Globin, catabolism

Fig. 1. Overview of intravascular heme catabolism. Hemoglobin, myoglobin, and other heme proteins are released into the circulation upon cellular destruction, and the heme moiety is oxidized by O2 to the ferric form (e.g., methemoglobin and metmyoglobin). Haptoglobin can bind a substantial amount of hemoglobin, but is readily depleted. Ferric heme dissociates from globin and can be bound by albumin or more avidly by hemopexin. Hemopexin removes heme from the circulation by a receptor-mediated transport mechanism, and once inside the ceU heme is transported to heme oxygenase for catabolism.

Correct answer = B. The cyclic heme molecule is oxidatively cleaved to form biliverdin. The catabolism occurs in the cells of the reticulo endothelial system, particularly the spleen, and results in the liberation of carbon monoxide. Protoporphyrinogen is an intermediate in the synthesis, not degradation, of heme. Hemo globin and tissue cytochromes are precursors of bilirubin. 

Phagocytic breakdown destroys older blood cells, primarily in the spleen, but also in the marrow (Fig. 99-2). Amino acids from the globin chains return to an amino acid pool heme oxygenase acts on the porphyrin heme structure to form biliverdin and to release its iron. Iron returns to the iron pool to be reused while biliverdin is further catabolized to bilirubin. The bilirubin is then released into the... 

In the human body CO is formed in the catabolism of heme after cleavage of globin (protein part) under catalysis by heme oxygenase (EC 1.14.99.3) with the help of O2 and NADPH. The a-methylene group of heme is quantitatively oxidized to CO. This is the only endogenous source of CO in the human organism. The green tetrapyrrole biliverdin IXa is formed as further reaction product. 

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