Tymoczko • Berg • Stryer © 2015 W. H. Freeman and Company Biochemistry: A Short Course Third Edition CHAPTER 3 Amino Acids © 2015 W. H. Freeman and Company

Chapter 3 Outline

Two Different Ways of Depicting How Biomolecules Will Be Used Fischer projections are useful for visualizing the constituent atoms of the molecule. Every atom is identified, and the bonds to the central atom are depicted as vertical and horizontal lines. The horizontal bonds are taken to project out of the plane toward the viewer, whereas the vertical bonds are assumed to project behind the viewer.

Two Different Ways of Depicting How Biomolecules Will Be Used Stereochemical renderings are useful for visualizing the shape of the molecule. Wedges are used to depict the direction of bond projection. A solid wedge shows the bond projecting toward the viewer out of the plane. A dashed wedge shows the bond projecting behind the plane, away from the viewer. The remaining bonds are depicted as straight lines.

An a-amino acid is composed of a central carbon atom called the a-carbon. The a-carbon is linked to an amino group, a carboxylic acid, a hydrogen atom, and a distinctive side chain called the R-group.

Most Amino Acids Exist in Two Mirror-Image Forms When four different groups are bonded to the a-carbon, the amino acids are chiral, which means that they exist as two mirror-image forms called the l isomer and the d isomer. Only the l isomers are found in proteins.

Figure 3. 1 The l and d isomers of amino acids Figure 3.1 The l and d isomers of amino acids. The letter R refers to the side chain. The l and d isomers are mirror images of each other.

All Amino Acids Have at Least Two Charged Groups Free amino acids in solution at neutral pH exist as dipolar ions. The amino group (NH3+) and the carboxyl group (COO-) are charged.

Figure 3. 2 Ionization state as a function of pH Figure 3.2 Ionization state as a function of pH. The ionization state of amino acids is altered by a change in pH. The zwitterionic form predominates near physiological pH, approximately 7.4 .

The 20 amino acids found in proteins contain unique side chains that vary in size, shape, charge, hydrogen-bonding capacity, hydrophobic character, and chemical reactivity. Amino acids have three-letter abbreviations and one-letter symbols.

Hydrophobic Amino Acids Have Mainly Hydrocarbon Side chains

Figure 3.3 Hydrophobic amino acids.

Polar Amino Acids Have Side Chains That Contain an Electronegative Atom

Figure 3.4 Polar amino acids

Positively Charged Amino Acids Are Hydrophilic The R group of lysine is topped with an amino group while that of arginine is topped with a guanidium group.

Figure 3.5 Positively charged amino acids.

Positively Charged Amino Acids are Hydrophilic Histidine has an imidazole side chain that can be uncharged or positively charged at neutral pH. Histidine is found at the active sites of many enzymes that require a proton donor or proton acceptor.

Figure 3. 6 Histidine ionization Figure 3.6 Histidine ionization. Histidine can bind or release protons near physiological pH.

Negatively Charged Amino Acids Have Acidic Side Chains

Figure 3.7 Negatively charged amino acids.

The Ionizable Side Chains Enhance Reactivity and Bonding

3.3 Essential Amino Acids Must be Obtained from the Diet Kwashiorkor is a form of malnutrition resulting from inadequate consumption of amino acids usually ingested as proteins.

Figure 3. 8 A child suffering from kwashiorkor Figure 3.8 A child suffering from kwashiorkor. Note the swollen belly and limbs. This swelling (edema) is due to fluid collecting in the tissues because there is not enough protein in the blood.