Back to dashboard Episode 51

DIP Episode 51 - Comprehensive USMLE Step 1 Biochemistry Review (Session 2 of 2)

MixedMedium4 - HighHas Q&A

Step 1Step 2MedicineUSMLEStep 1Step 2

Topic

Iron and heme metabolism; Glucose transport kinetics; Glycolysis, gluconeogenesis, and pyruvate fate; Lipid synthesis and breakdown (Beta oxidation)...

Key Takeaway

Understanding the regulatory mechanisms of major metabolic pathways—such as insulin-mediated dephosphorylation/activation, substrate inhibition (e.g., Malonyl CoA), and specific enzyme deficiencies (e.g., GSDs)—is critical for diagnosing biochemical disorders.

Runtime: — min

Published: 2018-09-25

Completed audio: Yes

Completed notes: Yes

Anki made: Yes

Section: BIOCHEMISTRY

Open Notion notes Episode source

Episode Notes

Source / episode info

Episode: 51
Title: Divine Intervention Episode 51 – Comprehensive USMLE Step 1 Biochemistry Review (Session 2 of 2)
Published: 2018-09-25
Source: Episode page

One-liner

This episode provides a comprehensive review of key metabolic pathways including iron/heme metabolism, glucose transport kinetics, glycolysis regulation, pyruvate fate (PDH complex), lipid synthesis and breakdown, cholesterol synthesis, and the spectrum of glycogen storage diseases.

High-yield summary

Iron Metabolism: Iron is absorbed in the duodenum only in the Fe^{2+} form; Vitamin C enhances this absorption. Hemochromatosis results from HFE mutations causing excessive iron reabsorption, treated with phlebotomy.
Glucose Transport: GLUT1 operates under zero-order kinetics (low K_M 5 mM), tracking normal blood glucose. GLUT2 has a high K_M, ensuring proportional uptake relative to plasma glucose levels.
Glycolysis Regulation: Glucokinase (liver) is induced by insulin and regulated by the GKRP, which sequesters it in the nucleus when bound by Fructose-6-Phosphate (F6 P).
Pyruvate Fate: Pyruvate can enter mitochondria via the PDH complex (requiring 5 cofactors: Thiamine, Lipoic acid, CoA, {FAD}, {NAD}^{+}) or be converted to OAA by Pyruvate Carboxylase (requires Biotin).
Glycogen Storage Diseases (GS Ds): Must differentiate the specific enzyme deficiency and clinical presentation: Pompe's (acid maltase/lysosomal, severe heart failure in infancy); Von Gierke’s (glucose-6-phosphatase/liver, hypoglycemia, lactic acidosis); McArdle’s (muscle glycogen phosphorylase/muscle, exercise myopathy).

Learning objectives

Describe the absorption mechanisms of iron, folate, and Vitamin B12.
Differentiate between various types of hyperbilirubinemia based on urine findings.
Explain the regulatory roles of insulin and glucagon on key metabolic enzymes (e.g., PFK2, Glucokinase).
Outline the cofactors required for Pyruvate Dehydrogenase Complex function.
Compare and contrast the clinical presentations and enzymatic defects of various Glycogen Storage Diseases (GS Ds).

Board exam buzzwords

Condition	Key Finding	Association	Board Exam Tip
Hemochromatosis	Iron overload	HFE mutation; excessive iron reabsorption	Treatment is phlebotomy, not chelation.
McArdle’s Disease (GSD5)	Exercise myopathy/cramps	Deficiency of muscle glycogen phosphorylase	Symptoms are restricted to skeletal muscle and do not cause hypoglycemia.
Von Gierke’s Disease (GSD1)	Hypoglycemia, Lactic Acidosis	Glucose-6-phosphatase deficiency in the liver	The hallmark is severe fasting hypoglycemia due to inability to release glucose.
GLUT2 Transporter	High K_M	Proportional glucose uptake relative to plasma levels	Allows the liver/pancreas to accurately track blood sugar changes.

