Question

One of the most important molecules in biochemical systems is adenosine triphosphate (ATP), for which the structure is ATP ATP is the principal carrier of biochemical energy. It is considered an energy-rich compound because the hydrolysis of ATP to yield adenosine diphosphate (ADP) and inorganic phosphate is spontaneous under aqueous biochemical conditions. (a) Write a balanced equation for the reaction of ATP with water to yield ADP and inorganic phosphate ion. [Hint: Hydrolysis reactions are just the reverse of condensation reactions(Section 22.8).] (b) What would you expect for the sign of the free-energy change for this reaction? (c) ADP can undergo further hydrolysis. What would you expect for the product of that reaction?

Short answer

(a) The balanced equation for the hydrolysis of ATP is: \[ATP + H_2O \rightarrow ADP + P_i\] (b) The free-energy change, \(\Delta G\), for this reaction is expected to be negative (exergonic). (c) In the further hydrolysis of ADP, the products are adenosine monophosphate (AMP) and an inorganic phosphate ion: \[ADP + H_2O \rightarrow AMP + P_i\]

Step-by-step solution

Understanding the Structure of ATP

Adenosine Triphosphate (ATP) consists of an adenine nucleotide (composed of adenine, ribose, and a phosphate group bound to the 5' carbon of the ribose) attached to three phosphate groups in a linear chain. The phosphates are connected to one another through a high-energy phosphate bond

Writing the Balanced Equation for ATP Hydrolysis

A hydrolysis reaction involves breaking down a compound using water. In the hydrolysis of ATP, water breaks the bond between the 2nd and 3rd phosphate groups, producing ADP (adenosine diphosphate) and an inorganic phosphate ion. The balanced equation for this reaction is: \[ATP + H_2O \rightarrow ADP + P_i\] Where \(P_i\) is the inorganic phosphate ion. (b)

Predicting the Sign of Free-Energy Change for the Reaction

The hydrolysis of ATP is an exergonic reaction, meaning it releases energy. This energy release is mainly because breaking the bond between the 2nd and 3rd phosphate groups results in a more stable state for the products (ADP and \(P_i\)). Thus, the free-energy change for this reaction, denoted as \(\Delta G\), is expected to be negative, as energy is released from the system. (c)

Predicting the Product of ADP Hydrolysis

As mentioned earlier, ADP is formed in the hydrolysis of ATP. ADP is similar in structure to ATP but has only two phosphate groups. ADP can also undergo hydrolysis through a similar mechanism. In this reaction, water would break the bond between the 1st and 2nd phosphate groups, producing adenosine monophosphate (AMP) and another inorganic phosphate ion. The balanced equation for the hydrolysis of ADP would be: \[ADP + H_2O \rightarrow AMP + P_i\]

Key concepts

Adenosine Triphosphate

Adenosine triphosphate, commonly referred to as ATP, is a crucial molecule in biochemistry because it serves as the principal energy carrier in cells. ATP is composed of three main parts:

• Adenine, a nitrogenous base.
• Ribose, a five-carbon sugar.
• Three phosphate groups connected in a linear chain.

These phosphate groups are linked through high-energy phosphate bonds. The energy stored in these bonds is what makes ATP such an important biological molecule. In cellular processes, ATP often transfers energy to other molecules by releasing one of its phosphate groups. This removal turns ATP into ADP, showcasing its role as an energy distributor. Understanding its structure helps us see how it's able to store and release energy efficiently.
Most biological reactions that require energy rely on ATP either directly or indirectly. Its role as an “energy currency” in cells highlights its significance in metabolic processes.

Free-Energy Change

When discussing chemical reactions like the hydrolysis of ATP, we often talk about the free-energy change, represented as \( \Delta G \). Simply put, this value tells us whether a reaction releases or absorbs energy.
In the case of ATP hydrolysis, the reaction releases energy, making it exergonic. This means the free-energy change \( \Delta G \) is negative. A negative \( \Delta G \) means that the products of the reaction, ADP, and an inorganic phosphate ion, are more stable than the reactants. The released energy is then available to do work in the cell, like muscle contractions or chemical synthesis.
This concept is crucial in understanding how cells harness energy from ATP. It ensures that cells can perform continuous work, using the energy given off by ATP's conversion to ADP and phosphate during hydrolysis.

ADP Hydrolysis

After ATP loses one phosphate group and turns into ADP (adenosine diphosphate), it can undergo another similar reaction called ADP hydrolysis. In this process, ADP further breaks down by losing another phosphate group, resulting in adenosine monophosphate (AMP) and an inorganic phosphate ion: \[ADP + H_2O \rightarrow AMP + P_i\]
This reaction, like ATP hydrolysis, involves the release of energy. However, the amount of energy released is less compared to ATP hydrolysis because the remaining phosphate bond in ADP is not as high-energy as the ones in ATP. This ensures a continuous, albeit smaller, supply of energy for cellular activities.
ADP hydrolysis plays a role in prolonged energy demand situations, helping sustain cellular activities when ATP levels dip.

Phosphate Groups

Phosphate groups in ATP are essential to its role as an energy carrier. Each ATP molecule contains three phosphate groups named the alpha, beta, and gamma phosphates.
The key to ATP's energy-carrying capacity lies in the bond between the beta and gamma phosphates. Breaking this "high-energy" bond during hydrolysis is what releases energy to drive cellular functions.
These phosphate groups not only play a direct role in energy transference but also participate in many biochemical pathways. They can be transferred to other molecules to activate them, effectively playing a signaling role in various cellular processes. This versatility in energy transfer and signal transduction via phosphate groups is at the heart of ATP's pivotal role in biology.