Amino Acids and Proteins 2: Chemistry of Amino Acids and Protein Structure

Proteins

are large, complex molecules that are critical for the normal functioning of the human body. They are essential for the structure, function, and regulation of the body's tissues and organs. Proteins are made up of hundreds of smaller units called amino acids that are attached to one another by peptide bonds, forming a long chain. You can think of a protein as a string of beads where each bead is an amino acid.

The double-bond character of the peptide bond

If you were asked to draw a peptide bond, you might draw a single bond between the nitrogen and the carbonyl carbon atoms. But in reality, this single bond is not a conventional single bond; in fact, it has a double-bond character. This double-bond character comes from the various resonance structures of a polypeptide. The next question would be: what are resonance structures? Resonance structures are different representations of the same molecule; the arrangement of the atoms remains the same, but the electrons are distributed differently amongst the atoms. Resonance structures exist when there is a possibility of movement of electrons between neighboring functional groups, as in the case of polypeptides.

Forces that keep the different protein structures together

In summary, the primary structure of a protein simply refers to the linear polypeptide with its amino acid sequence. The secondary structure is the folded version of the linear polypeptide stabilized by hydrogen bonding. The tertiary structure is formed by the coming together of several secondary structures that are held together by various types of interactions, and finally a quaternary structure is formed by the combination of several tertiary structures, again held together via different types of interactions.

The titration curve of glycine will look something like the one shown below; at pH=2.34(pKa ​1 forms A and B will be in equilibrium—i.e., the concentration of A equals the concentration of B. At pH = 9.6, forms and C will be in equilibrium - i.e., concentration of B equals concentration of C.

Mathematically, isoelectric point, pI, for glycine is calculated using the formula below:

Isoelectric point (pI) of amino acids

Isoelectric point is the point along the pH scale where the amino acid has a net zero charge. Consider glycine. Look at the equilibrium below; as we add hydroxide ions—in other words, raise the pH—different charged forms of glycine exist. Form B has a net zero charge and is called a zwitterion. Form A has a net charge of +1, and form C has a net charge of -1.

Mechanism of peptide bond formation: The simplest way to represent a peptide bond formation is as follows. Let's consider two amino acids with side chains, R1 and R2 respectively.

​​Step 1: The nucleophilic amino group of the second amino acid attacks the electrophilic carbonyl group of the first amino acid. Step 2: The carbonyl bond reforms with the elimination of hydroxide ion. Step 3: The hydroxide ion abstracts a proton—elimination of water—and the positive charge on nitrogen is neutralized. This results in the formation of a new bond—a peptide bond between the two amino acids. Please note that this is a very simplistic representation of the mechanism of a peptide bond formation. The mechanism gets complicated in the context of peptide-protein synthesis in biological systems where catalysts, cofactors, and enzymes are involved.

Let's draw three amino acids connected to each other through peptide bonds.

As illustrated above, electrons can move across the amide, O=C-N, bond generating the two structures A and B respectively. Structure C is the overall hybrid representation of the two resonance structures A and B, where the entire peptide bond, O=C-N, is shown to have a partial-double-bond character, represented by a solid line with a dotted line running parallel to to it. So, as a result of resonance, the bond between the carbonyl carbon and nitrogen acquires a partial-double-bond character, and, just like any double bond, rotation around this peptide bond is now restricted. Also, as with all double bonds, the atoms of the peptide bond have planar geometries. This planar geometry causes the peptide bond to be either in the cis or the trans configuration. In the cis configuration, the two alpha carbon atoms fall on the same side of the peptide bond. In the trans configuration, these groups are on opposite sides of the peptide bond.

L and D amino Acids

As shown above, L and D amino acids are mirror images of each other and are non-superimposable on each other, just like our left and right hands. By non-superimposable, we mean that when the mirror image of the object is placed over the original object, they do not have a perfect overlap. Pairs of amino acids like these are called enantiomers. Only L-amino acids are constituents of proteins. Our body synthesizes most of its own L-amino acids; these then get incorporated into proteins. Proteins are catalysts for most of the biochemical reactions that take place in our body. Along with DNA and RNA, proteins constitute the genetic machinery of living organisms. Proteins are often called the building blocks of life.

Quaternary structure:

Quaternary structure: When several tertiary structures come together, a quaternary protein structure is formed. For example, hemoglobin is a functional quaternary protein formed by the coming together of four tertiary structures, called globin proteins. The same forces of interactions operate in a quaternary structure as operate in a tertiary structure.

How are amino acids joined together?

Amino acids are joined together through peptide bonds. Peptide bonds are covalent bonds formed by the nucleophilic addition-elimination reaction between the carboxylic group of one amino acid and the amino group of another amino acid; this reaction releases a molecule of water as the by product. A peptide bond is essentially an amide bond.

Amino Acid Structure and its classification

An amino acid contains both a carboxylic group and an amino group. Amino acids that have an amino group bonded directly to the alpha-carbon are referred to as alpha amino acids. The simplest representation of an alpha amino acid is shown below.

Unlike the rigid peptide bond, the bond linking the amino group to the alpha carbon atom and the bond linking the alpha carbon atom to the carbonyl carbon are single bonds—as shown in the image below. These two bonds are free to rotate about the amide bonds, allowing the amino acids in the polypeptide chain to take on a variety of orientations.

The enhanced freedom of rotation with regards to these two bonds allows proteins to fold into a variety of shapes. These folded structures are referred to as secondary protein structures and are essentially of two types—alpha helix and *beta pleated sheets. These folded secondary structures are stabilized by the formation of hydrogen bonds between the amino acids.