Rapid review table

Topic	Key Point	Context	Exam Relevance
Iron Absorption	Fe^{2+} form only; Vitamin C required	Duodenum absorption	High-yield point for GI biochemistry questions.
Glucokinase Regulation	Inhibited by F6 P (via GKRP)	Liver glucose metabolism	Demonstrates allosteric regulation and nuclear sequestration.
PDH Complex Cofactors	5 cofactors required	Pyruvate -> Acetyl-CoA conversion	Essential for understanding the metabolic consequences of B1 deficiency (alcoholism).
GSD Differentiation	Muscle vs Liver defect	Specific enzyme location determines clinical presentation	Must know which GSD affects muscle (McArdle's) and which affects liver (Von Gierke's).

Board-speak -> diagnosis

Board-speak / Vignette phrase	Diagnosis / Concept	Why it fits
A young child presents with severe rhabdomyolysis and muscle cramps following intense exercise. Liver function tests are normal.	McArdle’s Disease (GSD5)	Deficiency of muscle glycogen phosphorylase; symptoms are restricted to skeletal muscle activity.
A patient develops hypoglycemia, lactic acidosis, and hyperlipidemia after fasting, with markedly elevated phosphate levels.	Von Gierke’s Disease (GSD1)	Defect in glucose-6-phosphatase in the liver, preventing glucose release into circulation.
A neonate presents with jaundice; urine contains increased urobilinogen but no direct bilirubin, and stools are dark/tea-colored.	Conjugated Hyperbilirubinemia (e.g., Dubin-Johnson)	Indicates an excretory problem in the liver (hepatocyte failure), leading to conjugated bilirubin backup into bile.
A patient with chronic alcoholism presents with signs of peripheral neuropathy and dermatitis, despite normal glucose levels.	Thiamine Deficiency (B1)	Alcohol impairs thiamine absorption/utilization; required for PDH complex and -ketoglutarate DH.
A metabolic screen reveals elevated plasma lactate and pyruvate in a patient presenting with muscle weakness after exercise.	Pyruvate metabolism defect (e.g., PDH deficiency)	Indicates an inability to efficiently convert pyruvate into Acetyl-CoA, causing backup of upstream metabolites.
A patient is treated for hypercholesterolemia with a drug that competitively inhibits HMG-CoA reductase and increases LDL receptor expression.	Statin therapy mechanism	Statins block the rate-limiting step in cholesterol synthesis (HMG-CoA -> Mevalonate).

Differential diagnosis / distinguishing features

Glycogen Storage Diseases (GS Ds)

Key Features	Distinguishing Findings	Next Step
Von Gierke’s (GSD1)	Hypoglycemia, Lactic Acidosis; Liver failure	Defect in Glucose-6-phosphatase. Severe fasting hypoglycemia.
Pompe’s Disease (GSD2)	Cardiomyopathy, severe infantile weakness	Deficiency of acid -1,4-glucosidase (lysosomal). Affects lysosomes globally.
McArdle’s Disease (GSD5)	Exercise myalgia/cramps; Normal blood glucose	Deficiency of muscle glycogen phosphorylase. Does not cause hypoglycemia.

Management pearls

Hemochromatosis: Diagnosis involves checking transferrin saturation and genetic testing (HFE). Treatment is regular phlebotomy to reduce iron stores.
Vitamin B7 Deficiency: Severe egg white consumption can lead to avidin binding, causing functional deficiency of Biotin, which is a cofactor for Pyruvate Carboxylase.
Statin Therapy: Statins are competitive inhibitors of HMG-CoA reductase and their mechanism leads to increased LDL receptor expression on the liver surface.
GSD Management: The management depends entirely on the specific enzyme defect (e.g., dietary carbohydrate restriction with frequent glucose supplementation for GSD1).

Don't miss

🚨

GLUT Transporters: GLUT2 has a high K_M, allowing it to operate in the linear, proportional range of the Michaelis-Menten curve relative to plasma glucose.

🚨

PDH Complex Cofactors: Remember the five cofactors (Thiamine, Lipoic acid, CoA, \text{FAD}, \text{NAD}^{+}) and their clinical relevance (e.g., Thiamine deficiency in alcoholism).

🚨

Skeletal Muscle Glycogenolysis: Skeletal muscle lacks glucose-6-phosphatase; therefore, it cannot raise blood glucose during fasting.

