Amino acid metabolism.

< p class s "tt1" > from the structure of amino acids, in addition to the side chain R group is different, there are α-amino and α base. The breakdown and metabolism of amino acids in the body is actually the metabolism of amino, carboxyl, and R-groups. The main way of amino acid decomposition metabolism is deaminogenic ammonia) and the corresponding α ketone acid; Amines can be oxidized by amine oxidase in the body, further decomposition to produce ammonia and the corresponding aldehyde and acid. Ammonia is a toxic substance to the human body, ammonia in the body mainly synthetic urea excreted, but also synthesis of other nitrogen-containing substances (including non-essential amino acids, glutamine, etc.), a small amount of ammonia can be directly excreted by urine. Ketoacids produced in part of the R-group can further oxidize and break down to produce CO2 and water, and provide energy, or they can be converted by a certain metabolic reaction to produce sugar or fat stored in the body. Because different amino acid structures are different, their metabolism also has its own characteristics.< the role of > amino acid metabolism in tissue organs, with the liver as the most important. Liver proteins are updated faster, amino acids are actively metabolized, most amino acids are decomposed and metabolized in the liver, while ammonia detoxification is mainly carried out in the liver. The breakdown and metabolism of branching amino acids is mainly carried out in muscle tissue.< the protein content of > protein in the food also affects the metabolic rate of amino acids. A high-protein diet induces the synthesis of enzyme systems associated with amino acid metabolism, which speeds up metabolism (Figure 7-1).

Figure 7-1 basic overview of amino acid metabolism < p class "biao1" >1, amino acid deamin< > ation effect < "center" class"tt3" > Figure 7-2 glutamate>dehydrogenase catalytic oxidation dehydrogenation reaction

the process by which amino acids are de-produced by α ketones under the catalysis of enzymes. This is the main way amino acids break down in the body. There are 20 kinds of amino acids involved in protein synthesis in human body, their structure is different, the way of deamining is also different, mainly oxidizing deamining, transamination, combined deamining and non-oxidizing deamination, etc., in order to joint deamine is the most important.

(i) oxidation deamination (i) oxidation deamination

oxidation deamination refers to the process by which amino acids are de-aminoated while oxidizing dehydrogenation under the catalysis of enzymes.

no oxygen dehydrogenase catalytic oxidation deamination action

< p class s "tt1" > glutamate mitochondrials by glutamate dehydrogenase (glutamate dehydrogonase) catalytic oxidation deamination. Glutamate dehydrogenase system does not require oxygen dehydrogenase, with NAD plus or NADP plus as coenzyme. The oxidation reaction is transferred from glutamate C alpha dehydrogenation to NAD(P) plus to form α aminoline diacids, which are then hydrolyzed to produce α ketone diacids and ammonia (Figure 7-2).< p class " tt1" > glutamate dehydrogenase as a variant enzyme. GDP and ADP are variant activators, and ATP and GTP are variant inhibitors.< in the body, glutamate > dehydrogenase catalytics reversible reactions. In general, the synthesis of glutamate is preferred (G' ≈30kJ,mal 1), as high concentrations of ammonia are harmful to the body, and this reaction balance point helps to maintain a low ammonia concentration. However, when glutamate concentration is high and NH3 concentration is low, it is beneficial to the α deamine and ketone diacin.

(ii) trans-amino action

< p-class> "transamination" refers totransaminase catalyzed by the transfer of α-amino acid amino acid amino acids to another α- is the process of ketone acid, the production of the corresponding α ketone acid and a new α-amino acid.< of amino acids in the body > deamined by transamination by trans-amino, according to the report. Of the 20 α-amino acids involved in protein synthesis, except glycine, lysine, suline and proline do not participate in transamination, the rest can be catalyzed by specific transaminase to participate in transamination. The most important amino subject of trans-amino action is α ketone diacin, which produces glutamate as a newly generated amino acid:

Amino is transferred to oxalic acid to produce α ketone diacin and tyrosine:

or to acetone acid. It α ketone diacin and alanine, and by a second transamination reaction, it regenerates α ketone diacin.

< p"center" >

thus having strong glutamic pyruvic transaminase (GPT) and glutamic transaminase (glutamic oxaloticticate trans aminase, GOT) activity in the body.

