A Carrier of Amino Groups

Vitamin Bg (pyridoxine also known as pyridoxol, 118) is an essential growth factor in the diet of many organisms and animals. It forms part of a coenzyme (pyridoxylphosphate) and it is a cofactor for a class of enzymes known as transaminases. A transaminase or an aminotransferase is an enzyme that catalyzes a type of reaction between an amino acid and an a-keto acid. The presence of elevated transaminase levels can be an indicator of liver damage. Vitamin Bg has both an aldehyde form (pyridoxal, 119) and an amine form (pyridoxamine, 120), and it is known that pyridoxal phosphate is a carrier of amino groups and sometimes amino acids. ... 

Section 16.8 MCAT Practice Pyridoxine (Vitamin B ), a Carrier of Amino Groups... 

The coenzyme form of pantothenic acid is coenzyme A and is represented as CoASH. The thiol group acts as a carrier of acyl group. It is an important coenzyme involved in fatty acid oxidation, pyruvate oxidation and is also biosynthesis of terpenes. The epsilon amino group of lysine in carboxylase enzymes combines with the carboxyl carrier protein (BCCP or biocytin) and serve as an intermediate carrier of C02. Acetyl CoA pyruvate and propionyl carboxylayse require the participation of BCCP. The coenzyme form of folic acid is tetrahydro folic acid. It is associated with one carbon metabolism. The oxidised and reduced forms of lipoic acid function as coenzyme in pyruvate and a-ketoglutarate dehydrogenase complexes. The 5-deoxy adenosyl and methyl cobalamins function as coenzyme forms of vitamin B12. Methyl cobalamin is involved in the conversion of homocysteine to methionine. 

Pyridoxal phosphate acts as an intermediate carrier of amino groups during transamination reactions. Write a series of reac-... 

Thiols are characterized by the sulfhydryl group (—SH). The amino acid cysteine is a thiol that is extremely important for maintaining the correct shapes of proteins. Coenzyme A is a thiol that serves as a "carrier" of acetyl groups in biochemical reactions. 

What are some common features in amino acid biosynthesis In the anabolism of amino acids, transamination reactions play an important role. Glutamate and glutamine are frequently the amino-group donors. The enzymes that catalyze transamination reactions frequently require pyridoxal phosphate as a coenzyme. One-carbon transfers also operate in the anabolism of amino acids. Carriers are required for the one-carbon groups transferred. Tetrahydrofolate is a carrier of methylene and formyl groups, and S-adenosylmethionine is a carrier of methyl groups. 

Ih a preliminary note, published in January, 1046, F. Schlenk and E. E. Snell (145a) report evidence pointing to the possibility that the system pyridoxal pyridoiamine may be involved in enzymatic transamination as an intermediary carrier of amino groups (see pp, 13-14). 

Vitamin B12 is a carrier of methyl groups in the conversion of amino acid homocysteine to methionine. 

Tetrahydrofolic acid then functions as a carrier of one-carbon groups for amino acid and nucleotide metabolism. The basic ring system is able to transfer methyl, methylene, methenyl, or formyl groups, and it utilizes slightly different reagents as appropriate. These are shown here for convenience, we have left out the benzoic acid-glutamic acid portion of the structure. These compounds are all interrelated, but we are not going to delve any deeper into the actual biochemical relationships. 

Fig. 1. Structure of CoA, composed of three parts a nucleotide pan derived from 3 -adenosine-5 -pbosphate, forming a phosphodiester bond with a 4-phospho derivative of pantothenic acid, and a third pan derived horn the amino acid, cysteine. The side chain SH group of the latter is ftee in this compound and is readily acylated, and thus able to act as a carrier for acyl groups in biochemical reactions in which it transfers that group between two substrates...

Tetrahydrofolate, a carrier of activated one-carbon units, plays an important role in the metabolism of amino acids and nucleotides. This coenzyme carries one-carbon units at three oxidation states, which are interconvertible most reduced—methyl intermediate—methylene and most oxidized—formyl, formimino, and methenyl. The major donor of activated methyl groups is -adenosylmethionine, which is synthesized by the transfer of an adenosyl group from ATP to the sulfur atom of methionine. -Adenosylhomocysteine is formed when the activated methyl group is transferred to an acceptor. It is hydrolyzed to adenosine and homocysteine, the latter of which is then methylated to methionine to complete the activated methyl cycle. 

The most important functions of pantothenic acid are its incorporation in coenzyme A and acyl carrier protein (AGP). Both CoA and AGP/4-phosphopantetheine function metabolically as carriers of acyl groups. Coenzyme A forms high-eneigy thioester bonds with carboxylic acids. The most important coenzyme is acetyl CoA. Acetic acid is produced during the metabolism of fatty acids, amino acids, or carbohydrates. The active acetate group of acetyl CoA can enter the Krebs cycle and is used in the synthesis of fatty acids or cholesterol. AGP is a component of the fatty acid synthase multienzyme complex. This complex catalyzes several reactions of fatty acid synthesis (condensation and reduction). The nature of the fatty acid synthase complex varies considerably among different species (91). 

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