🚨

FA Synthesis vs. Breakdown Location: FA synthesis occurs in the cytosol, while \beta-oxidation occurs within the mitochondria.

Integration & clinical reasoning

Insulin's Role: Insulin is a master regulator that promotes anabolic pathways by activating enzymes via dephosphorylation (e.g., PFK2 activation, Glycogen Synthase activation).
Citrate Shuttle: The buildup of ATP and NADH from TCA/ETC inhibits Isocitrate dehydrogenase, causing citrate to accumulate in the mitochondria and be shuttled out into the cytosol for fatty acid synthesis.
Pyruvate's Versatility: Pyruvate is a central hub: it can become lactate (regenerating \text{NAD}^{+}), Acetyl-CoA (TCA cycle/lipogenesis), or OAA (gluconeogenesis).

OMM / COMLEX integration

🦴

For COMLEX: know these viscerosomatics / Chapman points, but don't let OMM distract from emergent diagnosis and management.

Viscerosomatics: The metabolic defects discussed (e.g., muscle cramps in McArdle's) are excellent examples of viscerosomatic dysfunction, where a defect in one organ system affects others through systemic metabolite buildup.
OMM/OMT Contraindications: Not emphasized in this episode.

Concept connections / cross-references

Episode 51covers the metabolic regulation principles that were introduced in earlier episodes, particularly regarding hormonal control and enzyme activation. No explicit cross-references to other numbered Divine Podcast episodes are provided in this material.

High-yield association table

Condition	Association	Mechanism	Clinical Significance
Hemochromatosis	Iron overload	HFE mutation leads to excessive iron absorption/retention	Requires phlebotomy; risk of organ damage (liver, heart).
McArdle’s Disease	Exercise myopathy	Deficiency of muscle glycogen phosphorylase	Symptoms are limited to skeletal muscle and do not cause systemic hypoglycemia.
Von Gierke’s Disease	Hypoglycemia/Lactic Acidosis	Glucose-6-phosphatase deficiency in the liver	Severe fasting hypoglycemia due to inability to release free glucose into the bloodstream.
GLUT2 Transporter	High K_M	Allows proportional uptake of glucose relative to plasma concentration	Ensures accurate tracking of blood sugar levels by the liver and pancreas.

Key terms glossary

Term	Definition	Context	Example
Glucokinase (GK)	Hexokinase isoform found in the liver; high K_M	Regulates glucose uptake/phosphorylation specifically in the liver.	Induced by insulin, regulated by GKRP.
PDH Complex	Pyruvate Dehydrogenase Complex	Converts pyruvate to Acetyl-CoA within the mitochondria.	Requires 5 cofactors (B1, B5, B2, B3, {TPP}).
GKRP	Glucokinase Regulatory Protein	Inhibits glucokinase by binding it and sequestering it in the nucleus.	F6 P binding increases affinity for GK, promoting sequestration.
Acid Maltase (GSD2)	Acid -1,4-glucosidase	Enzyme deficient in Pompe's disease; breaks down glycogen within lysosomes.	Deficiency leads to lysosomal accumulation of glycogen, primarily affecting the heart.

Study optimization

Topic	Study Approach	Priority	Resources
GSD Differentiation	Create a comparison table (enzyme, location, primary symptom)	High	Use flowcharts or flashcards to compare GS Ds side-by-side.
Enzyme Regulation	Focus on the mechanism of activation/inhibition (e.g., dephosphorylation vs. allosteric inhibitor).	Medium-High	Practice drawing metabolic pathways and adding regulatory arrows ( or ).
Cofactor Requirements	Memorize the cofactors for complex enzymes (PDH, Pyruvate Carboxylase) and their associated vitamins.	High	Use mnemonics (e.g., "Thiamine likes alcohol").

Question pattern recognition

Differential Diagnosis: Comparing metabolic disorders with similar symptoms but different underlying enzymatic defects (e.g., GS Ds).
Regulatory Mechanism: Understanding how hormones or allosteric effectors control flux through pathways (e.g., insulin/glucagon effects on PFK2 and GK).
Clinical Correlation: Linking a biochemical defect to a specific organ system failure (e.g., Pompe's -> Cardiomyopathy; McArdle's -> Myopathy).

