1
2
3
4 """Calculate isoelectric points of polypeptides using methods of Bjellqvist.
5
6 pK values and the methos are taken from:
7
8 * Bjellqvist, B.,Hughes, G.J., Pasquali, Ch., Paquet, N., Ravier, F., Sanchez,
9 J.-Ch., Frutiger, S. & Hochstrasser, D.F.
10 The focusing positions of polypeptides in immobilized pH gradients can be predicted
11 from their amino acid sequences. Electrophoresis 1993, 14, 1023-1031.
12
13 * Bjellqvist, B., Basse, B., Olsen, E. and Celis, J.E.
14 Reference points for comparisons of two-dimensional maps of proteins from
15 different human cell types defined in a pH scale where isoelectric points correlate
16 with polypeptide compositions. Electrophoresis 1994, 15, 529-539.
17
18 I designed the algorithm according to a note by David L. Tabb, available at:
19 http://fields.scripps.edu/DTASelect/20010710-pI-Algorithm.pdf
20
21 """
22
23 positive_pKs = { 'Nterm': 7.5, 'K': 10.0, 'R': 12.0, 'H':5.98 }
24 negative_pKs = { 'Cterm': 3.55, 'D': 4.05, 'E': 4.45, 'C':9.0, 'Y':10.0 }
25 pKcterminal= {'D':4.55, 'E':4.75}
26 pKnterminal = {'A':7.59, 'M':7.0, 'S':6.93, 'P':8.36, 'T':6.82, 'V':7.44, 'E':7.7}
27 charged_aas = ('K', 'R', 'H', 'D', 'E', 'C', 'Y')
28
29
30
32 - def __init__(self, ProteinSequence, AminoAcidsContent):
35
36
37
39 charged = {}
40 for aa in charged_aas:
41 charged[aa] = float(AminoAcidsContent[aa])
42 charged['Nterm'] = 1.0
43 charged['Cterm'] = 1.0
44 return charged
45
46
47 - def _chargeR(self, pH, pos_pKs, neg_pKs):
48 PositiveCharge = 0.0
49 for aa, pK in pos_pKs.iteritems():
50 CR = 10**(pK-pH)
51 partial_charge = CR/(CR+1.0)
52 PositiveCharge += self.charged_aas_content[aa] * partial_charge
53
54 NegativeCharge = 0.0
55 for aa, pK in neg_pKs.iteritems():
56 CR = 10**(pH-pK)
57 partial_charge = CR/(CR+1.0)
58 NegativeCharge += self.charged_aas_content[aa] * partial_charge
59
60 return PositiveCharge - NegativeCharge
61
62
64 pos_pKs = dict(positive_pKs)
65 neg_pKs = dict(negative_pKs)
66 nterm = self.sequence[0]
67 cterm = self.sequence[-1]
68 if nterm in pKnterminal.keys():
69 pos_pKs['Nterm'] = pKnterminal[nterm]
70 if cterm in pKcterminal.keys():
71 neg_pKs['Cterm'] = pKcterminal[cterm]
72
73
74 pH = 7.0
75 Charge = self._chargeR(pH, pos_pKs, neg_pKs)
76 if Charge > 0.0:
77 pH1 = pH
78 Charge1 = Charge
79 while Charge1 > 0.0:
80 pH = pH1 + 1.0
81 Charge = self._chargeR(pH, pos_pKs, neg_pKs)
82 if Charge > 0.0:
83 pH1 = pH
84 Charge1 = Charge
85 else:
86 pH2 = pH
87 Charge2 = Charge
88 break
89 else:
90 pH2 = pH
91 Charge2 = Charge
92 while Charge2 < 0.0:
93 pH = pH2 - 1.0
94 Charge = self._chargeR(pH, pos_pKs, neg_pKs)
95 if Charge < 0.0:
96 pH2 = pH
97 Charge2 = Charge
98 else:
99 pH1 = pH
100 Charge1 = Charge
101 break
102
103
104 while pH2 - pH1 > 0.0001 and Charge!=0.0:
105 pH = (pH1 + pH2) / 2.0
106 Charge = self._chargeR(pH, pos_pKs, neg_pKs)
107 if Charge > 0.0:
108 pH1 = pH
109 Charge1 = Charge
110 else:
111 pH2 = pH
112 Charge2 = Charge
113
114 return pH
115