Examen Integrado Bioquímica common mistakes
Examen Integrado Bioquímica · Understand the traps and how to avoid them.
When asked for the rate-limiting enzyme of a pathway, the candidate names the enzyme that catalyzes the committed step or the most regulated step without checking whether the question asks specifically for the slowest step; for glycolysis they answer phosphofructokinase-1 for every wording, including items about the committed step versus the rate-limiting step.
Textbooks often conflate 'committed', 'regulatory', and 'rate-limiting', so candidates memorize one enzyme per pathway and apply it to all three question types.
What to do instead
For each pathway, build a three-column table: committed step enzyme, rate-limiting/slowest enzyme, and allosteric regulators. For glycolysis note hexokinase/glucokinase (committed entry), PFK-1 (rate-limiting), and pyruvate kinase; for gluconeogenesis note pyruvate carboxylase (committed) versus fructose-1,6-bisphosphatase (rate-limiting). Answer only the column the stem names.
On Michaelis-Menten and inhibition items, the candidate predicts Km and Vmax changes from the inhibitor name alone without deriving them from the Lineweaver-Burk plot or the velocity equations; they label any inhibitor that lowers Vmax as noncompetitive even when the plot shows parallel lines.
Memorized mnemonics ('competitive raises Km, noncompetitive lowers Vmax') break down for uncompetitive and mixed inhibition, and candidates skip the algebraic check under time pressure.
What to do instead
For every inhibitor item, draw the double-reciprocal plot: intersecting on the y-axis = competitive (Vmax unchanged, Km up); parallel lines = uncompetitive (both Km and Vmax down by the same factor); intersecting left of the y-axis = mixed; intersecting on the x-axis = noncompetitive (Km unchanged, Vmax down). Confirm with the velocity equation before selecting.
When computing ATP yield, the candidate uses a single fixed P/O ratio (often 3 per NADH, 2 per FADH2) for every tissue and condition and ignores the glycerol phosphate versus malate-aspartate shuttle, so answers for brain versus liver or for cytoplasmic versus mitochondrial NADH are wrong.
Older textbooks teach integer P/O ratios, and candidates do not track where reducing equivalents are generated or how they enter the mitochondrion.
What to do instead
Before summing ATP, trace each NADH/FADH2 to its source: glycolysis NADH uses a shuttle (glycerol phosphate gives FADH2, ~1.5 ATP; malate-aspartate gives NADH, ~2.5 ATP), pyruvate dehydrogenase and TCA give mitochondrial NADH (~2.5), succinate dehydrogenase gives FADH2 (~1.5). State the assumed P/O values explicitly in the workup and adjust for the shuttle named in the stem.
The candidate answers lipoprotein questions by memorizing 'LDL is bad cholesterol' and cannot assign apolipoproteins, receptors, or enzyme cofactors to the correct particle; they mix up apoB-48 and apoB-100, or assign LCAT to VLDL and lipoprotein lipase to HDL.
Clinical shorthand ('good/bad cholesterol') displaces the structural facts, and the apolipoprotein table is learned as an undifferentiated list rather than tied to particle function.
What to do instead
Rebuild the table by particle: chylomicrons (apoB-48, apoA, apoC-II, apoE), VLDL (apoB-100, apoC-II, apoE), IDL, LDL (apoB-100 only, ligand for LDL receptor), HDL (apoA-I, apoA-II, LCAT activator). Anchor each enzyme: lipoprotein lipase on capillary endothelium activated by apoC-II; LCAT on HDL activated by apoA-I; CETP exchanges cholesteryl esters. Test yourself by particle, not by apolipoprotein name.
On urea cycle and amino acid items, the candidate cannot state which nitrogen enters as free ammonia versus as aspartate, or where the cycle is regulated, and they confuse the transport of nitrogen from muscle (alanine) with the transport from other tissues (glutamine).
The cycle is memorized as a list of five reactions without tracking nitrogen atoms or compartmentation, so questions that ask 'which nitrogen' or 'which tissue' defeat the list.
What to do instead
Draw the cycle labeling each nitrogen: carbamoyl phosphate supplies the nitrogen that becomes urea's first amino group (from ammonia via CPS-I in mitochondria), aspartate supplies the second (from oxaloacetate, via argininosuccinate synthetase). Note CPS-I is the regulated, NAG-activated step. Separately map nitrogen carriers: alanine from muscle (glucose-alanine cycle), glutamine from most tissues to kidney/intestine, and ammonia detoxified to urea in liver.