< p class s"tt1" > trans-amino action is reversible, the reaction in the G'≈0, so the equilibrium constant is about 1. The direction of the reaction is taken from the relative concentration of the four reactants. Therefore, trans-amino action is also an important way to synthesize certain amino acids (non-essential amino acids) in the body.

2. Trans-amino action process:

Trans-amino action process can be divided into two stages:

(1) the amino acid of an amino acid is transferred to the enzyme molecule, producing the corresponding ketone acid and aminoase:

< p class s "tt1" >(2) NH2 is transferred to another ketone acid (e.g. α ketone diacids) to produce amino acids and release enzyme molecules:

> <>p class s"tt1" >in order to transmit the NH2 gene, transaminase requires the participation of its aldehyde-based coenzyme-phosphate acetaldehyde (pyridoxal-5'-phosphate, PLP). During the transamino process, coenzyme PLP is converted to pyridoxamine (pyridoxamine 5' phosphate, PMP). PLP forms Schiff alkali by shrinking the lysine omega amino in its aldehyde base and enzyme molecules and co-prices binding subenzyme molecules.

Esmond Snell, Alexande Branstein and David Metgler reveal that transamination is a ping-pong mechanism, with two phases in three steps (Figure 7-3).

Figure 7-3 PLP dependent enzyme-promoting transamination reaction

First stage: amino acids into ketoic acids

(1) amino acid acts on the enzyme PLp Schiff alkali C atom, forming an amino acid PLp Schiff alkali by transimination or trans Schiffigation, while restoring the NH2 group of lysine in the enzyme molecule.

(2) removes the amino acid α hydrogen by the enzyme-active bit lysine catalysis, and re-discharges the amino acid Lp Schiff alkali molecule into a α ketone PMP schiff alkali by a resonant and stable intermediate product that adds protons to the 4th C atom of the PLP.

< p class s"tt1" > (3) hydrolysed to produce PMP and α-ketoic acid.

Phase II: α-ketone acid into amino acids

In order to complete the transamination reaction cycle, coenzyme must be changed from PMP form to E-PLp-Schiff form, this process also includes three steps, for the reverse process of the above reaction.

< p class s "tt1" >(1) PMP with a α-ketoacide effect to form α-ketoacate-Schiff alkali. < p class- "tt1" >(2) molecular rearm, α-ketoacide-PMp-Schiff base into amino acid-PLP-Schiff base.

(3) enzyme activity point lysine omega-NH2 group attacks the amino acid -PLp-Schiff alkali, by trans-amino to produce an active enzyme -PLP Schiff alkali, and release the formation of new amino acids.

trans-amino reaction, coenzyme converts between PLP and PMP, acting as an amino vector in the reaction, and amino transfer between α-ketone acid and α-amino acid. It can be seen that there is no net NH3 generation in trans-amino reactions.

< the physiological significance of the

"tt1" >3. trans-amino >-class-tt1> trans-amino plays a very important role. The type and quantity of non-essential amino acids in the body can be regulated by transamination to meet the demand for non-essential amino acids when proteins are synthesized in the body.

transamino action is also an important part of the combined deamino action, thus accelerating the transformation and transport of ammonia in the body, ticking the body's sugar metabolism, lipid metabolism and amino acid metabolism of the interrelation.

< p class s "biao2" > (iii) joint deamination

combined deamination is the main way of deamination in the body. There are two main reaction pathways:

1. A combined deamination act, catalyzed by L-glutamate dehydrogenase and transaminase: the α-ammonia of an amino acid is first catalyzed by transaminase. The base is transferred to α-ketone diacid to produce glutamate, and then, under the action of L-Glutamate dehydrogenase, glutamate oxidized deaminate produces α-ketone diacid, while α-ketone diacids continue to participate in transamination.

L-glutamate dehydrogenase is mainly distributed in the liver, kidneys, brain and other tissues, and α-ketone diacin participation in the transamination role is common in organizations, so this kind of combined deamination is mainly carried out in liver, kidney, brain and other tissues. The combined desamine reaction is reversible and therefore can also be called combined ammonia.

2. purine nucleotide cycle: L glutamate dehydrogenase activity in skeletal and cardiomyopathy tissue is very low and therefore cannot be deamined by the combined deamination reaction in the above form. However, skeletal and heart muscles are rich in adenosine deaminase (adenylat).