: The stimulus threshold decreases. This reasoning assumes a deblocking of receptors that could explain the process. In mental illnesses such as depression or schizophrenia, there may be a disturbance in this process, which could explain at least part of the respective symptoms. In this context, initial studies support the effect of D-serine as an antidepressant.