Test yourself

Common mistakes to avoid

🚫

Confusing the location of glycogen phosphorylase activity (muscle vs. liver) and its associated clinical syndromes.

🚫

Forgetting that GLUT2's high K_M is key to understanding proportional glucose uptake in the liver.

🚫

Misidentifying which metabolic process requires phlebotomy as treatment (Hemochromatosis).

Common traps

⚠️

The "Straight Line" Trap: Assuming all transporters operate linearly; remember that GLUT1 operates under zero-order kinetics due to its low K_M.

⚠️

GSD Overlap Trap: Mistaking the symptoms of McArdle's (muscle only) for Von Gierke’s (systemic/liver).

⚠️

Bilirubin Pattern Trap: Confusing obstructive jaundice (dark urine, pale stools) with hepatocellular failure.

Original transcript with highlights

Divine Intervention Episode 51 Comprehensive Step 1 Biochemistry Review (Session 2) Some PGY1 Absorbing Fe/Breaking Down Heme -Fe is absorbed (also only carries O2) in the 2+ form only. Vit C encourages this process (what are 2 other HY functions of Vit C that have been discussed?). -HFE regulates this process. A HFE mutation can cause too much Fe reabsorption (hemochromatosis, tx w/phlebotomy). What should your first step in diagnosis be? -I’d encourage you to also try recalling the relationship b/w Fe2+/Fe3+ w/pathologies. -It is HY to know the breakdown pathway for heme and the different diseases that could arise from issues along that pathway (as well as the associated kind of hyperbilirubinemia)-> Hemolytic anemia, Newborn jaundice, TMP-SMX toxicity, Crigler Najjar (T1 and 2), Gilbert’s, Dubin Johnson, Rotor, Obstructive process, etc. -Remember that Fe is absorbed in the duodenum, folate is absorbed in duodenum/jejunum, B12 is absorbed in the terminal ileum (re-Crohn’s association). Heme Breakdown

Another Step 1 Worthy Question/Thought Can you explain these lesions? Option A-Increased urine bilirubin, decreased urine urobilinogen, increased direct bilirubin, dark/tea colored urine, acholic stools. Option B-Increased urine urobilinogen, no urine bilirubin, increased indirect bilirubin, normal colored urine, dark colored stools. Some General Principles (make thy life super easy!) -Insulin works through tyrosine kinase receptors. Insulin is a dephosphorylator. -Glucagon works through G protein coupled receptors which activate PKA. Glucagon is a phosphorylator. -If you know this, you can easily reason that if an enzyme is activated by insulin, the activated form must be a “dephosphorylated form” of the enzyme (and vice versa for glucagon). -Carboxylase enzymes are ABC enzymes (they use ATP and Biotin, hence the AB). C stands for carboxylase (and CO2). -Kinase enzymes as a rule add phosphate groups to stuff. GLUT Transporters

Other Important Stuff -As a correlation from prior blocks, remember that GLUT1 transporters operate under zero order kinetics by virtue of their low KM (approx. 5 mM) which tracks along with normal blood glucose levels. -GLUT 2 transporters have a KM that is much higher than normal blood glucose levels. If you consider the Michaelis Menten curve, this is ideal b/c the transporters will operate on the “straight line” portion which essentially guarantees “proportional” glucose uptake that tracks along with blood glucose levels. -Why are GLUT2 transporters bidirectional? -GLUT4 transporters are insulin dependent. Muscle has the unique ability to express GLUT4 transporters in an “insulin independent” fashion in the setting of exercise. Glycolysis Broken Down Part 1

Glucokinase vs. Hexokinase -Hexokinase has a low KM and VMAX. -Glucokinase has a high KM and VMAX. Glucokinase is also induced by insulin. Glucokinase is regulated by a regulatory protein under the auspices of F-6-P and glucose. Glucokinase Regulatory Protein -Is an inhibitor of glucokinase (GK). -Binds GK and sends it to the nucleus (where it is inactive). -GKRP has the ability to bind both F6 P and glucose. -When bound by F6 P, GKRP has a higher affinity for GK (which sequesters GK by taking it to the nucleus). -When bound by glucose, GKRP has a much lower affinity for GK (which brings it back to the cytoplasm for reaction). Glycolysis Broken Down Part 2

