Questions & explanations
1. How do you tell apart an aminoacidopathy from an organic acidemia?
Aminoacidopathies, like PKU, involve defects in amino acid breakdown, leading to high levels of specific amino acids in blood. Symptoms are often chronic, with intellectual disability if untreated. Organic acidemias, like methylmalonic acidemia, involve defects in the breakdown of amino acids, fats, or carbohydrates, leading to buildup of organic acids. They often present with acute metabolic crises: vomiting, lethargy, and metabolic acidosis. Diagnosis: aminoacidopathies are detected by plasma amino acid analysis; organic acidemias by urine organic acid analysis. Both can cause high ammonia, but organic acidemias more often cause severe acidosis. Treatment differs: aminoacidopathies often require protein restriction, while organic acidemias may need carnitine and specific vitamins.
2. How would you design an experiment to tell apart whether an allosteric enzyme follows the MWC or KNF model?
One way is to measure the binding of ligands to the enzyme at different concentrations and see the shape of the binding curve. The MWC model predicts a sigmoidal curve that is symmetric and can be described by a single equilibrium constant for the T-to-R transition. The KNF model also gives a sigmoidal curve, but the shape depends on the number of subunits and the order of binding. Another method is to use techniques like X-ray crystallography or NMR to see if all subunits change shape at once (MWC) or one by one (KNF). Also, studying mutant enzymes with only one functional subunit can help: in MWC, such a mutant would still show cooperativity if the T-R equilibrium is intact, while in KNF, cooperativity would be lost.
3. How can periodate oxidation tell apart a 1→4 linked glucose chain from a 1→6 linked chain?
In a 1→4 linked chain, each glucose has free -OH groups on carbons 2, 3, and 6. Periodate will cleave between carbons 2-3 and 3-4? Actually, careful: For a 1→4 linked glucose, the 4-OH is used in the bond, so it is not free. The free -OHs are on C2, C3, and C6. Periodate cleaves between C2-C3 and C3-C4? But C4 is linked, so no cleavage there. Actually, periodate cleaves between C2-C3 because both have -OH. C3-C4: C4 has no free -OH, so no cleavage. So each glucose consumes 1 mole of periodate. In a 1→6 linked chain, the 6-OH is used, so free -OHs on C2, C3, and C4. Then periodate cleaves between C2-C3 and C3-C4, consuming 2 moles per glucose. By measuring periodate consumed, you can tell the linkage.
4. Why is newborn screening done soon after birth?
Newborn screening is done soon after birth to find serious but treatable disorders before they cause harm. For example, phenylketonuria (PKU) is a disorder where the body cannot break down an amino acid called phenylalanine. If not treated, it builds up and causes brain damage. Early detection allows a special diet to prevent this. Similarly, congenital hypothyroidism (low thyroid hormone) is detected by measuring thyroid-stimulating hormone (TSH) in a blood spot. Treatment with thyroid hormone pills prevents intellectual disability. The screening uses a few drops of blood from the baby's heel, collected on a special card. It is a simple test that saves lives.
5. How does a transmembrane protein's topology (which side of the membrane the N- and C-termini face) get determined during translocation?
Topology is determined by the orientation of the signal sequence and stop-transfer sequences. For a single-pass transmembrane protein, if the signal sequence is at the N-terminus and is not cleaved, it acts as a signal-anchor that inserts into the membrane in a specific orientation. The part of the protein that follows the signal sequence will be translocated across the membrane. A stop-transfer sequence, a stretch of hydrophobic amino acids, stops further translocation and anchors the protein in the membrane. The orientation of the signal sequence (whether it inserts with the N-terminus facing the cytosol or the lumen) decides the final topology.
6. How do you determine Ki from experimental data?
You measure the initial reaction rate at several substrate concentrations, with and without the inhibitor. Then you plot the data, often as a Lineweaver-Burk plot (1/rate vs 1/substrate). For competitive inhibition, the lines cross on the y-axis, and the slope change gives Ki. For uncompetitive inhibition, lines are parallel, and the intercept change gives Ki'. You can also use nonlinear regression to fit the Michaelis-Menten equation including inhibition terms. The Ki value is the inhibitor concentration that halves the enzyme's affinity or catalytic rate, depending on type. Software like GraphPad Prism can calculate Ki directly from the data.
