One of the most important substances for the human body are amino acids. Without them, the metabolism can not form proteins, liver metabolism, growth, building the skin, nails and hair and the function of the nervous system would not be possible. Amino acids are building blocks for the synthesis of proteins and serve as basic building blocks for gluconeogenesis, which is the metabolic pathway for the formation of glucose. Thus, amino acid metabolism encompasses all biochemical processes in the body in the composition, breakdown, and conversion of amino acids.
What is amino acid metabolism?
Amino acid metabolism includes all biochemical processes in the body in the composition, breakdown and conversion of amino acids, e.g. lysine. The human body can produce some amino acids itself and obtains others from food. The processes involved are complex. Enzymes and co-enzymes are used. If such enzymes are defective or insufficiently formed, this leads to disturbances in amino acid metabolism, which in turn results in serious diseases. Individual amino acids also form precursors of hormones and neurotransmitters or serve in the composition of nitrogen-containing compounds.
Function and task
Basically, amino acids are divided into essential and non-essential. Amino acid metabolism includes their formation and breakdown. Essential amino acids cannot be formed by the human organism itself, because in the course of evolution the enzymes suitable for them were gradually lost and synthesis is rather cumbersome. They are taken in with food and are called leucine, isoleucine, leucine, methionine, lysine, phenylalanine, tryptophan, threonine and valine. In amino acid metabolic disorders, other amino acids also become essential, e.g., tyrosine in phenylketonuria. Non-essential amino acids can be formed by the body itself. These include aspartic and glutamic acids, alanine and serine. The formation takes place in the liver and kidneys, for the processes in the liver the body needs alpha-keto acids which are provided by beta-oxidation from fatty acids. Alpha-keto acids serve a transfer of amino groups through amino acids. The body obtains a derivative from vitamin B6 called co-enzyme pyridoxal phosphate, which plays an important role in the transfer. In amino acid metabolism, the co-enzyme is also responsible for dehydration, transamination and decaroxidation into biogenic amines. Dehydration is a process of splitting off hydrogen from molecules, referring to organic compounds that have been oxidized. This occurs in an endothermic reaction, but can also be converted to an exothermic one by oxygen, which then forms water. In transamination, the alpha-amino group of an amino acid is shifted to an alpha-keto acid, forming a new alpha-keto acid and amino acid. The enzymes required for this process are called transaminases. Acceptors are ketoglutarate, oxaloacetate and pyruvic acid. The process that takes place in this way is always reversible. Decarboxylation is the splitting off of a carboxyl group of a molecule and the resulting formation of carbon dioxide. Biogenic amines are thus assembled, which in turn are basic molecules bearing amino groups. In the case of non-essential amino acids, the aldehyde group of the co-enzyme pyridoxal phosphate forms a Schiff’sche base with the amino group of the acid, with the elimination of water. The Schiff’sche base is thus stabilized by a cationic group inside the enzyme. In the pyridine ring of the co-enzyme, an electrophilic action of nitrogen continues to take place and the formation of a ketimine formation, which is not stable and is transformed. This can occur either via transamination or deamination. The latter involves the cleavage of an amino group from an amino acid and is further used as ammonia. This is important for blood PH levels, for example, but at low concentrations is recognized by the brain as toxic and is therefore converted in the liver to urea, which is then excreted.
Diseases and ailments
If defects in amino acid metabolism occur, serious damage to the organism and associated diseases result. Most are congenital gene effects transmitted by inheritance. Many of them are rather rare and their manifestations depend on whether enzymes are missing or only their function is disturbed.In amino acid metabolism, several processes take place, which in turn require different enzymes. Thus, the disorders can also vary. This makes the diagnosis of such diseases all the more difficult. Symptoms are not always known and are also difficult to identify. Most defects manifest themselves in the fact that intermediate products can no longer be converted into amino acids. Likewise, the precursors to this process may be blocked. The consequences of this are an accumulation of such intermediates, as a result of which the functions can no longer take place due to the amino acid deficiency. Thus, tissue and organ damage soon occur or failure symptoms develop. Under these conditions, amino acid transport is disturbed. Amino acids are no longer excreted in the urine, but are transported back into the organism. The resulting diseases include albinism, alkaptonuria and phenylketonuria. In all three manifestations, the metabolism of tyrosine is disturbed. In albinism, the human body can no longer produce the skin pigment melanin for this reason. The skin, eyes and hair are therefore very pale, pink to white. Tyrosine is produced by the amino acid phenylalanine, which is obtained from food. If this is missing, not only melanin but also the thyroid hormone thyroxine or messenger substances such as catecholamines can no longer be produced. In alkaptonnuria, the enzyme homogentisine oxigenase is missing. This converts homogentisic acid into maleylacetoacetic acid. As alkapton, this is excreted in the urine or stored in the body. This results in inflammation due to crystal or calcium deposits in the joints, as well as kidney stones or cardiac dysfunction. A possible diagnosis is the addition of bases, which causes the urine to turn black. Phenylketonuria is the most common amino acid metabolism disorder. It is caused by a defect in the enzyme phenylalanine hydroxylase, which is responsible for converting phenylalanine into tyrosine. If this does not take place, phenylalanine accumulates in the tissues and blood and begins to damage the brain. This affects and delays overall physical development. Seizures and mental retardation can occur. For this defect, there is a diet enriched with special amino acids, which, applied in the first two months of a child’s life, leads to normal development. The diet must be followed until puberty.
