MLT GlobeChemistry: Proteins & electrophoresis
125 Chemistry MCQs on Proteins & electrophoresis with answers and explanations.
Twenty standard amino acids share the same backbone. What gives each one its distinctive chemical behaviour?
All amino acids carry identical amino and carboxyl groups on the α-carbon; the variable R group determines size, charge, polarity and reactivity.
A protein that buffers well near pH 7.4 would be rich in which residue?
The imidazole group of histidine has a pKa close to 6–7, so it can accept or donate protons at physiological pH, as in haemoglobin buffering.
Hair and nails are tough because their main structural protein is heavily cross-linked by disulphide bonds. Which protein is this?
α-Keratin is very rich in cysteine, whose disulphide bridges give hair and nails strength. Collagen is rich in glycine and proline instead.
The inhibitory neurotransmitter GABA is formed by which reaction?
Glutamate decarboxylase, a pyridoxal phosphate enzyme, removes the α-carboxyl of glutamate to yield γ-aminobutyrate.
Which class of biomolecule shows the greatest structural and functional diversity in a cell?
Twenty amino acids combined in any sequence and length give proteins almost limitless variety, serving as enzymes, receptors, transporters and structural elements.
A serum sample left in a boiling water bath turns cloudy as its proteins precipitate. This is because heat:
Heat denaturation breaks hydrogen and hydrophobic interactions, unfolding the protein so it aggregates. The primary sequence (peptide bonds) stays intact.
Which amino acid contains a thiol (–SH) group capable of forming disulphide bridges?
Cysteine's side chain ends in –SH; two cysteines oxidise to form cystine. Serine has –OH and tyrosine a phenolic –OH.
The pigment melanin is synthesised in melanocytes from which amino acid?
Tyrosinase hydroxylates tyrosine to DOPA and then DOPA-quinone, the start of melanin synthesis; its absence causes albinism.
Which of these is an amino acid that the human body can make for itself?
Proline is formed from glutamate, so it is non-essential. Leucine, lysine and tryptophan must come from the diet.
Which amino acid has its nitrogen in a ring as a secondary amine (an imino acid)?
Proline's side chain loops back to bond its α-nitrogen, forming a pyrrolidine ring; this makes it an imino acid and a helix breaker.
Two cysteine residues join to form cystine through oxidation of their:
Oxidation links the –SH groups of two cysteines into a disulfide (–S–S–) bond. Cysteine has no hydroxyl side chain, and amino/carboxyl groups form peptide bonds.
The first complete amino acid sequence of a protein, insulin, was determined by:
Sanger sequenced insulin using end-group labelling (FDNB) and fragment overlap. Edman later devised stepwise N-terminal degradation; Pauling described the α-helix.
The α-helix of proteins is stabilised mainly by:
Each backbone carbonyl hydrogen-bonds to the amide N–H four residues along. Disulfide, ionic and hydrophobic interactions mainly stabilise tertiary structure.
Which amino acid cannot be made in the human body and is the limiting amino acid in most cereal proteins?
Lysine is essential and scarce in wheat, rice and maize. Glutamate and serine are made from glycolytic or Krebs intermediates; tyrosine is made from phenylalanine.
In the collagen triple helix, which amino acid occupies every third position?
The Gly-X-Y repeat is needed because only glycine's tiny side chain fits inside the tight triple helix. Proline and hydroxyproline are common in X and Y positions but not every third residue.
When myoglobin's fractional oxygen saturation is plotted against pO2, the curve is:
Myoglobin has a single heme and no cooperativity, so binding is hyperbolic. The sigmoidal curve belongs to tetrameric hemoglobin with cooperative binding.
Which amino acid contains sulfur in a thioether side chain and begins almost every newly made protein?
Methionine (CH3–S–CH2–) is encoded by the start codon AUG. Cysteine is the other sulfur amino acid; tyrosine, lysine and histidine contain no sulfur.
Which of these amino acids must be supplied in the human diet?
Valine is an essential branched-chain amino acid. Alanine is made by transamination of pyruvate, serine from 3-phosphoglycerate, and glycine from serine.
At pH 7.4, which amino acid carries a negatively charged side chain?
Aspartate's side-chain carboxyl (pKa about 3.9) is ionised at physiological pH. Lysine is positive, histidine is mostly uncharged (pKa ~6), and proline's side chain is non-polar.
Which dietary protein has the highest protein efficiency ratio and is often used as the reference protein?
Whole egg protein has an ideal essential amino acid pattern and near-complete digestibility, giving the highest PER and biological value. Milk, meat and fish proteins are high quality but rank slightly lower.