Compare the structure of the nucleoside triphosphate CTP...

Compare the structure of the nucleoside triphosphate CTP with the structure of ATP. (FIGURE CAN'T COPY)
Now predict the $K_{\mathrm{eq}}^{\prime}$ and $\Delta G^{\prime \circ}$ for the following reaction: $$\mathbf{A T P}+\mathbf{C D P} \rightarrow \mathbf{A D P}+\mathbf{C T P}$$

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Key Concepts

Structure of Nucleoside Triphosphates

Nucleoside triphosphates are composed of a nucleoside (a nitrogenous base linked to a sugar) and a chain of three phosphate groups. The energetic capacity of these molecules largely comes from the high-energy bonds between the phosphate groups, while the specific chemical identity of the base (adenine in ATP versus cytosine in CTP) mainly distinguishes the molecules without dramatically altering their energy-transfer role.

Phosphoryl Transfer Reactions

Phosphoryl transfer reactions involve the transfer of a phosphate group from one nucleoside triphosphate to a nucleoside diphosphate. In the reaction ATP + CDP ? ADP + CTP, the phosphoryl group is exchanged between the nucleotide pairs. Such reactions are typically isodesmic in nature, meaning the similar types of bonds are broken and formed, so there is little net change in free energy.

Thermodynamic Equilibrium and Free Energy

The equilibrium constant, K?q, and the standard Gibbs free energy change, ?G°, are related by the equation ?G° = -RT ln K?q. Because the phosphoryl transfer reaction in question involves very similar chemical species on both sides of the equation, the reaction is nearly thermoneutral. This implies that K?q is close to 1, and ?G° is approximately zero under standard conditions.

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