Amino acid reductases and

Table 12.1 summarizes the selenoenzymes isolated from Clostridia, which fall into three major categories. The first group is the amino acid reductases, such as GR, sarcosine reductase (SR), betaine reductase (BR), and proline reductase (PR). The second class of selenoenzymes includes FDH. Although a number of clostridial species are suspected to contain a... 

Iwasaki, N., Noji, S., and Shidara, S. (1975). Achromobacter cycloclastes nitrite reductase. The function of copper, amino acid composition, and ESR spectra. J. Biochem. (Tokyo) 78, 355-361. 

Methotrexate (Amethopterin) is a folic acid antagonist that binds to dihydrofolate reductase, thus interfering with the synthesis of the active cofactor tetrahydrofolic acid, which is necessary for the synthesis of thymidylate, purine nucleotides, and the amino acids serine and methionine. 

Affinities between NOSs and BH4 are stronger than those between aromatic amino acid hydroxylases and BH4, so the purified NOS from animal tissues still contain 0.2-0.5 BH4 molecules per heme moiety [128]. BH4 tightly binds to endothelial and neural NOSs with dissociation constants in the nanomolar range, and this binding is reported to stabilize the dimeric structure of NOS [129-131], whereas aromatic amino acid hydroxylases do not have BH4 in the proteins. BH4 functions as a one electron donor to a heme-dioxy enzyme intermediate. The BH4 radical remains bound in NOS and is subsequently reduced back to BH4 by an electron provided by the NOS reductase domain [128]. 

The cell-free system prepared from washed embryos has much higher translational activity than the conventional system (compare Fig. 3A and B). When 5 -capped dihydrofolate reductase (DHFR) mRNA containing 549 nt of 3 UTR with a pA tail was incubated with newly prepared as well as conventional extract, there was almost linear kinetics in DHFR synthesis over 4 h, compared with the regular system, which ceased to function after 1.5 h. Further, when washed extract in the reaction volume was increased to 48%, amino acid incorporation occurred initially at a rate twice that of 24% extract, and then stopped after 1 h. However, this pause was caused by a shortage of substrates rather than an irreversible inactivation of ribosomes or factors necessary for translation addition of amino acids, ATP, and GTP after cessation of the reaction (arrow) restarted... 

K Nakamura, S Kondo, Y Kawai, N Nakajima, A Ohno. Amino acid sequence and characterization of aldo-keto reductase from baker s yeast. Biosci Biotechnol Biochem 61 375-377, 1997. 

Aminopterin and amethopterin are 4-amino analogues of folic acid (Fig. 11.5) and as such are potent inhibitors of the enzyme dihydrofolate reductase (EC 1.5.1.3) (Blakley, 1969). This enzyme catalyses the reduction of folic acid and dihydrofolic acid to tetrahy-drofolic acid which is the level of reduction of the active coenzyme involved in many different aspects of single carbon transfer. As is clear from Fig. 11.6, tetrahydrofolate is involved in the metabolism of (a) the amino acids glycine and methionine (b) the carbon atoms at positions 2 and 8 of the purine ring (c) the methyl group of thymidine and (d) indirectly in the synthesis of choline and histidine. 

As shown in Figure 10.10, tetrahydrobiopterin activates molecular oxygen by forming a peroxypterin that reacts with an iron atom in the active site, yielding Fe=0 that reacts with the amino acid substrate, and hydroxypterin, which then undergoes dehydration to yield dihydrobiopterin. Dihydrobiopterin is reduced back to tetrahydrobiopterin by dihydrobiopterin reductase dihydrofolate reductase (Section 10.3.3) does not have any significant activity toward dihydrobiopterin (Fitzpatrick, 1999). 

The metabolism of folic acid involves reduction of the pterin ting to different forms of tetrahydrofolylglutamate. The reduction is catalyzed by dihydtofolate reductase and NADPH functions as a hydrogen donor. The metabolic roles of the folate coenzymes are to serve as acceptors or donors of one-carbon units in a variety of reactions. These one-carbon units exist in different oxidation states and include methanol, formaldehyde, and formate. The resulting tetrahydrofolylglutamate is an enzyme cofactor in amino acid metabolism and in the biosynthesis of purine and pyrimidines (10,96). The one-carbon unit is attached at either the N-5 or N-10 position. The activated one-carbon unit of 5,10-methylene-H folate (5) is a substrate of T-synthase, an important enzyme of growing cells. 5-10-Methylene-H folate (5) is reduced to 5-methyl-H,j folate (4) and is used in methionine biosynthesis. Alternatively, it can be oxidized to 10-formyl-H folate (7) for use in the purine biosynthetic pathway. 

Epinephrine (adrenaline) (Figure 32-7) is synthesized from tyrosine by conversion of tyrosine to 3,4-dihydro-xyphenylalanine (dopa) by tyrosine-3-monooxygenase (tyrosine hydroxylase) in the cytosol. The mixed-function oxidase requires molecular oxygen and tetrahydro-biopterin, which is produced from dihydrobiopterin by NADPH-dependent dihydrofolate reductase. In the reaction, tetrahydrobiopterin is oxidized to dihydrobiopterin, which is reduced to the tetrahydro form by NADH-dependent dihydropteridine reductase. These reactions are similar to the hydroxylations of aromatic amino acids (phenylalanine and tryptophan), in which an obligatory biopterin electron donor system is used (Chapter 17). 

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