Question 1

Which of the following correctly defines the term pKa?&#10;A) equilibrium constant for the dissociation of HA to A&#8722; and H+&#10;B) ion constant of water&#10;C) negative log of the concentration of H+&#10;D) pH at which a molecule is neutrally charged&#10;E) pH at which an equivalent distribution of acid and conjugate base exist in solution

Accepted Answer

The logarithmic measure of the acid dissociation constant of an acid or base, termed pKa, is defined as the pH at which the protonated and unprotonated molecular species are at equal concentrations. With respect to this question the protonated species can be represented as HA while the unprotonated species would be A₋. pKa is the pH at which there is an equivalent distribution of an acid and its conjugate base in solution, indicating the point where the acid is half dissociated.

Question 2

Which of the following correctly defines the isoelectric point (pI) of an amino acid or protein?&#10;A) the equilibrium constant for the ionization of the substance&#10;B) the ion constant of water&#10;C) negative log of the concentration of H+&#10;D) pH at which a molecule is electrically neutral&#10;E) pH at which an equivalent distribution of acid and conjugate base exists in solution

Accepted Answer

The isoelectric point is that pH at which a substance exhibits no net charge. In other words, all the negative and positive charges, say for instance in a protein, are equal in number such that the molecule is electrically neutral. The isoelectric point (pI) is the pH at which an amino acid or protein carries no net electrical charge, meaning it is electrically neutral.

Question 3

The blood contains many compounds that serve to buffer the pH of the fluid such as bicarbonate and phosphate ions. Which of the following most correctly defines the meaning of the term buffering?&#10;A) a solution containing a large concentration of a base such that the pH will not change significantly when an acid is added&#10;B) a solution containing a large concentration of an acid such that the pH will not change significantly when more acid is added&#10;C) a solution or substance which resists changes in pH when small quantities of an acid or base are added to it&#10;D) pH at which a molecule or solution is neutrally charged&#10;E) pH at which an equivalent distribution of acid and conjugate base exists in solution

Accepted Answer

A buffer is a molecule that tends to either bind or release hydrogen ions in order to maintain a particular pH. More precisely, a buffer is defined as a mixture of a conjugate acid-base pair that can resist changes in pH when small amounts of strong acids or bases are added to it Buffering refers to the ability of a solution to resist changes in pH when small amounts of acid or base are added.

Question 4

Which of the following best describes the characteristics of polar amino acids?&#10;A) ionizable in water&#10;B) more likely to be exposed to water than to be found in the interior of a folded protein&#10;C) partially charged due to the oxygen atom in their carboxyl group&#10;D) partially charged due to fairly consistent sharing of electrons among atoms in their R group&#10;E) positively charged

Accepted Answer

Polar amino acids are defined as those whose R groups are capable of forming hydrogen bonds with water. Due to this property they are also said to be hydrophilic (water loving) and, therefore, are most often found exposed to the aqueous environment on the surface of proteins as opposed to buried in the interior.

Question 5

Which one of the following amino acids may be considered a hydrophobic amino acid at physiological pH of 7.4?&#10;A) arginine&#10;B) aspartic acid&#10;C) glycine&#10;D) isoleucine&#10;E) threonine

Accepted Answer

Isoleucine is a hydrophobic amino acid because its side chain is nonpolar and uncharged at physiological pH.  Hydrophobic amino acids are those with side chains that do not like to reside in an aqueous environment. For this reason, these amino acids are more often found buried within the hydrophobic core of a protein, or within the lipid portion of a membrane.

Question 6

The greatest buffering capacity at physiological pH would be provided by a protein rich in which of the following amino acids?&#10;A) alanine&#10;B) cysteine&#10;C) histidine&#10;D) proline&#10;E) tyrosine

Accepted Answer

Histidine contains an imidazole ring as its R group. The nitrogen in this ring possesses a pKa around 6.0, thus it is able to accept or donate a proton at physiological pH. This fact makes the amino acid an ideal buffering component of a protein containing several histidine residues. Histidine has a side chain with a pKa around physiological pH (approximately 6.0), allowing it to effectively buffer changes in pH in biological systems.

Deck 1: Amino Acids, Carbohydrates, Lipids, Nucleic Acids