Classification of KEGG atom types

Atom type	Description	Class	Frequency

C1a	R-CH3	C	16473
C1b	R-CH2-R	C	20193
C1c	R-CH(-R)-R	C	4964
C1d	R-C(-R)2-R	C	698
C1x	ring-CH2-ring	C R	14010
C1y	ring-CH(-R)-ring	C R	27376
C1z	ring-CH(-R)2-ring	C R	4463
C2a	R=CH2	C CP	634
C2b	R=CH-R	C CP	3965
C2c	R=C(-R)2	C CP	1914
C2x	ring-CH=ring	C CP R	2964
C2y	ring-C(-R)=ring or ring-C(=R)-ring	C CP R	3722
C3a	R≡CH	C CP	43
C3b	R≡C-R	C CP	282
C4a	R-CH=O	C CC	350
C5a	R-C(=O)-R	C CC	3595
C5x	ring-C(=O)-ring	C CC R	2257
C6a	R-C(=O)-OH	C CC	3190
C7a	R-C(=O)-O-R	C CC	1691
C7x	ring-C(=O)-O-ring	C CC R	869
C8x	ring-CH=ring	C A	19905
C8y	ring-C(-R)=ring	C A	20511
C0		C	8

N1a	R-NH2	N	2440
N1b	R-NH-R	N	3003
N1c	R-N(-R)2	N	374
N1d	R-N(-R)3+	N	105
N1x	ring-NH-ring	N R	806
N1y	ring-N(-R)-ring	N R	1464
N2a	R=N-H	N	230
N2b	R=N-R	N	163
N2x	ring-N=ring	N R	357
N2y	ring-N(-R)+=ring	N R	14
N3a	R≡N	N	119
N4x	ring-NH-ring	N A	785
N4y	ring-N(-R)-ring	N A	840
N5x	ring-N=ring	N A	2131
N5y	ring-N(-R)+=ring	N A	59
N0		N	194

O1a	R-OH	O	18369
O1b	N-OH	O	198
O1c	P-OH	O	3111
O1d	S-OH	O	332
O2a	R-O-R	O	4199
O2b	P-O-R	O	2481
O2c	P-O-P	O	502
O2x	ring-O-ring	O	5853
O3a	N=O	O OP	134
O3b	P=O	O OP	2248
O3c	S=O	O OP	941
O4a	R-CH=O	O OP	350
O5a	R-C(=O)-R	O OP	3595
O5x	ring-C(=O)-ring	O OP	2862
O6a	R-C(=0)-OH	O OP	6384
O7a	R-C(=O)-O-R	O OP	3382
O7x	ring-C(=O)-O-ring	O OP	1738
O0		O	127

S1a	R-SH	S	100
S2a	R-S-R	S	420
S2x	ring-S-ring	S	261
S3a	R-S-S-R	S	45
S3x	ring-S-S-ring	S	48
S4a	R-SO3	S	267
S0		S	223

P1a	P-R	P	112
P1b	P-O	P	2158

X	"F, Cl, Br, I"	X	1419
Z		Z	261
R	R	RR	0