The candidate confuses DNA replication and transcription directionality and enzyme roles, assigning primase to RNA synthesis in transcription, or claiming DNA polymerase synthesizes 3'-to-5' or that the lagging strand is made continuously.
The words 'polymerase' and 'primase' are learned by name rather than by template direction and product, and the leading/lagging distinction is memorized as a slogan.
What to do instead
Fix the invariant: all nucleic acid polymerases read the template 3'-to-5' and synthesize the new strand 5'-to-3'. Then attach the enzyme to its job: primase makes a short RNA primer (replication only), DNA pol III extends, DNA pol I removes the RNA primer, RNA polymerase makes RNA without a primer. Leading strand is continuous toward the fork; lagging strand is discontinuous as Okazaki fragments.
On gene regulation items, the candidate treats all operons as inducible and all repressors as binding the operator only when bound by a corepressor, so they mispredict lac and trp responses; for the lac operon they ignore CAP-cAMP and claim glucose presence increases transcription.
The operon is learned as one story ('lactose induces, glucose represses') without separating negative control (repressor) from positive control (CAP-cAMP), and without distinguishing inducible from repressible systems.
What to do instead
For each operon, answer three questions: is it inducible or repressible, what is the repressor's default state, and what is the positive regulator. lac: inducible, repressor bound by default (lactose/allo-lactose releases it), CAP-cAMP activates only when glucose is low. trp: repressible, repressor inactive by default, tryptophan is the corepressor, attenuation adds a second layer. Predict transcription only after combining both controls.
When interpreting clinical lab values, the candidate reads a single abnormal number in isolation and names a diagnosis without checking the pattern (e.g., calling any high bilirubin 'liver disease' without fractionating direct versus indirect, or diagnosing diabetes from a single random glucose without confirmatory criteria).
Exam items supply a vignette with several values, but candidates anchor on the most salient abnormal value and skip the pattern-recognition step that distinguishes prehepatic, hepatic, and posthepatic causes.
What to do instead
Adopt a fixed reading order for every lab vignette: (1) list all abnormal values, (2) compute derived indices (direct/total bilirubin ratio, anion gap, AST/ALT ratio, transferrin saturation), (3) match the pattern to a category before naming a disease. For bilirubin: indirect-predominant = hemolysis or conjugation defect; direct-predominant with elevated ALP = cholestasis; mixed with high transaminases = hepatocellular. Apply explicit diagnostic criteria (e.g., fasting glucose ≥126 mg/dL or HbA1c ≥6.5% confirmed on a second occasion).
On integrated metabolism items (fed versus fasted, exercise, diabetes), the candidate describes pathways in isolation and cannot state which pathway is active in which tissue at a given time, or which enzyme is the reciprocal control point; they claim the liver performs glycolysis and gluconeogenesis simultaneously at high rates.
Pathways are studied as separate chapters, so the candidate never builds a tissue-by-state matrix, and reciprocal regulation (e.g., PFK-1 versus FBPase-1 controlled by fructose-2,6-bisphosphate) is not connected to hormonal signals.
What to do instead
Build a matrix: rows = tissues (liver, muscle, adipose, brain, RBC), columns = states (fed, fasted, exercise, uncontrolled diabetes), cells = active pathways plus the key regulatory enzyme. Anchor reciprocal pairs: PFK-1/FBPase-1 (fructose-2,6-bisphosphate, insulin versus glucagon), acetyl-CoA carboxylase/malate enzyme, glycogen synthase/glycogen phosphorylase. Note that liver gluconeogenesis is active in fasting while glycolysis is suppressed, and that muscle lacks glucose-6-phosphatase.
On laboratory technique items, the candidate selects a method by name without matching it to the analyte's properties; they choose SDS-PAGE to measure a protein's native quaternary structure, or use a competitive assay for a large analyte, or claim a Western blot quantifies absolute concentration.
Techniques are memorized as a list of acronyms, and the underlying separation or detection principle (charge, size, antigen-antibody, mass) is not linked to the question being asked.
What to do instead
For each technique, state the principle and then the question it answers: SDS-PAGE separates by size after denaturation (not native structure); native PAGE preserves complexes; isoelectric focusing separates by pI; Western blot detects and identifies a specific protein but is semi-quantitative; ELISA formats depend on analyte size (sandwich for large, competitive for small haptens); mass spectrometry gives mass and structure. Choose the technique only after naming what property must be measured.