Table 3.

Average packing values and the percent occurrence of amino acids in the helices of membrane and soluble proteins^*

	Membrane proteins		α-Bundle proteins†		α-Nonbundle proteins†		αβ proteins†
	Packing value	Occurrence (%)	Packing value	Occurrence (%)	Packing value	Occurrence (%)	Packing value	Occurrence (%)
Gly	0.512	7.35	0.451	2.61	0.432	4.39	0.441	2.81
Ala	0.471	11.12	0.455	10.18	0.443	12.58	0.449	13.28
Ser	0.484	5.26	0.402	4.24	0.388	4.61	0.379	6.26
Cys	0.463	0.69	0.516	2.69	0.543	2.41	0.525	1.08
Val	0.408	9.44	0.450	5.58	0.463	7.17	0.464	6.80
Thr	0.441	6.93	0.406	4.81	0.396	5.56	0.388	4.43
Asp	0.417	1.37	0.352	4.10	0.340	3.37	0.321	6.91
Asn	0.432	1.75	0.366	3.60	0.382	3.80	0.368	3.35
Leu	0.402	15.80	0.482	13.22	0.461	13.46	0.449	11.23
Ile	0.412	8.30	0.474	4.88	0.454	6.73	0.476	6.37
Lys	0.322	1.94	0.288	6.29	0.293	5.12	0.276	6.70
Arg	0.381	2.44	0.324	6.86	0.339	5.63	0.296	5.62
Glu	0.418	1.52	0.323	8.34	0.305	6.07	0.300	8.32
Gln	0.406	1.14	0.364	6.71	0.337	4.68	0.302	3.89
Met	0.454	4.34	0.427	2.40	0.461	2.78	0.408	2.38
Pro	0.461	2.51	0.344	1.84	0.356	2.56	0.302	1.73
His	0.453	2.63	0.448	2.19	0.437	2.12	0.322	1.51
Phe	0.403	9.25	0.471	3.53	0.497	2.71	0.449	3.13
Tyr	0.387	2.97	0.464	4.31	0.415	2.41	0.388	0.76
Trp	0.427	3.24	0.507	1.63	0.466	1.83	0.472	3.46
Average‡	0.431		0.411		0.411		0.388

^*The occluded surface calculations were carried out on full protein structures, but the packing values reported represent the helical residues only. 

^† The soluble proteins were classified according to Michie et al. (26). 

^‡ The average packing value for each protein class was determined by summing up the packing values of all amino acids in their helical sections and dividing the sum by the total number of amino acids in the helical section of each protein class.