311 



Amino Acids and Related Compounds 



Cysteine is a component of glutathione and of penicillin. 

 Methionine is important in transmethylation reactions. The 

 entire topic of one-carbon metabolism cannot be reviewed here. 

 The transfer of methyl groups from methionine to oxygen and 

 nitrogen atoms, and probably to carbon atoms in biosynthetic 

 sequences requires ATP, and the active complex has been iden- 

 tified as S-adenosylmethionine. 



NH2 



^'^^M 



© "^\ Jl J 



OOC— CH— CH2— CH2— S— CH2 ^N-^-N^ 

 NH2 CH3 



OH OH S-Adenosylmethionine 



Several labile methyl group compounds (choline, betaine, ser- 

 ine) probably can contribute the methyl group of methionine 

 by way of the proper coenzymes (Bj^, THFA). Some neurospora 

 mutants have been found which seem to synthesize methionine 

 from cysteine and, ultimately, from aspartic acid. The follow- 

 ing scheme has been suggested: 



ATP TPN 

 HOOC— CH,— CH— COOH > H2O3P— OOC— CH.— CH— COOH > 



I 



NH2 

 Aspartic Acid 



OHC— CH,— CH— COOH 



NH.j 

 Aspartic ^-Semialdehyde 



TPN 



NHo 

 /i-Aspartyl Phosphate 



HOCH2— CHo— CH— COOH, 



NH.. 

 Homoserine 



/ 



HS— CH>— CH,— CH— COOH CH,— CH,— CH— COOH 



NH, S NH2 



Homocysteine CH,— CH— COOH 



NH, 

 Cystathionine 



CH3— S— CH2— CH2— CH— COOH 



CH3— CH— CH— COOH 



OH NH2 

 Threonine 



CH2SH 

 CH— NH2 



COOH 



Cysteine 



NH2 



Methionine 



