Protein Evolution

Total Information in Insulin A and B Chains

The techniques used in the previous section can easily be extended to calculate the total information in insulin. Figure 4.3 was generated using a software package designed to align the amino acid sequences of similar proteins, Clustal X. Instead of using the three letter abbreviation for each amino acid, the single letter abbreviation is used to conserve space. The letters across the first row are the amino acid sequence of insulin in chickens, the second row snakes, and the last row flounders. The columns are aligned by the Clustal X program so that similar amino acids appear in the same column. The dashes represent gaps inserted by the Clustal program to align the sequences.

Figure 4.3: The Amino Acid Sequence of Insulin

Single letter amino acid abbreviations used in figure 4.3:
A = Ala    C = Cys    D = Asp    E = Glu
F = Phe    G = Gly    H = His    I = Ile
K = Lys    L = Leu     M = Met    N = Asn
P = Pro    Q = Gln    R = Arg    S = Ser    
T = Thr    V = Val     W = Trp    Y = Tyr   

Insulin is composed of two chains, A and B. Referring to the ruler at the bottom of figure 4.3, the B chain is comprised of amino acids 2-31, and the A chain is comprised of amino acids 32-52. The columns are aligned in such a way to match up the amino acids when they are the same. For example, the last amino acid on each row is always N. This means that N (the amino acid Asparagine or Asn) is conserved. Notice that even in columns where the amino acids differ, the variability is still quite low. Insulin is a highly conserved protein.

   Tables 4.2 and 4.3 calculate the information in insulin as it exists today. To help with understanding, a few sample calculations are described first.

Example calculation for table 4.2: refer to figure 4.3, and find the 10th column. Notice that it contains A (alanine) in flounders, S (serine) for most species, and P (proline) in mice. This means that all three of these amino acids are acceptable at this position. The information content is calculated as follows (refer to table 4.1): information = 3.32 x log[64 possible outcomes/ (4+6+4) observed outcomes] = 2.2 bits. Thus, position 10 in table 4.2 is assigned 2.2 bits. The remaining positions used the same technique to assign information.

Example calculation for table 4.3: refer to figure 4.3, column 42. This column is always C (cysteine), thus, the information is as follows: information = 3.32 x log [ 64/2] = 5 bits. The total information for each chain is the sum of the information at each position. The sum of columns 3 and 6 in table 4.3 is 81 bits.


Table 4.2: Information in Insulin (B chain only)

total = 108 bits

Table 4.3: Information in Insulin (A chain only)

total = 81 bits


   The total information today in insulin is the sum of the information found in both chains or 81 +108 = 189 bits. It is important to keep in mind that this number has absolutely nothing to do with the probability of insulin evolving.



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