Question 21.8: Number of Possible Peptides If you had only 5 different amin...
Number of Possible Peptides
If you had only 5 different amino acids to use (Ala, Lys, Ser, Glu, Arg), how many different tetrapeptides could you synthesize? Give two of them.
Strategy
To calculate the total number of possibilities, you take the number of available amino acids and raise it to the power of the number of amino acids in the peptide.
Using the information given, the total number of possibilities for a tetrapeptide starting with 5 different amino acids would be 5^{4}, or 625. Thus, you have a large choice of ways of showing two of them, including the boring: Ala-Ala-Ala-Ala. If you really were asked to draw all possible combinations for a question like this, the strategy would be to start with something simple, like AAAA, and then systematically change something, such as:
AAAA
AAAK
AAAS
AAAE
AAAR
KKKK
KKKA
etc.
■ Quick Check 21.7
How many different decapeptides can be synthesized starting with 6 different amino acids?
■ Quick Check 21.8
Which would give you more possible combinations?
(a) Synthesizing dipeptides from five possible different amino acids, or
(b) Synthesizing pentapeptides from only two possible amino acids