Amino acids are organic compounds containing both a carboxylic acid and amino group. Based on the number of acid groups and amine groups the amino acid may be acidic or basic. Most of them will form extensive hydrogen bonding with water and hence are soluble in it. If the amino group and carboxylic acid group are present in the same carbon atom, then the amino acid is called as α-amino acid. They are building blocks of proteins and the order of amino acids determine the general characteristic of protein.
The amino acids are classified as essential and non essential amino acids. The amino acids which are synthesized within our body and not to be given through regular diet are called as non essential amino acids. Glycine is an example for non-essential amino acid. But the amino acid which is not synthesized in our body and has to be supplied by regular diet is called as essential amino acid. Lysine is an example for essential amino acid.
Food components containing protein are sources of many amino acids as the protein can be degraded to give amino acid easily.
Lysine is an essential amino acid with formula NH2
)-COOH. The structure of lysine is given here.
It contains two basic amino group and one acidic carboxylic acid group. Hence it is basic in nature. It forms extensive hydrogen bonding due to the presence of two amino groups. There are substituted lysine at the terminal amino group like N-methyl lysine and N,N dimethyl lysine etc. Similarly hydroxyl lysine is also available.
Lysine optically active amino acid due to the presence of asymmetric carbon atom. The asymmetric carbon atoms in the lysine are marked here.
As the second carbon atom is attached four different groups and the molecule does not possess any symmetry elements, it is optically active. The isomer which rotates the plane polarized light in the clock wise direction is called as dextro rotatory and the one which rotates the plane polarized light in the anticlockwise direction is called as laevo rotatotry. A pair of dextro and laevo isomers are called as enantiomers
The enantiomers of lysine are given here. They have non superimposable mirror image relationship. They rotate the plane polarized light in the same magnitude but in different directions.
The another way of naming the stereo isomer is representing the configuration with respect to glyceraldehyde. If the configuration resembles with that of D-glyceraldehyde then it is named as D isomer and the other form is known as L isomer.
The naming convention of D and L is not common in amino acids. We are following only dextro and laevo rotatory (d and l). Lysine is available in two enantiomeric forms, d-lysine and l-lysine. d-lysine is synthesized from l-lysine by racemisation. It is synthetic form of lysine. It is used in the preparation of poly-d-lysine which is used as coating material to improve the cell attachment.
Lysine is the important ingredient in animals feeds because it is the important supplement for the animals that are grown for meat like chicken, pigs etc. Lysine assist in muscle formation and increase the flesh content in the animals. Hence the high value animals feeds that are rich in protein like maize can be replaced by low protein content foods if it is added with synthetic lysine.
- Lysine is essential amino acid and is useful in the synthesis of many proteins.
- It is also used in calcium absorption, building of muscles, recovery of muscles after injury.
- It also affects the production of hormone, enzyme and antibodies. Cheese, eggs, beans and potato are found to have high amount lysine.
- In animals lysine deficiency creates immune deficiency.
- Excess of lysine will give increased level of cholesterol and the tendency to gallstones.
- They may also affect the blood pressure to a moderate level and may lead to stroke.
Animals cannot synthesis lysine of their own. Hence it has to be given through diet on regular basis. But plants can synthesis lysine and the process involves the following steps.
- Aspartic acid (aspartate ion) is converted to L-aspartyl 4 phosphate. This process is accompanied by the release of energy by one ATP molecule.
- This is further converted to β-aspartyl semialdehyde. Energy from NADPH is used for this conversion.
- An enzyme called as dihydrodipicolinate synthase is used to add a pyruvate group and subsequently two water molecules are removed. This give 2,3 dihydrodipicolinate.
- This is further reduced to 2,3,4,5 tetrahydrodipicolinate by the energy from one NADPH molecule.
- Another enzyme called as tetrahydrodipicolinate N-acetyltransferase breaks the ring leading to the formation of N-acyl-2-amino-6-oxopimelate. Two water molecules and one syccinyl Co enzyme A are used in this reaction.
- This is finally converted to L-lysine by the enzymes suddinyl diaminopimelate aminotransferase, succinyl dizminopimelate desuccinylase, diaminopimelate epimerase and finally dizminopimelate decarbosylase. Hence the naturally available lysine is L-lysine.
In animals especially mammals, lysine is metabolized to give acetyl-coA by transamination through an trans-aminating process with α ketoglutarate.
Similarly lysine can also be degraded to cadavarine by decarboxylation.
Another derivative of lysine called as allysin is used to produce elastin and collagen.