Glycolysis Broken Down Part 3

Glycolysis Broken Down Part 4

Some Other Important Stuff -Overall, glycolysis gives rise to the rule of 2s (2 AT Ps, 2 NADH, and 2 Pyruvates). Pyruvate has multiple fates; -It can form lactate under the action of lactate DH. This step regenerates NAD to keep the Glyceraldehyde-3-P DH step working. -Pyruvate can go into mitochondria to receive special attention from the PDH complex ultimately leading to Acetyl-co A formation. -Pyruvate can receive special attention from Pyruvate carboxylase (what is a HY cofactor utilized by this enzyme???) to form OAA that can reverse course in gluconeogenesis (through subsequent PEPCK action). Galactose Metabolism

Fructose Metabolism

Other Important Stuff -You should by now notice that the main job of a kinase is to phosphorylate stuff. “Phosphorylated stuff” cannot leave cells. They also tend to be osmotically active. This explains the many problems associated with the “2nd enzyme” in the 2 metabolic pathways we just discussed. -Note the temporal association of the diseases presented to breastfeeding and introduction of fruits.

Tender Loving Care For Nancy -In cells with mitochondria, pyruvate can receive special attention from the PDH complex as a means of making Acetyl CoA that can be used in the TCA cycle. -It is HY to know the cofactors for this complex (Thiamine-Vit B1, Lipoic acid, CoA-Vit B5, FADH2-Vit B2, and NADH-Vit B3). -This pathway generates 1 NADH and 1 CO2 per pyruvate. -Acetyl CoA/Ca inhibits this complex. Low energy indicators like ADP and NAD activate this complex (hence ATP and NADH should inhibit). -Some Step 1 correlates->there is an alcoholic story with thiamine. The TLCFN cofactors are also used by alpha ketoglutarate DH and branched chain ketoacid DH. Transketolase in the HMP shunt relies on the T part of the mnemonic. Arsenic poisoning is bad b/c it screws up lipoic acid (consider in a rice water stool/garlic breath Q). The TCA Cycle

TCA Cycle Takeaways -Makes (per pyruvate) CO2, NADH, FADH2, ATP (2311). -The VOMIT pathway feeds into succinyl-co A (Valine, Odd Chain F As, Methionine, Isoleucine, Threonine). -It is also HY to know certain keto acid, AA relationships; OAA is the keto acid to aspartate. Pyruvate is the keto acid to alanine. Alpha ketoglutarate is the keto acid to glutamate. The ETC in a Nutshell

Glycogen Formation and Breakdown In a Nutshell

Other Important Stuff With Glycogen -UDP glucose is the intermediate that comes right before Glycogen. As a rule, you want “activated monomers” coming together to form a polymer since bond formation requires energy. UDP glucose is an activated monomer (where have we seen this before???). As an aside, think of epinephrine as being equivalent to glucagon. -Skeletal muscle lacks glucose-6-phosphatase. Skeletal muscle can participate in glycogenolysis but the buck stops at glucose-6-phosphate which is simply used in glycolysis. Skeletal muscle does not raise blood glucose in fasting! -The alpha-1,4 bonds in glycogen are broken down to G-1-P. There is 1 reaction where free glucose can be formed from a pathway that operates only in lysosomes (enzyme is alpha-1,4-glucosidase also known as acid maltase b/c the lysosome is acidic). A deficiency of this enzyme constitutes Pompe’s disease. For Step 1, consider in any super young kid with heart failure. Other Important Stuff With Glycogen -With exercise, epinephrine can activate beta receptors that can essentially accomplish the same job as glucagon since these receptors are G-protein coupled. In addition, Ca complexed with calmodulin can also activate glycogen phosphorylase kinase.