7. What is the difference between allosteric and non-allosteric regulation?
Allosteric regulation involves a molecule binding at a separate site from the active site, changing the enzyme's shape and activity. Non-allosteric regulation usually means direct competition at the active site (competitive inhibition) or irreversible binding. Allosteric enzymes often have multiple subunits and show cooperativity, while non-allosteric enzymes follow simple Michaelis-Menten kinetics. Allosteric regulation is reversible and fine-tunes enzyme activity in response to cellular signals. Non-allosteric regulation is often simpler, like a competitive inhibitor blocking the active site. Both are important for controlling metabolism.
8. A child has episodes of vomiting, lethargy, and metabolic acidosis. What type of inborn error might this be and what test helps diagnose it?
These symptoms suggest an organic acidemia, such as methylmalonic acidemia or propionic acidemia. These are inborn errors where the body cannot break down certain amino acids or fats, leading to buildup of organic acids. A key diagnostic test is urine organic acid analysis by gas chromatography-mass spectrometry (GC-MS). This test detects abnormal organic acids like methylmalonic acid. Blood tests may show high ammonia and metabolic acidosis with an increased anion gap. Treatment includes a special low-protein diet, carnitine supplements, and sometimes vitamin B12. Early diagnosis and management prevent severe episodes and long-term damage.
9. A baby with maple syrup urine disease has a sweet-smelling urine. What biochemical pathway is affected and what is the emergency treatment?
Maple syrup urine disease (MSUD) affects the breakdown of branched-chain amino acids (leucine, isoleucine, valine). The enzyme branched-chain alpha-keto acid dehydrogenase is deficient. This causes buildup of these amino acids and their keto acids, which smell like maple syrup. Emergency treatment for a metabolic crisis includes stopping protein intake and giving high-calorie fluids with glucose and lipids to prevent catabolism. Hemodialysis may be needed to rapidly remove toxic metabolites. Long-term treatment is a special diet low in branched-chain amino acids and careful monitoring. Without treatment, MSUD causes brain damage and death.
10. How does next-generation sequencing (NGS) improve genetic testing compared to older methods?
Next-generation sequencing (NGS) can sequence millions of DNA fragments at once, allowing analysis of many genes or even the whole genome quickly and cheaply. Older methods like Sanger sequencing are slower and only sequence one gene at a time. NGS is useful for diagnosing heterogeneous disorders, like inherited cancer syndromes, where many genes could be responsible. It can also detect mutations at very low levels, useful in cancer for identifying tumor DNA in blood (liquid biopsy). However, NGS generates large amounts of data that require complex analysis. It has revolutionized genetic testing by making comprehensive screening possible.
11. A woman with breast cancer has a tumor that tests positive for estrogen receptors. What does this mean for treatment?
Estrogen receptor-positive breast cancer means the cancer cells have receptors that bind to estrogen, which can stimulate their growth. Treatment often involves blocking estrogen's effect. Drugs like tamoxifen block the estrogen receptor on cancer cells, preventing estrogen from binding. Another approach is to lower estrogen levels using aromatase inhibitors (like letrozole) in postmenopausal women. These treatments can slow or stop cancer growth. They are often given for 5-10 years after initial treatment. Hormone therapy is effective only for receptor-positive cancers. It is less toxic than chemotherapy and is a key part of treatment.
12. Why do some inborn errors cause high ammonia levels and how is it treated?
High ammonia (hyperammonemia) occurs when the urea cycle is defective, as in ornithine transcarbamylase (OTC) deficiency. The urea cycle normally converts toxic ammonia into urea for excretion. If an enzyme is missing, ammonia builds up. Symptoms include vomiting, lethargy, and coma. Emergency treatment includes stopping protein intake, giving intravenous glucose and lipids, and using medications like sodium benzoate or sodium phenylacetate to help remove ammonia. Hemodialysis may be necessary. Long-term treatment includes a low-protein diet and supplements like arginine or citrulline. Early diagnosis and treatment prevent brain damage.