(Choice A) Heterochromatin refers to condensed DNA that has a low level of transcriptional activity, whereas euchromatin (loosely arranged) has high levels of transcriptional activity. Histone acetylation promotes the formation of euchromatin; this process is impaired in Huntington disease, but is unrelated to the pathogenesis of Parkinson disease.
(Choice B) Vitamin K-<lependent gamma-carboxylation is critical for the functioning of clotting factors II, VII, IX, and X and of anticoagulative proteins C and S. Warfarin inhibits reduction of vitamin K to its active form, which in turn prevents carboxylation of vitamin K-dependent clotting factors.
(Choice C) One step in the hepatic processing of bilirubin includes bilirubin conjugation with glucuronic acid in the endoplasmic reticulum. Patients with Crigler-Najjar syndrome lack the enzyme needed to catalyze bilirubin glucuronidation, causing severe unconjugated hyperbilirubinemia.
(Choice D) Phosphorylation, or the addition of a phosphate group (PO.;3-) to a protein or other organic molecule, is commonly involved in the regulation of enzymatic activity. Hyperphosphorylation of tau proteins can contribute to abnormal protein aggregation in Alzheimer disease, not Parkinson disease.

(Choice B) Arachidonic acid can be ingested or synthesized from phospholipids in the cell membrane. Its eicosanoid derivatives (eg, prostanoids, leukotrienes) are important modulators of inflammation. It is not affected by carnitine levels.
(Choice C)
Glutathione is a tripeptide- that can be synthesized from amino acids (glutamate, cyste-ine, and glycine). It is an important antioxidant and plays a role in DNA synthesis and repair.
(Choice D) Homocysteine is an amino acid that is synthesized from methionine. Using vitamin cofactors, it can be converted to cysteine (pyridoxine) or recycled into methionine (cobalamin).
(Choice E) Lactate is produced from pyruvate under anaerobic conditions. Patients with carnitine deficiency synthesize lactate normally but may produce increased lactate during times of catabolic stress due to inability to utilize fatty acids for energy production.
(Choice F) Palmitate is a fatty acid that can be ingested or synthesized from carbohydrates. Palmrtate synthesis occurs in the cytosol and would not be affected by carnitine deficiency.

(Choice B) Binding of a ligand to the extracellular domain of a transmembrane receptor can result in activation of the cAMP or phosphatidylinositol second messenger system, activation of receptor tyrosine kinases, opening of ion channels, and activation of the calcium-calmodulin system. The extracellular domain of these receptors is typically composed of hydrophilic amino acids.
(Choice C) Activation of G protein-coupled receptors utilizing the cAMP second messenger system results in increased synthesis of intracellular cAMP and subsequent activation of protein kinase A. Protein kinase A then phosphorylates cytosolic proteins that translocate into the nucleus and bind to DNA promoter regions to modulate transcription.
(Choice D) Intracellular iron-containing proteins (hemeproteins) include hemoglobin, myoglobin, and cytochrome oxidase. Heme is a complex of protoporphyrin IX and iron. Hemoglobin A, the most common hemoglobin in adults, consists of 2 alpha and 2 beta globin chains held together by noncovalent interactions. Each subunit contains stretches of alpha helices that form a heme-binding pocket lined by nonpolar amino acids.
(Choice E) Insulin and IGF-1 bind to receptors that possess tyrosine kinase on their intracellular domain (receptor tyrosine kinases). These receptors dimerize upon ligand binding to the extracellular domain, which subseq uently activates tyrosine kinase. Tyrosine kinase phosphorylates tyrosine residues, triggering a downstream cellular response.