Nonheme iron
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Nonheme Iron: Insights from Recent Research
Oxygen Activating Nonheme Iron Enzymes
Recent studies have significantly advanced our understanding of oxygen-activating nonheme iron enzymes. Thirteen crystal structures of protocatechuate 3,4-dioxygenase complexes have provided detailed insights into the enzyme active site changes during catalysis. A common structural motif, the 2-His-1-carboxylate facial triad, has been identified in several mononuclear nonheme iron enzymes, such as isopenicillin N synthase and tyrosine hydroxylase . Additionally, high-valent intermediates in methane monooxygenase and ribonucleotide reductase have been characterized, expanding the known enzymes with nonheme diiron sites to include alkene monooxygenase and alkane hydroxylase .
Nonheme Iron Absorption and Dietary Factors
Influence of Meat on Nonheme Iron Absorption
The addition of small amounts of pork meat to a phytate-rich meal significantly increases nonheme iron absorption. A study involving 45 healthy women demonstrated that while 25 grams of meat did not significantly enhance absorption, 50 and 75 grams of meat increased absorption by 44% and 57%, respectively . This suggests that incorporating moderate amounts of meat into meals can effectively boost nonheme iron absorption.
Adaptation to Iron Supplementation
Iron supplementation has been shown to reduce nonheme iron absorption from food, but not heme iron absorption. In a study with healthy individuals, nonheme iron absorption decreased significantly after 12 weeks of supplementation with 50 mg of ferrous sulfate per day, while heme iron absorption remained unchanged . This indicates a physiological adaptation mechanism that regulates nonheme iron absorption in response to increased iron intake.
Differential Absorption and Utilization
Research using a rat model of iron overload revealed that nonheme iron absorption is significantly higher than heme iron absorption. Elevated hepatic hepcidin levels were found to reduce the expression of duodenal iron transporters, impacting nonheme iron absorption more strongly than heme iron absorption . This study also highlighted the differential tissue utilization of absorbed heme and nonheme iron, with more nonheme iron being recovered in red blood cells and more heme iron in the spleen .
Nonheme Iron in Oxidation Reactions
Mononuclear Nonheme Iron(IV)-Oxo Complexes
Mononuclear nonheme iron enzymes generate high-valent iron(IV)-oxo intermediates that play crucial roles in oxidative transformations. The first spectroscopic characterization of such an intermediate in taurine: α-ketoglutarate dioxygenase (TauD) was achieved in 2003, revealing a 2-His-1-carboxylate facial triad motif . Subsequent studies have extensively investigated the structure, spectroscopy, and reactivity of these intermediates, leading to significant advancements in understanding their role in oxidation reactions .
Crystallographic Characterization
A high-resolution crystal structure of a synthetic nonheme iron(IV)-oxo complex has confirmed the existence of a terminal iron-oxygen bond in a nonporphyrin ligand environment. This finding supports proposed mechanisms of nonheme iron catalysis and challenges the traditional heme paradigm .
Nonheme Iron and Health
Iron Intake and Diabetes Risk
A study from the China Health and Nutrition Survey examined the association between dietary iron intake and diabetes risk. It found that nonheme iron intake followed a reverse J-shaped curve in men and an L-shaped curve in women, suggesting that sufficient intake of nonheme iron might be protective against diabetes, while excessive intake could increase the risk . No significant associations were observed between heme iron intake and diabetes risk .
Nutritional Contributions
Both heme and nonheme iron contribute to human nutrition, with nonheme iron being present in a variety of foods. The bioavailability of nonheme iron is influenced by dietary factors, such as the presence of enhancers and inhibitors. Studies have shown that nonheme iron bioavailability is less critical in the context of a varied Western diet compared to single meal absorption studies Cook1991Carpenter1992.
Conclusion
The research on nonheme iron has provided valuable insights into its absorption, utilization, and role in enzymatic reactions. Understanding the factors that influence nonheme iron absorption and its impact on health can help in developing dietary strategies to improve iron status and prevent related health issues.
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