7 topics with concise reviews: definitions, key facts, and class questions.
In this section, we examine the lifecycle of heme—from its multi-step assembly to its utilization—as well as the turnover of iron in the body. This is classic biochemistry that is encountered daily in clinical practice in the form of anemias, poisonings, and jaundice.
The material covers both normal physiology and pathology. We will break down heme biosynthesis step by step, along with the genetic errors along this pathway, known as porphyrias. We will separately study iron transport and storage, as well as clinical disorders of its metabolism. Finally, we will trace the entire pathway of heme catabolism to understand the mechanisms behind various types of jaundice, paying special attention to physiological and pathological neonatal jaundice.
What is most frequently asked in class? Instructors will undoubtedly require you to know the key and regulatory enzymes of heme biosynthesis (primarily ALAS), iron metabolism marker proteins (ferritin, transferrin), and the differential diagnosis of jaundice based on blood and urine bilirubin fractions. Learn these cascades with clinical application in mind from the start—this is the foundation you cannot do without in clinical years.
Which vitamins are essential for normal heme biosynthesis?
Pyridoxal phosphate is a required cofactor for the first reaction of synthesis. This is a derivative of vitamin B6, without which the enzyme 5-aminolevulinate synthase cannot function.
Why is a porphyria attack often accompanied by skin lesions?
Porphyrinogens accumulate in the skin. In the light, they turn into porphyrins, react with oxygen, and form active radicals that destroy cells.
Why is copper needed in iron metabolism?
Copper is a cofactor for ferroxidase (ceruloplasmin). Without this enzyme, iron cannot be oxidized to Fe3+ upon entering the blood and cannot bind to the carrier protein transferrin.
Why is ascorbic acid added to iron preparations?
Vitamin C reduces ferric iron to ferrous iron. This is critically important for its absorption in the gastrointestinal tract and also helps release iron stores from ferritin.
Why did indirect bilirubin get this name?
In the blood, this type of bilirubin is transported in a tight complex with albumin. Due to the protein shell, it cannot directly interact with the diazo reagent in laboratory tests.
Why does stool turn white (acholic) in obstructive jaundice?
Due to biliary tract obstruction, bile pigments do not enter the intestine at all. Consequently, stercobilin is not formed, which normally gives stool its brown color.
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