-As discussed in an earlier slide, you need to know the GS Ds for Step 1 (not your exam, so ignore this); GSD1 (Von Gierke’s, glucose-6-phosphatase deficiency)-no muscle issues, liver issues (since this is where G-6-P’ase lives), hypoglycemia b/w meals since gluconeogenesis is gone, severe lactic acidosis since the Cori cycle does not work. GSD2 (Pompe’s, acid maltase deficiency)-glycogen accumulates in lysosome, kid dies of heart problems before or around age 2. GSD3 (Cori’s, debranching enzyme deficiency)-fasting hypoglycemia that is not as severe as type 1 since there are other alternatives, muscle and liver problems since both kinds of tissue have the affected enzyme. GSD5 (McArdle’s disease, deficiency of the glycogen phosphorylase isoform found only in muscle)-muscle cramps in a young athlete with exercise, no liver problems since the liver isoform is just fine. Gluconeogenesis In a Nutshell

Other Important Stuff With Gluconeogenesis -For Step 1, remember the association of serious egg white consumption (too much avidin) with a deficiency of Vitamin B7 which is needed for pyruvate carboxylase function.

Pentose Phosphate Pathway In a Nutshell

Rapid Review-The Roles of Insulin (KNOW THIS!!!) -Induces Glucokinase in Pancreatic Beta Cells/Liver -Dephosphorylates and Activates PFK2 in Glycolysis -Dephosphorylates and Activates Pyruvate Dehydrogenase indirectly -Activates Acetyl CoA Carboxylase which is the RLS of FA Synthesis -Induces genetic expression of FA Synthase -Upregulates GLUT4 targeting to the membranes of Adipocytes/Skeletal Muscle -Activates Glycogen Synthase by dephosphorylation

Intro To Fatty Acid Metabolism/FA Synthesis -Any reason why cis unsaturated fats are better than trans unsaturated/saturated fats? -In a patient with steatorrhea, how would you differentiate between a pancreatic enzyme deficiency/bile non-delivery as a cause in comparison with malabsorption? -To cook F As (in the cytoplasm), you need a few ingredients -> Acetyl co A (from the mitochondria through citrate shuttle), ATP (from glycolysis, TCA, ETC), CO2 (everywhere), NADPH (from the HMP shunt and from the Malic enzyme). -When the cell has a lot of energy from the TCA cycle and ETC -> ATP and NADH build up -> inhibit Isocitrate dehydrogenase -> Citrate builds up -> to cytoplasm. -RLE here is Acetyl co A carboxylase (induced by insulin, also activated by dephosphorylation, activated by citrate as well ->what is the feedforward activating mechanism in glycolysis?).ACCASE deactivation is via glucagon med. phosphorylation.

FA Synthesis

Triglyceride Synthesis/Breakdown -Triglyceride synthesis requires 2 ingredients -> FF As and Glycerol-3-P. -We already know how to make FF As. Glycerol-3-P is made primarily in the liver and adipocytes. -There is only 1 source of glycerol-3-P in adipocytes (Glycerol-3 P DH). There are 2 sources in liver (Glycerol-3 P DH and Glycerol kinase). -The breakdown of T Gs requires lipases (e.g. pancreatic lipase, lipoprotein lipase, hormone sensitive lipase, etc). -One other kind of TG (modified a bit) is a glycerophospholipid. These “lipids” are also broken down by special lipases (PLA1, PLA2, PLC, etc).

Cholesterol Synthesis -RLE is HMG CoA reductase (insulin activated, glucagon deactivated) which is inhibited by statins (competitive inhibition, increased LDL-R expression). -There are nice tie ins here with statin myopathy (farnesyl PP), dolichol phosphate synthesis (farnesyl PP), and weird similarities with the fungal ergosterol synthesis pathway. -Dolichol PO43- carries sugars into the ER for N-linked glycosylation (attachment to asparagine residues). -7 alpha hydroxylase helps with making bile acids from cholesterol. After playing their life roles, bile acids move back in enterohepatic recirculation to the liver (and inhibit 7-AH). This nicely explains how bile acid binding resins lower blood cholesterol levels.

Cholesterol Synthesis (Cytosol)

FA Breakdown (Beta Oxidation) -Occurs in the mitochondria (unlike synthesis in the cytosol). We don’t want both processes to be operating at the same time so malonyl co A from FA synthesis inhibits the transport mechanism of F As (CAT1) into the mitochondria. -Step 1 of BOX involves activating the FA by losing ATP (kinda like glycolysis).

