Amino Acid Structures Trivia Quiz

Answer: Glycine

Chiral carbons arise when a carbon is attached to four different groups. This leads to optical isomerism, which results in either D or L isomers. Most naturally occurring amino acids are found in the L-isomer, rather than the D-isomer, which is prominent in sugars (e.g. D-glucose, D-galactose, etc.).

Glycine is represented by the one letter code G. It is usually grouped with the hydrophobic amino acids, though its R group is too small to make any significant contribution to hydrophobic interactions. Glycine has large conformational freedom, meaning it does not tend to form alpha helices, and so is rare in stretches of amino acids which form these helices.

Answer: Histidine

Histidine was assigned the letter H and its R group features a ringed imidazole group, meaning that it possesses metal-binding ability. Histidine can be converted into histamine in the body, which is important in regulating the inflammatory response during infection.

Histidine, like arginine, lysine, glutamate and aspartate, can become ionised, though the side chain of histidine has a pKa value near to the physiological pH, meaning that it is capable of acting as either an acid or a base. This property makes histidine very important to enzyme catalysis and it is the most common amino acid found in enzyme active sites.

Answer: Valine

Valine is hydrophobic and its single letter code is V. It is involved in anaplerotic reactions, going through many conversions, eventually to succinyl-CoA, which can enter the citric acid cycle and so provide energy to the cell when other fuel sources are scarce.

Answer: Lysine

Lysine was given the single letter code K because this was the letter closest to L in the alphabet that had not already been taken by another amino acid. Lysine usually exists as a positively charged amino acid and so is likely to be found on the exterior of a protein, facing the polar solvent (water) rather than in the hydrophobic interior.

It is particularly prominent in histones - the proteins responsible for "packing" DNA into chromosomes. Lysine can be modified in many ways to regulate gene expression, such as via methylation or acetylation. Lysines can also be modified to control a protein's lifespan, such as by ubiquitylation, which communicates to the cell that the protein should be destroyed.

Answer: Tryptophan

Tryptophan was designated its single letter code using a similar logic to tyrosine, arginine, and phenylalanine - i.e. according to its principal initial sound, rather than its first letter. For example, tYrosine is Y, aRginine is R, PHenylalanine is F, and tryptophan is...well, W (tWiptophan, apparently). Like the other aromatic amino acids, tryptophan is important in synthesising several chemicals in the body, but is also useful in analytical biochemistry. Tryptophan absorbs light at 280 nm, and so by knowing the average amount of tryptophan in a protein, the amount of light at 280 nm absorbed by a sample can be used to deduce the concentration of the protein in the solution being analysed.

Answer: Methionine

Methionine is designated by the letter M. Like cysteine, it contains sulphur and so is prone to oxidation. Methionine holds the grand title of being coded for by the "start codon". Each amino acid is coded for by one or more codons (three letter sequences), and the one which codes for methionine also tells a cell where to start translating a protein. Methionine is also relatively flexible and so is often found at binding sites of proteins. Methionine is the precursor of SAM, which is an important methyl group donor. Methyl groups are important in regulating protein activity, as well as deciding which genes are expressed in which cells.

This latter case is an example of epigenetics.

Answer: Glutamate

Glutamate is an acidic amino acid and so is usually negatively charged at pH7. It is designated by the letter E. It is the most common excitatory neurotransmitter in the human body and its action at neurones is strongly linked to learning and memory.

Answer: Tyrosine

Tyrosine is designated by the one letter code Y, since T was taken by threonine. Though threonine was actually the most recently discovered amino acid, the one letter code naming system (as introduced by Margaret Oakley Dayoff) was implemented after the discovery of all 20 amino acids. Tyrosine is an aromatic amino acid and is essentially a phenylalanine with an -OH group attached, meaning that its R group is a phenol group.

Answer: Alanine

Alanine has a methyl group as its R group and its single-letter code is A. It is used in alanine scanning to probe protein structure and function. It is also important in the de novo synthesis of glucose in times of starvation - a process known as gluconeogenesis.

Answer: Proline

Proline is distinct as being the only amino acid that is not a primary amine. Instead, it is a secondary amine since the amino nitrogen is bonded to two alkyl groups rather than just one. Proline is a cyclic amino acid, and so is more limited in its conformational freedom than other amino acids.

This means it is very unlike glycine (which has large conformational freedom) though both are rarely found in alpha helices. Despite its apparent limitations, proline is essential to collagen structure. A failure to adequately hydroxylate proline leads to a weakness in collagen and causes scurvy.

