Learning Strategies in Biochemistry: Protein Sequence
Essential Review From Organic Chemistry
If you are not familiar with the following topics, look them up in
the index of your organic chemistry text, and review them thoroughly:
Amines: structure and ionization
Carboxylic acids: structure and ionization
Amides: structure, synthesis, and hydrolysis
Chirality
Essential Memory Work: The 20 Amino Acids
Sorry, but there's no way around it: if you are
going to be able to read and retain any material in biochemistry, you'll just have to memorize
the structures of those 20 amino acids, along with their 3-letter abbreviations. This
is essential vocabulary in biochemistry, like learning to conjugate the verb to be
in a foreign language. And while you're at it, you'd better memorize the ionization
states of all ionizable groups at pH's above and below their pKa's: the groups you
need to know are the alpha amino and alpha carboxyl groups, and the side chains of
arginine, histidine, lysine, aspartic acid, glutamic acid, cysteine, and tyrosine.
These are the only amino acids whose side chains ionize near physiological pH. Each group
will exist predominantly in its protonated state if the pH is lower than the pKa of the
group, and predominantly in the unprotonated state when the pH is higher than the pKa of
the group.
Carry out this memory work BEFORE you try to read the chapter. Most
texts provide a table of the amino-acid structures, and another table of the ionizable
groups, showing their structures and charges when protonated and unprotonated. To make you
work easier, first memorize the general structure of an alpha amino acid, then memorize
all the side chains as they exist at pH 7, and finally, memorize how the ionizable groups
(listed in boldface above) change as the pH changes. Then begin studying the chapter.
The authors of your text, and your instructor as well, will quickly start
treating you like a biochemist, which means that they will use the names of amino acids
freely, and will assume that you translate names into structures automatically. Refresh
you memory on the structures regularly, especially before each exam. There's nothing more
frustrating than not being able to get started on a ten-point exam problem simply because
you can't come up with the structure of an amino acid.
Quiz for USM Students
(At first class on this topic -- counts as 1/10 of one exam)
1. Draw the complete structure of the two amino acids whose 3-letter abbreviations are
given on the board. Draw each in the ionization state that predominates at pH 7.
2. One of these two amino acids possesses an ionizable side chain. Draw the complete
structure of its side chain at pH 2.0. Then draw the complete structure of the side chain
at pH 12.0.
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