Eighteen  distinctly  different  amino  acids  have  been  isolated  from  pro-
tein, and  it  may  assist  in  the  conception  of  our  problem  if  we  place  these
amino  acids  side  by  side  and  link  them  together  in  the  manner  described
above.  This  is  done  in  the  accompanying  chart  compiled  by  D.  D.  Van
Slyke,  in  which  also  various  other  important  facts  concerning  the  chem-
istry of  the  amino  acids  are  incidentally  added.
At  the  lower  part  of  each  formula  will  be  seen  the  characteristic  car-
boxyl and  amino  groups  of  neighboring  acids  linking  together  the  ter-
minal carbon  atoms.  The  upper  vertical  bond  of  this  carbon  atom  is  con-
nected with  the  characteristic  group  of  the  amino  acid,  which  may  be  very
simple  and  represented  only  by  hydrogen,  as  in  glycocoll,  or  highly  com-
plex and  including  a  ring  formation,  as  in  tryptophane.  It  will  further
be  observed  that  there  may  be  other  amino  groups  connected  in  various
positions  in  this  radicle.  This  is  particularly  the  case  in  the  first  three  of
the  amino  acids  in  the  table  —  namely,  the  basic  amino  acids.  In  lysine
the  extra  amino  group  reacts  with  nitrous  acid,  liberating  free  nitrogen


602


METABOLISM


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NEUTRAL  AMINO  ACIDS  CONTAIN  ONE  AMINO  OROUP  AND  ONE  CARBOXYL  GROUP.