Overview of FA Breakdown (Note special regulation of HSL)

A Step 1 Worthy Series of Questions -CAT deficiency presents with muscle weakness. Why is that? -It so happens that GSD5 (McArdle’s disease) also presents with muscle weakness. -How would you differentiate these two? -MCAD deficiency is classically said to present with hypoketotic hypoglycemia. What is the mechanism behind the “hypoketosis”? What is the mechanism behind the “hypoglycemia”? What is one characteristic “acid” that rises in the blood? -Is there some way to differentiate MCAD deficiency from CAT deficiency? Your friends at the NBME love this! This is a SUPER HY slide to understand and explain based on your understanding of the biochemistry.

Some Thoughts on Ketone Body Synthesis -Takes place in the mitochondria. Don’t get dinged on Step 1 for not recognizing the difference between the cytosolic HMG CoA Synthase (cholesterol synthesis) and the mitosolic HMG CoA synthase (KB synthesis) -The liver has the ability to make ketone bodies. It however, lacks the ability to use ketone bodies (thiophorase). -This is a relatively mute point, but in fasting states, the liver does not use AcCoA generated from BOX in the TCA cycle. It uses it almost exclusively for KB synthesis. Why might this be? -However, extrahepatic tissues have the ability to use K Bs to generate energy (Acetyl co A) which can be fed into the TCA cycle. Why can they do this?

Ketone Body Synthesis and Utilization

References -First Aid for The USMLE Step 1 2017

Practice questions — USMLE style

Question 1 — Biochemistry

A 45-year-old man presents with fatigue, abdominal pain, and evidence of liver cirrhosis. Laboratory studies reveal elevated serum ferritin levels and signs of iron overload in multiple organs. Genetic testing suggests a mutation in the HFE gene. The physician suspects hereditary hemochromatosis. Which diagnostic step is most appropriate to confirm the diagnosis and guide initial management?

A) Measuring transferrin saturation index (TSAT)
B) Performing liver biopsy for Prussian blue staining of iron deposits
C) Ordering genetic testing for mutations in the Transferrin Receptor 2 gene
D) Initiating high-dose oral Vitamin C supplementation immediately

Answer: A. The primary initial diagnostic step for suspected hemochromatosis is measuring the transferrin saturation index (TSAT). Elevated TSAT (>45%) strongly suggests excessive iron absorption and deposition. While genetic testing (HFE mutation) supports the diagnosis, the biochemical confirmation of iron overload via elevated TSAT guides the immediate need for treatment (phlebotomy).

Question 2 — Physiology

A biochemist is studying glucose uptake in various tissues using different GLUT transporters. They observe that one transporter maintains a near-linear relationship between blood glucose concentration and glucose uptake across physiological ranges, suggesting it operates under zero-order kinetics relative to normal blood sugar levels. Another transporter, however, has a much higher Michaelis constant (KM) than typical blood glucose concentrations, allowing its activity to remain proportional to changes in circulating glucose. Which pair of GLUT transporters best fits this description?

A) GLUT1 and GLUT4
B) GLUT2 and GLUT3
C) GLUT1 and GLUT2
D) GLUT4 and GLUT5

Answer: C. GLUT1 is known for its low KM (approx. 5 mM), allowing it to operate under zero-order kinetics that track normal blood glucose levels, making it crucial for basal uptake in many tissues. GLUT2 has a high KM relative to normal blood glucose levels; therefore, the Michaelis-Menten curve operates on the "straight line" portion, ensuring proportional glucose uptake that tracks with changes in circulating glucose concentrations.

Question 3 — Biochemistry

A young boy is admitted to the emergency department after an episode of severe vomiting and fasting for 12 hours. Physical examination reveals lethargy, and blood gas analysis shows profound hypoglycemia accompanied by low levels of ketone bodies (hypoketotic hypoglycemia). The physician suspects a defect in fatty acid oxidation. Which enzyme deficiency best explains this clinical picture?

A) Deficiency in carnitine palmitoyltransferase I (CPT-I), leading to impaired mitochondrial entry of long-chain fatty acids.
B) Deficiency in acetyl-CoA carboxylase, impairing the synthesis of malonyl-CoA and subsequent beta-oxidation.
C) Deficiency in medium-chain acyl-CoA dehydrogenase (MCAD), resulting in inability to oxidize medium-chain fatty acids.
D) Deficiency in branched-chain $\alpha$-keto acid dehydrogenase complex, leading to accumulation of branched amino acid metabolites.

