. Biochemistry of plants and animals, an introduction. Biochemistry. 172 GENERAL BIOCHEMISTRY of carbon dioxide, three of DPNH, and one molecule of TPNH by the reactions of the citric acid cycle. The energy thus accumulated in the DPNH and TPNH is made available by the reactions of the next section. Whenever the supply of hexoses exceeds that required for immediate needs, some of it is metabolized as far as acetyl co- enzyme A, then converted to fatty acids. The proteins required for growth and tissue repair depend in part upon three key amino acids derived directly from the pyruvate, a-ketogl
. Biochemistry of plants and animals, an introduction. Biochemistry. 172 GENERAL BIOCHEMISTRY of carbon dioxide, three of DPNH, and one molecule of TPNH by the reactions of the citric acid cycle. The energy thus accumulated in the DPNH and TPNH is made available by the reactions of the next section. Whenever the supply of hexoses exceeds that required for immediate needs, some of it is metabolized as far as acetyl co- enzyme A, then converted to fatty acids. The proteins required for growth and tissue repair depend in part upon three key amino acids derived directly from the pyruvate, a-ketoglutarate, and oxalacetate. The resulting amino acids are alanine, glutamic, and aspartic acids. These various connections with the citric acid cycle are discussed later. Electron transfer. The fourth and fifth stages in the oxidation of foods and substrates as outlined on page 167 involve the transfer of electrons from DPNH and TPNH through a series of electron ac- ceptors to oxygen. During these reactions three molecules of ATP are formed from ADP and inorganic phosphate for each DPNH oxidized by this route. The present knowledge of the reactions in this system is quite limited, and probably there are differences in different species. However, the steps known for the higher plants and animals may be similar down to the point of transfer of electrons to oxygen. These oxidations can be summarized as follows, starting with either TPNH or DPNH: TPNH + DPN+ ^ TPN+ + DPNH DPNH + FAD + H3O+ ^ DPN+ + FADH2 + H2O FADH2 + 2 cytochrome • Fe+++ + 2H2O ^ FAD + 2 cytochrome • Fe++ -f 2H3O+ 4 cytochrome • Fe++ + 4H3O+ + O2 4 cytochrome • Fe+++ + 6H2O FAD = flavine adenine dinucleotide and has the structure. OH OH OH 0 0~ ,N I I I II I </ CH2C — C—C—CH2OP —O—P —OCHj O, N H H H 0_ O. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly
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