Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 438

Introductory Biochemistry

Spring 2010

Lecture Notes: 8 February

© R. Paselk 2006
 
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Amino Acids 3

Amino Acid Chemistry: All aa's share two chemically functional groups, the carboxyl group and the amino group. Thus they will share the chemical reactions of these groups familiar from organic chemistry. Many of these reactions are exploited in the laboratory manipulation of amino acids, peptides, and proteins. Note that these reactions are also common to the side chains of asp, glu (-COOH), and lys (-NH2). Another side-chain with important chemistry is cys (-SH). Biologically the most important reactions are those required for protein formation, particularly the peptide bond.

pKa's: Note that the pKa's for carboxylic acids tend to have values of about 5, while the pKa of the amino acid -COOH is around 2. What's going on? The shift in pKa can be assigned to the nearby protonated amine. Recall that 'naked' charges are very unstable, while nearby counter-charges stabilize them. Also, from organic chemistry you may recall that negative charges can be stabilized by inductive effects of nearby electron withdrawing groups, such as a protonated, positively charged, nitrogen. Because of the extra intervening carbons the side chain -COOH's of asp and glu are not similarly stabilized, and thus have pKa values closer to the expected 5. Of course we would also expect analogous effects of the negative charge on the carboxyl group on the charged amine.

Peptides and Protein Primary Structure

 
Now that we have looked at peptide bond formation, we next want to look at the structure of this bond and the sequence of amino acid residues (primary structures) of proteins. (Note that "residue" refers to the remainder of a molecule after it is incorporated into a polymer.)

 

3-D Structure of Proteins

Overview: Proteins are commonly large (MW > 6,000), globular molecules serving many functions.

Proteins are complex systems - difficult to understand at a fundamental structural level. Thus we search for patterns using normal perceptual tools: regularity, clustering, cleavage/separation/emptiness.

We are then able to discern alpha helices, beta sheets, beta turns, and "random" regions. 310 helical regions show up with computer searches. None of these is necessarily more or less random than others, they are simply easier or more difficult for us to perceive as ordered. They exist through our rationalization. Often structural elements also appear to serve a functional role, thou this is through our dissection of the molecular machine.

Look at theoretical possibilities resulting from the available bond angles around the peptide bond system

Let's go back and look at overall shape and interpret it. Look for substructures that recur in various molecules. Perhaps we see a globule is made of subglobules. Look closer and we see alpha helices and beta structures. Finally we can discern aa residues.

In order to understand and categorize their organization, protein structure has been divided into four hierarchical levels and a couple of sublevels:


Pathway Diagrams

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Last modified 8 February 2010