Amino acid metabolism

Amino acid degradation
PLP is the help group for aminotransferases
The excretion of ammonium ions
Destination of degraded amino acid in the metabolism
Degradation of phenylalanine and tyrosine in acetylacetate and fumarate
A blockade in the degradation of phenylalanine can to lead mental retardation

In contrast to fatty acids (see fatty acid metabolism) and glucose (see glycogen metabolism), amino acid cannot be stored. Amino acid can not be excreted as well. An excess of amino acids is used as a fuel. For that the alpha-amine group (as NH3) is removed and the carbon skeleton that is left is converted so that it can be used further in the metabolism. The largest part of the amine groups of an excess of amino acids is converted into urea, while their carbon skeletons are transformed in acetyl CoA, acetoacetyl CoA, pyruvate or a intermediary compounds of the citric acid cycle. From the citric acid cycle amino acids can be converted into fatty acids, ketone bodies and glucose.

Alpha-amine groups are converted into ammonium ions by the deamination of glutamate.

In mammals the degradation of amino acids especially takes place in the liver. Aminotransferases (transaminases) catalyse the transfer of an amine group from an amino acid to a keto acid.



Examples:
Aspartate aminotransferase (ASAT = GOT: glutamate oxalate transaminase) catalyse :
aspartate + alpha-ketoglutarate glutamate + oxaloacetate
Alanine aminotransferase (ALAT = GPT: Glutamate pyruvate transaminase) catalyse:
alanine + alpha-ketoglutarate glutamate + pyruvate

Glutamatedehydrogenase catalyse the reaction by which ammonium ions (NH₄⁺) are formed from glutamate with NAD⁺ or NADP⁺ as the oxidising coenzyme. The enzyme is lowered in activity by GTP and ATP and is activated by GDP and ADP. Thus, a lowering of the energy status in the cell accelerates the oxidation of amino acid.

Pyridoxal phosphate forms a Schiff base as a intermediate product with aminotransferases. Pyridoxal phosphate (PLP) is the help group for all aminotransferases. PLP is a transformations product of pyridoxine (vitamin B6). During the transamination, PLP is temporary converted in pyridoxamine phosphate (PMP).



In the absence of an amino acid as a substrate the aldehyde group of PLP is bound to the amine group (Schiff base bond) by a particular lysine residue on the active side of the aminotransferase. At the presence of an amino acid substrate a new Schiff base bond is formed. The alpha-amine group of the amino acid takes the place of the epsilon-amine group of the lysine residue on the active side. In other words: an internal aldimine becomes an external aldimine.



The external aldimine loses a proton of its alpha-carbon atom under the formation of a quinon-like intermediate product. Protonation gives a ketimine with a double bond between the N-atom and the C-atom of the substrate. In the aldimine on the other hand there was a double bond between the N-atom and the carbonyl carbon atom of PLP. The ketimine then is hydrolysed in an alpha-keto acid and PMP.



Another (second) alpha-keto acid can undergo the opposite reaction under the formation of another (second) amino acid.
Then the total reaction is:

amino acid (1) + alpha-keto acid(2) amino acid (2) + alpha-keto acid(1)

A part of the NH₄⁺ that is formed in the degradation of amino acid is used for the biosynthesis of nitrogen compounds. In most of the land living vertebrate animals (including human) the excess NH₄⁺ is converted in urea and in that form it is excreted. You can read more about this metabolic process on the page urea cycle. In birds and living reptiles NH₄⁺ is converted in urine acid for the excretion. In many in water living animals NH₄⁺ is excreted directly in the water.

The carbon atoms of degraded amino acids are found in important intermediate products of the metabolism. The strategy of the amino acid degradation is the formation of important intermediate products that can be converted in glucose or can be oxidised in the citric acid cycle. The carbon skeletons of the twenty amino acids are brought back to only seven molecules: pyruvate, acetyl CoA, acetoacetyl CoA, alpha-ketoglutarate, succinyl CoA, fumarate and oxaloacetate. Amino acids that are degraded to acetyl CoA or acetoacetyl CoA are called ketogenic amino acids because they can be converted in ketone bodies. The amino acids that are converted in the remaining of the seven molecules are called glucogenic amino acids because they can be converted in phospho-enol pyruvate and from there in glucose (via the gluconeogenesis).
Destinations of the carbon chain of amino acids. Glucogenic amino acids are in a red frame, the ketogenic amino acids in a yellow frame. Some amino acids (ile, try, phe, tyr) are as well glucogenic as ketogenic amino.



The C3 family: alanine, serine and cysteine is converted in pyruvate. Pyruvate is the entry point for alanine, serine, cysteine, glycine, threonine and tryptophan. Cysteine can be converted in pyruvate through different paths, by which the sulphur atom returns in H₂S, SO₃^2-, or SCN^-. Also carbon atoms of the amino acids glycine, threonine and tryptophan can be converted in pyruvate.
The C4 family: aspartate and asparagine are converted in oxaloacetate. Aspartate can also be converted in fumarate by the urea cycle. Fumarate is an entry point for half of the carbon atoms of tyrosine and phenylalanine.
The C5 family: glutamine, proline, arginine and histidine are converted to alpha-ketoglutarate via glutamate. Alpha-ketoglutarate is the entry point of glutamine, proline, arginine and histidine that are first converted to glutamate.
Succinate CoA is an entry point for single apolar amino acids by the transformation of methionine, isoleucine and valine via propionyl CoA in succinate CoA. The pathway of propionyl CoA to succinate CoA is also present in the oxidation of fatty acids with an odd number carbon atoms. Fatty acids with an odd number of carbon atoms are thus partial glucogenic; three of their carbon atoms can go in glucose.

The first step in the degradation of phenylalanine is the hydroxylation of phenylalanine to tyrosine by the enzyme phenylalanine hydroxylase. This enzyme is called a monooxygenase or mixed-function oxygenase because one atom of O₂ is found in the product (tyrosine) and the other in H₂O.



The second step is the transamination of tyrosine in p-hydroxyphenylpyruvate. This alpha-keto acid reacts with O₂ and forms homogentisate (the acid rest of homogentisin acid). The enzyme p-hydroxyphenylpyruvate hydroxylase is a dioxygenase because both atoms of O₂ are built in the product (one in the ring and one in the carboxyl group).
The aromatic ring is then broken by O2 under formation of 4-maleylacetylacetate. This reaction is catalysed by another dioxygenase enzyme.
4-Maleylacetylacetate is then isomerised to 4-fumarylacetylacetate. Ultimately 4-fumarylacetylacetate is hydrolysed in fumarate and acetylacetate. See the figure below for the path of the degradation of phenylalanine and tyrosine.

The absence or insufficient activity (deficiencies) of phenylalaninehydroxylase (or its cofactor tetrahydrobiopterine) causes an accumulation of phenylalanine in all bodies liquids and therefore an excretion in the urine of the transformation product phenylpyruvate (phenylketon).



The biochemical background of the mental retardation is still a riddle.
An early diagnosis is essential and is reached by screening on newborn. The concentration phenylalanine in the blood, obtained by a heel prick, is determined.
The frequency of phenylketonuria is 1 on 20,000 of all newborn. The illness is inherited autonomic recessive.
The mental retardation is prevented by a diet with little phenylalanine. For that proteins with a low content of phenylalanine, like the milk protein casein, is hydrolysed. From the hydrolysate, phenylalanine is removed by adsorption.

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Literature
[1]. Stryer, Lubert;- Biochemistry - fourth edition; New York: W. H. Freeman and Company (1995). ISBN 0-7167-2009-4