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Figure 2.16. Inositol trisphosphate, a multiply phosphorylated polyalcohol.

Figure 2.12b shows adenosine with a chain of three phosphate groups attached to the 5' carbon of the ribose. This important molecule is adenosine triphosphate (ATP), and we will meet it many times in the course of this book. The three phosphate groups are denoted by the Greek letters a, j, and y. Phosphorylated nucleosides are called nucleotides.

Figure 2.15 shows another important molecule formed from adenosine. Here adenosine and a nicotinamide nucleotide are joined through their phosphate groups. The resulting molecule, called nicotinamide adenine dinucleotide (NAD+), plays a critical role in the cellular energy budget.

Molecules with several OH groups can become multiply phosphorylated. Figure 2.16 shows inositol trisphosphate (IP3), an important messenger molecule we will meet again in Chapter 16. Both ATP and IP3 have three phosphate groups, but to indicate the fact that in ATP these are arranged in a chain, while in IP3 they are attached to different carbons, we use the prefix tri in adenosine triphosphate and the prefix tris in inositol triphosphate. Similarly, a compound with two phosphates in a chain is called a diphosphate, while one with one phosphate on each of two different carbons is called a bisphosphate.

AMINO ACIDS, POLYPEPTIDES, AND PROTEINS

Amino acids contain both a COOH group, which readily gives an H+ to water and is therefore acidic, and a basic NH2 group, which readily accepts H+ to become NH3+. Figure 2.17a shows two amino acids, leucine and 7-amino butyric acid (GABA), in the form in which they are found at normal pH: the COOH groups have each lost an H+ and the NH2 groups have each gained one, so that the molecules bear both a negative and a positive charge.

We name organic acids by labeling the carbon that bears the carboxyl group a, the next one j, and so on. When we add an amino group, making an amino acid, we state the letter of the carbon to which the amino group is attached. Hence leucine is an a-amino acid

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