Standard state, biochemical

Many biological processes involve hydrogen ions the standard state of an H+ solution is (by definition) a 1 mol L"1 solution, which would have a pH of nearly 0, a condition incompatible with most forms of life. Hence, it is convenient to define the biochemical standard state for solutes, in which all components except H+ are at 1 mol L-1, and H+ is present at 10 7mol L (i.e., pH 7). Biochemical standard-state free energy changes are symbolized by AG0, and the other thermodynamic parameters are indicated analogously (AH0, AS0, etc.). 

Since the components of the reaction are in their biochemical standard states (see Sec. 10.2), the above value for E is, in fact, defined as 0 for this half-cell reaction (note the prime). 

Under biochemical standard-state conditions, the free energy of ATP synthesis is given by the expression... 

For convenience of calculations, biochemists therefore define a different standard state, in which the concentration of H+ is 10-7 M (pH 7) and the water is 55.5 M. Physical constants based on the biochemical standard state are called standard transformed constants and are written as AG° or K to distinguish them from the untransformed constants used by chemists and physicists. By con-... 

The relatively large negative value of AG ° indicates that the overall reaction occurs spontaneously under biochemical standard conditions. The biochemical standard state corresponds more nearly to typical conditions in a cellular environment. The first step in the metabolic process is the conversion of glucose into glucose-6-phosphate ... 

This is arbitrarily assigned a standard reduction potential Eo= 0.0 V. At the biochemical standard state of pH 7, the hydrogen half-cell has an Eq = —0.421 V. 

Why do we need a modified standard state for biochemical applications Since biochemical reactions do not occur naturally at a hydrogen-ion concentration of 1 M, a biochemical standard state (AG° ) is often used, where the [H+] is setatlX10- M(pH = 7.0). 

H+] = 10-7 biochemical standard state mol dm denoted by prime on symbol ... 

Free energy changes associated with a biochemical reaction are determined at a standard state, with all reactants and products at 1M. Many biomolecules are unstable in acid, so the biochemical standard state is set at pH = 7.0 rather than at pH = 0 (IM acid), the standard state for chemical reactions. Biochemical standard free energies of reaction are given as AG° to where the indicates this change in standard conditions. 

Free energy changes for systems described in terms of biochemical standard states are symbolized with a prime, such as AG°, AH°, AS°. ... 

Standard redox potential for the mediator redox reaction at the biochemical standard state (V.shp.)... 

These reactions occur in cells, not in the gas phase, so the specification of the conditions of the reaction are different. A biochemical standard state includes the requirement that an aqueous solution be neutral (that is, neither acidic nor basic), with a pH of 7. We use the prime symbol on a state function to imply that it refers to a reaction at the biochemical standard state. For the ATP ADP reaction in equation 2.61, the H° is —24.3 kj per mole of ATP reacted. 

Under biochemical standard states, the potential for the reaction... 

---