0.20 Bis2a 06.0 energy in living systems v1.2  (Page 4/15)

Phosphorylation

Recall that, in some chemical reactions, enzymes may bind to several substrates that react with each other on the enzyme, forming an intermediate complex. An intermediate complex is a temporary structure, and it allows one of the substrates (such as ATP) and reactants to more readily react with each other; in reactions involving ATP, ATP is one of the substrates and ADP is a product. During an endergonic chemical reaction, ATP forms an intermediate complex with the substrate and enzyme in the reaction. This intermediate complex allows the ATP to transfer its third phosphate group, with its energy, to the substrate, a process called phosphorylation. Phosphorylation refers to the addition of the phosphate (~P). This is illustrated by the following generic reaction:

When the intermediate complex breaks apart, the energy is used to modify the substrate and convert it into a product of the reaction. The ADP molecule and a free phosphate ion are released into the medium and are available for recycling through cell metabolism.

How do cells generate atp

All cells require energy in the form of mobile packets. As we saw earlier in the class, these packets can be used to couple thermodynamically unfavorable reactions to drive the formation of specific products. Remember it costs energy to build, and the primary role of a cell is to make two cells. For what ever reason, almost 3.25 billion years of evolution has favored ATP as that mobile form of energy. It is the energy held within the phosphoanhydride bonds (the terminal or gamma and beta Phosphates) that store the energy. As shown in Figure 3, many common cellular compounds have such bonds and can also be used as an internal energy source with specific enzymes. Such compounds include all of the NTPs as well as common intermediates such as Phosphoenol pyruvate (PEP), which we will discuss later in glycolysis. Remember Not all carbon-Phosphate bonds are high energy, such as glucose-6-phosphate (figure 3).

Because ATP is the primary source of mobile energy in the cell, a variety of mechanisms have ermerged over the 3.25 billion years of evolution to form ATP. The majority of these mechanism are modifications on two themes: direct synthesis of ATP or indirect synthesis of ATP. However all of the known reactions fall into two basicmechanisms: Substrate Level Phosphorylation (SLP) and oxidative phosphorylation    and will be discussed in detail below and in the next few modules. Suffice it to say both mechanisms rely on the transfer of potential energy from one high energy compound (often called the energy source) to ADP, to synthesize ATP. This is either done directly, as in the case of Substrate level phosphorylation, or indirectly, as is the case for oxidative phosphorylation.

Substrate level phosphorylation