Amino Acids and Proteins Questions and Answers

Answer: C

The monomer unit of protein is amino acid.

Proteins are polymers of amino acids joined by peptide bonds, formed between the carboxyl group of one amino acid and the amino group of the next. Twenty standard amino acids make up the proteins of the body.

Nucleic acids are themselves polymers, of nucleotides, not the units of proteins. Acetic acid (as acetyl-CoA) is the building block of fatty acids. Sulphuric acid is a mineral acid with no place in protein structure.

Answer: B

The amino acid with an imidazole ring is histidine.

Histidine's side chain is an imidazole ring, a five-membered ring with two nitrogen atoms. Its pKa of about 6 lets it gain or lose a proton near body pH, which makes histidine an important buffer, as in haemoglobin, and a common catalytic residue in enzymes.

Serine has a simple hydroxyl (-CH2OH) side chain. Tyrosine carries a phenol ring, which has no nitrogen. Arginine has a guanidino group, which contains nitrogen but is not a ring.

Answer: C

The essential amino acid present in low proportion in rice is lysine.

Cereal proteins, including those of rice and wheat, are poor in lysine, which is their first limiting amino acid. Pulses are rich in lysine but poor in methionine, which is why a cereal and pulse meal such as rice and dal gives a better-balanced protein than either alone.

Methionine is the limiting amino acid of pulses, not of rice. Tryptophan is the classic deficiency of maize, linked with pellagra. Histidine is not the limiting amino acid of rice.

Answer: B

The shared intermediate is fumarate.

In the urea cycle, argininosuccinate is split by argininosuccinate lyase into arginine and fumarate. Fumarate enters the TCA cycle, where it is converted to malate and then oxaloacetate, which can form aspartate to re-enter the urea cycle. This link is called the aspartate-argininosuccinate shunt, or the Krebs bicycle.

Citrulline, ornithine and arginine are intermediates of the urea cycle only and do not take part in the TCA cycle.

Answer: A

The polar pair is serine and asparagine (spelt aspargine in the paper).

Serine carries a hydroxyl group (-CH₂OH) and asparagine an amide group (-CH₂CONH₂) in its side chain. Both can form hydrogen bonds with water, so both are polar, uncharged amino acids.

Alanine, valine and methionine have hydrocarbon or thioether side chains and are classed as non-polar (hydrophobic). Each of the other options pairs at least one of these with a polar amino acid, or two non-polar ones together.

Answer: A

Canavanine is incorporated in the place of arginine.

L-canavanine, found in jack bean and other legume seeds, is a structural analogue of arginine in which a CH₂ group of the side chain is replaced by an oxygen atom. Arginyl-tRNA synthetase cannot tell the two apart well, so canavanine is charged onto tRNA^Arg and built into proteins at arginine positions. The altered proteins misfold, which is why canavanine is toxic to insects that eat the seeds.

Cysteine, methionine and glycine have quite different side chains and are not mimicked by canavanine.

Answer: C

The amino acid is histidine.

Histidine biosynthesis starts like purine biosynthesis, from PRPP (5-phosphoribosyl-1-pyrophosphate), and it uses ATP as a building block: the pathway condenses PRPP with ATP and releases AICAR (5-aminoimidazole-4-carboxamide ribonucleotide), an intermediate of the purine pathway that goes back into purine synthesis. The two pathways are therefore closely linked.

Proline is made from glutamate, leucine from pyruvate, and tryptophan from chorismate in the shikimate pathway (it also uses PRPP, but not a purine-like route).

Answer: A

Aspartame is L-aspartyl-L-phenylalanine methyl ester.

It is a dipeptide made from the natural L forms of aspartic acid and phenylalanine, with the carboxyl group of phenylalanine esterified with methanol. It is about 200 times sweeter than sucrose. Because it releases phenylalanine on digestion, products containing it carry a warning for people with phenylketonuria.

The sweet taste depends on the exact stereochemistry. Isomers containing a D-amino acid, as in the other three options, are different compounds and are not the sweetener sold as aspartame.

Answer: D

Epinephrine is synthesised from tyrosine.

The catecholamine pathway in the adrenal medulla and in nerve cells runs:

tyrosine → L-DOPA (tyrosine hydroxylase) → dopamine (DOPA decarboxylase) → norepinephrine (dopamine beta-hydroxylase) → epinephrine (phenylethanolamine N-methyltransferase).

Tyrosine itself can be made from phenylalanine. Tryptophan is the precursor of serotonin and melatonin, not of the catecholamines. Glycine contributes to haem, creatine and purines, and proline to collagen, but neither gives rise to epinephrine.

Answer: B

The defective enzyme is the branched chain dehydrogenase.

Valine, leucine and isoleucine are first transaminated to their branched-chain α-keto acids, which are then oxidatively decarboxylated by the mitochondrial branched-chain α-keto acid dehydrogenase complex. In maple syrup urine disease this complex is deficient, so the keto acids build up in blood and urine, giving the urine its sweet smell and causing brain damage if untreated.

Homogentisate oxidase deficiency causes alkaptonuria, and oxaloacetate decarboxylase and acetoacetate carboxylase are not involved in branched-chain amino acid breakdown.