The Amino Acid Trivia Quiz

Answer: Carboxyl group

An acid is defined as a proton donor. A proton is a hydrogen ion and some hydrogen ions are lost more easily than others. The more easily a hydrogen ion is lost, the more acidic the species is. As seen in the picture, the carboxylic acid (referred to in this instance as the carboxyl group) is a carbon atom double-bonded to an oxygen atom and singly-bonded to an -OH (hydroxyl) group.

It is the hydrogen of the -OH group that is lost and which therefore confers this acidity.

Answer: Three

Nitrogen is in group 5 of the periodic table and so has five electrons in its valence shell. Nitrogen obeys the octet rule and therefore forms chemical bonds with other atoms in order to achieve eight electrons in its valence shell in an attempt to become more stable. By forming chemical bonds with two hydrogen atoms and a carbon atom, the nitrogen atom now has six bonding electrons (three of its own valence electrons and one electron from each hydrogen/carbon atom), with two electrons left over as a "lone pair". 6 + 2 = 8.

Answer: The variable group

The R group varies between amino acids and it is this group which defines which amino acid is which. The types of R group can be roughly broken down into hydrophobic, aromatic, polar, acidic, and basic categories. There are some amino acids which can fit into several categories (for example, histidine can be aromatic, acidic, and basic) and some which do not fit clearly into any category (e.g. proline).

Answer: The alpha carbon

All naturally occurring amino acids have the acid and amino groups bonded to the same carbon. Because the alpha carbon is bonded to four different chemical groups, it is therefore said to be chiral. Chiral compounds display optical isomerism (i.e. they can exist in two different structural forms that are mirror images of one another).

The only amino acid that is not chiral is glycine since its alpha carbon is attached to two groups that are the same (both are hydrogen atoms). The most infamous example of a chiral compound is thalidomide. One isomer was the effective version which prevented morning sickness, whereas the other isomer caused devastating birth defects.

Answer: A zwitterion

The net charge of a zwitterionic amino acid is zero (unless the R group is acidic or basic), since the negative charge on the acidic group and the positive charge of the amino group balance each other perfectly. Nevertheless, there is a localisation of electrical charge, and many zwitterions can interact with one another via ionic bonds to form giant lattice structures.

Answer: It will gain a hydrogen ion to give an overall charge of +1

The R group of lysine is a hydrocarbon chain with an amino group at the end. Much like the amino group bonded to the alpha carbon, it is likely to use the lone pair of the nitrogen atom to form a covalent bond with a hydrogen ion. Lysine is one of four of the 20 standard amino acids that exists as a charged molecule at pH7.

The others are aspartic acid (-1), glutamic acid (-1), and arginine (+1).

Answer: Hydrophobic

The hydrophobic (or aliphatic) amino acids have no formal charge and very little polarity, and so are practically chargeless. They include alanine, valine, leucine and isoleucine. These amino acids tend to be found on the inside of proteins, away from the hydrophilic solvent (water), as is consistent with the hydrophobic collapse theory. For more information on protein folding, please take my quiz "Solving Levinthal's Paradox".

Answer: Condensation reaction

The clue here is in the picture. Whilst these ions are lost so that a chemical bond can be formed between two amino acids, the ions concomitantly join together to form a water molecule. One water molecule is formed per covalent bond formed, which can be several thousand for large proteins.

Answer: A peptide bond

The lone pair of the amino group's nitrogen becomes partially delocalised, as do the p-electrons of the C=O double bond. This results in something which is halfway between a single and a double bond, and so rotation is restricted. The peptide bond is therefore said to be planar and is always drawn exactly horizontal in diagrams.

Answer: Hydrogen

A hydrogen bond is a non-covalent interaction between the lone pair of an electronegative atom (e.g. oxygen or nitrogen) and a hydrogen atom that is covalently bonded to an electronegative species. Here, the lone pair is the hydrogen bond acceptor and the hydrogen atom is the hydrogen bond donor. Linus Pauling described the hydrogen bond as the most important interaction in biochemistry, and he may not have been far off. Hydrogen bonding is essential to just about everything in life - it gives water its special properties, it holds DNA together, and it allows proteins to fold and function properly.