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FIGURE 18-17 Conversions of one-carbon units on tetrahydrofolate.

The different molecular species are grouped according to oxidation state, with the most reduced at the top and most oxidized at the bottom. All species within a single shaded box are at the same oxidation state. The conversion of N5,N10-methylenetetrahydrofolate to N5-methyltetrahydrofolate is effectively irreversible. The enzymatic transfer of formyl groups, as in purine synthesis (see Fig. 22-33) and in the formation of formylmethionine in prokaryotes (Chapter 27), generally uses N10-formyltetrahydrofolate rather than N5-formyltetrahydrofolate. The latter species is significantly more stable and therefore a weaker donor of formyl groups. N5-formyltetrahydrofolate is a minor byproduct of the cyclohydrolase reaction, and can also form spontancously. Conversion of N5-formyltetrahydrofolate to N5, N10-methenyltetrahy-drofolate, requires ATP, because of an otherwise unfavorable equilibrium. Note that N5-formiminotetrahydrofolate is derived from histidine in a pathway shown in Figure 18-26.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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