MLT GlobeChemistry: Proteins & electrophoresis – page 4
125 Chemistry MCQs on Proteins & electrophoresis with answers and explanations.
Molecular chaperones such as Hsp70 mainly:
Chaperones prevent aggregation and allow proteins to refold; the final shape is dictated by the amino acid sequence, not a template. Irreparably misfolded proteins are degraded by the ubiquitin–proteasome system.
Which essential branched-chain amino acid accumulates, with its keto acid, in maple syrup urine disease?
Leucine, isoleucine and valine are essential branched-chain amino acids whose keto acids cannot be decarboxylated in MSUD. Cysteine, tyrosine and aspartate are non-essential.
Pulses such as lentils are relatively poor in which amino acid, so they are best combined with cereals?
Pulses are limited in sulfur amino acids (methionine), while cereals are limited in lysine; eaten together they complement each other.
The CD antigens used to immunophenotype leukocytes by flow cytometry are mostly:
Most CD markers (e.g. CD3, CD4, CD19) are membrane glycoproteins recognised by monoclonal antibodies, and they mediate cell recognition and signalling. Proteoglycans and GAGs are mainly extracellular matrix components.
Glycine is formed mainly from which amino acid by serine hydroxymethyltransferase?
The enzyme transfers serine's hydroxymethyl carbon to tetrahydrofolate, leaving glycine and supplying one-carbon units. Choline and betaine are minor sources.
Silk fibroin, which forms stacked β-sheets, is composed largely of which small amino acid?
Fibroin has repeating Gly-Ala/Ser sequences; small side chains allow sheets to pack closely. Bulky tryptophan and helix/sheet-breaking proline are rare.
Which electron-carrying protein uses haem, not FAD or FMN, as its prosthetic group?
Cytochrome c is a haem protein that shuttles electrons between Complexes III and IV. Complex I (FMN), succinate dehydrogenase (FAD) and xanthine oxidase (FAD) are flavoproteins.
Which amino acid is purely glucogenic?
Arginine is converted via glutamate to alpha-ketoglutarate, a gluconeogenic precursor. Leucine and lysine are purely ketogenic, and isoleucine is both.
A neonate is well at birth, then feeds poorly, becomes lethargic and has urine smelling of burnt sugar by the end of the first week. What is the likely diagnosis?
BCKD deficiency causes branched-chain amino acids and alloisoleucine to build up within days of feeding. PKU and alkaptonuria do not cause an acute neonatal crisis.
'Beta-gamma bridging' on serum protein electrophoresis is characteristic of:
In cirrhosis, polyclonal IgA rises and runs between the beta and gamma zones, filling the dip between them. Albumin is also usually low.
Low albumin with a markedly increased alpha-2 band on electrophoresis suggests:
In nephrotic syndrome small proteins such as albumin are lost in urine, while the large alpha-2 macroglobulin is kept and the liver makes more of it, so the alpha-2 band stands out.
A narrow, sharp band in the gamma region of serum protein electrophoresis should next be confirmed by:
A narrow band suggests a monoclonal protein. Immunofixation identifies the heavy chain and light chain type and confirms monoclonality, as in myeloma or MGUS.
The Kjeldahl reference method converts measured nitrogen to protein using a factor of 6.25. This factor assumes that serum proteins contain on average:
Serum proteins average about 16% nitrogen by mass, so protein = nitrogen × 100/16 = nitrogen × 6.25. The factor itself is not the nitrogen percentage.
In a patient with severe inflammation and low albumin, bromocresol green (BCG) gives a higher albumin result than bromocresol purple (BCP). The main reason is that BCG:
BCG binds some non-albumin proteins (acute-phase alpha and beta globulins), so it overestimates albumin, especially when albumin is low; short reading times reduce this. BCP is more specific for albumin.
Serum protein electrophoresis is accidentally run on heparinized plasma instead of serum. What artifact is most likely?
Fibrinogen in plasma migrates between beta and gamma and can mimic a small monoclonal band. Repeating on serum, or immunofixation, resolves the question.
A 50-year-old with acute bacterial pneumonia has serum electrophoresis done on day 3. Which pattern is expected?
The acute-phase response raises alpha-1 antitrypsin (alpha-1) and haptoglobin (alpha-2), while albumin, a negative acute-phase protein, falls. A narrow gamma spike suggests a monoclonal protein.
In serum immunofixation electrophoresis, how are the separated proteins identified?
After electrophoresis, antisera to IgG, IgA, IgM, kappa and lambda are applied to separate lanes; immune complexes precipitate and stay after washing, then are stained. Unbound proteins wash away.
When urine containing Bence Jones protein is heated, the protein typically:
Free light chains show this unusual heat behaviour: they come out of solution at 40–60 °C and go back into solution near boiling. Precipitation in the cold describes cryoglobulins, not Bence Jones protein.
A patient with cadmium exposure has increased urine beta-2 microglobulin with little albumin. This pattern indicates:
Low-molecular-weight proteins like beta-2 microglobulin are normally filtered and reabsorbed by proximal tubules; tubular damage lets them escape into urine. Glomerular damage mainly causes albuminuria.
Paired CSF and serum samples are run on isoelectric focusing. Several IgG bands appear in CSF that are absent from serum. This finding indicates:
Oligoclonal bands present in CSF but not serum show IgG made inside the CNS, supporting multiple sclerosis. Identical bands in both fluids would reflect passive transfer from blood.