Answer: Arginine

Arginine is designated by the letter R and is a basic amino acid, therefore usually existing in a positively charged state at pH7. Its R group is an aliphatic chain tipped with a guanidinium group, which confers basicity. Arginine is common in regions of proteins which bind to DNA. Arginine is also the biological precursor of nitric oxide, which has a wide variety of uses, including in the immune response and as a vasodilator. Arginine is converted (in a multi-step reaction) into citrulline (a non-proteinogenic amino acid), with nitric oxide as a by-product.

Answer: Threonine

Threonine beat tyrosine and tryptophan to the honour of being designated the letter T. It has a polar R group and can therefore form hydrogen bonds with water molecules and other polar compounds. Threonine is commonly phosphorylated and is important in intracellular signalling. Since no threonine proteases were known of before, it came as a surprise to find that the proteasome (the most important cellular component in protein degradation) uses threonine proteases rather than the more ubiquitous serine or aspartate proteases.

Answer: Isoleucine

Isoleucine, as its name suggests, is an isomer of leucine (i.e. it has the same chemical formula, but a different structure). As a branched hydrophobic amino acid, isoleucine is common in the interior of proteins. Protein folding is driven by the hydrophobic effect, which is essentially an effort to get hydrophobic amino acids like isoleucine into the centre away from the polar solvent.

Answer: Phenylalanine

Phenylalanine is designated by the letter F (P was taken). It has a bulky aromatic group, meaning that it has delocalised electrons in its cyclic ring. Despite being fairly stable, phenylalanine can undergo a large number of reactions in the body to form other amino acids (like tyrosine), hormones (like serotonin) and other molecules (such as acetyl CoA, which can be used in the synthesis of fatty acids, or in the citric acid cycle to produce energy).

Answer: Cysteine

Cysteine is designated the letter C and its name is derived from the Greek word for bladder, since it was first isolated from kidney stones. Cysteine also has its own class of proteases. It is a sulphur-containing amino acid and so is prone to oxidation. Oxidation of some cysteine residues in proteins may indicate to the cell that the protein is old or damaged and so should be removed. On the other hand, oxidation of cysteine is central to the formation of disulphide bridges, which are important in the folding process of proteins, as well as in the maintenance of their structure in vivo.

The formation of these disulphide bridges in mammals is catalysed by protein disulphide isomerase (PDI).

Answer: Asparagine

Asparagine is named after asparagus, since this is where it was first derived. Asparagine was actually the first amino acid discovered, in 1806. We would wait a further 132 years for all 20 amino acids to be found. The precursor to asparagine synthesis is aspartate, which is aminated in a similar fashion to how glutamate is converted to glutamine.

Indeed, the amino group required for the synthesis of asparagine is often derived from glutamine (therefore concomitantly forming glutamate).

Answer: Serine

Serine is designated by the letter S and was first isolated from silk protein, in which it is abundant. It is commonly phosphorylated by kinases such as MAP kinases, and plays an essential role in intracellular signalling. Serine plays a multitude of roles in the cell, being the key amino acid in common proteases such as trypsin and chymotrypsin, as well as being involved in the synthesis of lipids which form the cell membrane.

Answer: Aspartate

Aspartate is acidic and is represented by the letter D. It is commonly involved in what are known as anaplerotic reactions, being converted into other biomolecules. Among these molecules are oxaloacetate, ornithine and citruline. All three of these biomolecules are involved in the citric acid cycle and so aspartate can be used to generate substrate for this cycle in order to provide energy to the cell. Aspartate is also involved in pyrimidine synthesis, which is important in generating both T and C nucleotides that form DNA.

Answer: Leucine

Leucine's single letter designation is L. Maple Syrup Urine disease arises from mutations to genes coding for an enzyme complex that is involved in the metabolism of branched chain amino acids - mainly leucine, but also isoleucine and valine. Mutations to any gene coding for any of the four subunits of this enzyme complex may lead to this disease, which is named for the distinctive sweet odour of the patient's urine.

Answer: Glutamine

Glutamine is designated by the letter Q. It is one of the most commonly found amino acids in the blood, and functions as a non-toxic transporter of ammonia. Ammonia may build up in the body following the degradation of nucleotides or amino acids, and this build up is toxic. Glutamine is usually transported via the blood to the liver, where it is converted by enzymes in the mitochondria to glutamate, releasing ammonia, which the liver cells can then metabolise further to form urea, which is released in the urine.