Answer: C. MCAD deficiency is the most common defect causing hypoketotic hypoglycemia during fasting. The body cannot efficiently break down medium-chain fatty acids for energy, leading to severe hypoglycemia and an inability to produce adequate ketone bodies (hypoketosis). While CPT-I deficiency also causes liver failure and hypoglycemia, MCAD specifically impairs the oxidation of medium chains, which is a classic presentation differentiating it from other defects.

Question 4 — Biochemistry

A neonate presents with progressive muscle weakness and cardiomyopathy. The physical exam suggests a lysosomal storage disorder. Biochemical analysis reveals accumulation of glycogen within lysosomes due to a deficiency in acid $\alpha$-1,4-glucosidase (acid maltase). This enzyme is critical for breaking down the $\alpha$-1,4 bonds found in glycogen polymers within acidic compartments. What is the most likely diagnosis?

A) Von Gierke’s disease (GSD I)
B) McArdle’s disease (GSD V)
C) Pompe’s disease (GSD II)
D) Cori’s disease (GSD III)

Answer: C. The clinical presentation of cardiomyopathy and muscle weakness in a neonate, combined with the specific deficiency of acid $\alpha$-1,4-glucosidase (acid maltase), is diagnostic for Pompe's disease (Glycogen Storage Disease Type II). This enzyme deficiency leads to glycogen accumulation specifically within the lysosomes.

Quick fire review

What is the primary difference in regulation between hexokinase and glucokinase?

Hexokinase has a low $\text{K}_{\text{M}}$ and $\text{V}_{\text{MAX}}$, while Glucokinase has a high $\text{K}_{\text{M}}$ and $\text{V}_{\text{MAX}}$.

What is the functional consequence of GLUT2 having a high $\text{K}_{\text{M}}$?

It allows glucose uptake to be proportional to blood glucose levels, operating on the "straight line" portion of the Michaelis-Menten curve.

Which metabolic process requires the cofactor Thiamine ($\text{B}_1$), Lipoic acid, $\text{CoA}$ ($\text{B}_5$), $\text{FAD}$ ($\text{B}_2$), and $\text{NAD}^{+}$ ($\text{B}_3$)?

The Pyruvate Dehydrogenase (PDH) complex.

What is the key difference in how skeletal muscle handles glucose-6-phosphate compared to the liver?

Skeletal muscle lacks Glucose-6-phosphatase, meaning it cannot raise blood glucose levels via glycogenolysis; G6 P is simply used in glycolysis.

How does insulin activate Acetyl-CoA Carboxylase (AC Case)?

Insulin activates AC Case by promoting its dephosphorylation.

What molecule inhibits the transport of fatty acids into the mitochondria for beta oxidation?

Malonyl CoA, which acts as a feedback inhibitor.

Which enzyme is responsible for converting pyruvate to OAA in gluconeogenesis, and what cofactor does it require?

Pyruvate carboxylase; requires Biotin ($\text{B}_7$).

What are the three main fates of pyruvate after glycolysis?

Lactate (via lactate DH), Acetyl-CoA (via PDH complex into mitochondria), or OAA (via Pyruvate Carboxylase).

Which specific GSD deficiency leads to glycogen accumulation in lysosomes, primarily affecting the heart and causing severe infantile cardiomyopathy?

Pompe’s disease ($\text{GSD}2$), due to acid maltase deficiency.

What is the primary mechanism by which statins lower blood cholesterol levels?

They are competitive inhibitors of HMG-CoA reductase (the rate-limiting enzyme in cholesterol synthesis).

Why does the liver have a unique role regarding ketone bodies compared to extrahepatic tissues?

The liver synthesizes ketone bodies but lacks Thiophorase, meaning it cannot use them for energy. Extrahepatic tissues can use K Bs and feed them into the TCA cycle.

What is the activated monomer intermediate used in glycogen synthesis?

UDP glucose.

Which transporter operates under zero-order kinetics due to its low $\text{K}_{\text{M}}$ (approx. 5 mM)?

GLUT1 transporter.

Quick recall / Anki-style questions