PROTEIN METABOLISM 



469 



liecithins 



.y 



^~(K)CCH2N(CII;;)... 

 ^ bi'tainf 



>i 



HOCH.CHoN(CH3)3 



choline 



3(-CH3)^ 



Proteins 



Purines 



Nucleic acids i 



Porphyrins 



Bile pigments 



C0o + ( rH:,OH)j 



^.- — 5-2(-CIl3) 



rH3NH2rH:.roo" 



IiOCIl2CH2NHj 

 cthanolaminc 



y 



li<N(HoC"00 



(-I'll. ,UH)'«; ^ 



y 



pliosphaliilyl 

 ellianulaniines 



CHoCHCOO" 

 NH3+ 



CHoCHCOO" 



"I ^ 

 NH3+ 



cystine 



CH,)S(CH.,).,CHCOO" 



NH;,+ 



lucUiiuiiiuu 



HOCH2CH2CHCOO" 

 NH3+ 



honioserine 



CHaCHaCHCOO" 



NH3+ 



a-aminobutyric acid 



HSCH2cn2cncoo 



NH."*" 



huinocyslcinc 



rH2CHC00- 

 S NH3'*" 

 CH2CH2CHC00~ 

 NH3+ 



cystathionine 

 CH3CHOHCHCOO" 



NH3'^ 



threonine 



/ 



/ 



^ CH3CH2COCOO" 



a-kc(obutyratc 



\ 

 -A 



Tricarbuxylic acid cycle 



FIGURE 20-3. A l!)59 summary ol llie iiliii/alioii of glycine, serine, llirconinc, and 

 llic siilfnr amino acids. Tlic literature may be consulted for the intervcninf;; slc])s, 



enzymes, and coiactois omitted here. 



Hemoglobin 



This protein is taken as an example of the complex proteins con- 

 taining porphyrins that occiu" in the diet of both herbivorous and 

 carnivorous animals. Furthermore, members of this group are im- 



