Biochemical standard potentials

Biochemical standard potential — (/ ) Most biochemical processes proceed at pH values that are not too far from 7, and most biochemical redox systems involve also a proton transfer, i.e., they can be written as Ox+ H+ + me < H Red " m )+. Thus it became customary to define the potential of those redox systems at pH 7.0 as the biochemical standard potential E. E is related to the (chemical) - standard potential E as follows E = E - 0.414 fV1 —. 

The biochemical methane potential of 54 fruits and vegetable wastes samples and eight standard biomass samples were determined by Gunaseelan (2004) to compare the extents and the rates of their conversion... 

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. 

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

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

We saw in Section 4.2 that in biochemical work it is common to adopt the biological standard state (pH = 7, corresponding to neutral solution), rather than the thermodynamic standard state (pH = 0). To convert standard potentials to biological standard potentials, , we must first consider the variation of potential with pH. The two potentials differ when hydrogen ions are involved in the half-reaction, as in the fumaric acid/succinic acid couple fum/suc with fum = HOOCCH=CHCOOH and sue = HOOCCH2CH2COOH, which plays a role in the citric acid cycle (Case study 4.3) ... 

Zero Point on the Biochemical Redox Scale. Since hydrogen ions participate in most biochemical redox reactions, their concentration should also be set at 1 M in order to measure the standard potentials. Of course, this is not feasible because the enzymes are not active at such a low pH. Furthermore, it seems more reasonable to choose physiological conditions. In biochemistry, therefore, it is customary to use standard potentials Eo referring to pH of 7. At that pH, the hydrogen electrode has a potential difference of —0.42 volts with respect to the hydrogen electrode of pH 0. 

The water-soluble and fat-soluble vitamins in the parenteral multivitamin mix are essential cofactors for numerous biochemical reactions and metabolic processes. Parenteral multivitamins are added daily to the PN. Patients with chronic renal failure are at risk for vitamin A accumulation and potential toxicity. Serum vitamin A concentrations should be measured in patients with renal failure when vitamin A accumulation is a concern. Previously, vitamin K was administered either daily or once weekly because intravenous multivitamin formulations did not contain vitamin K. However, manufacturers have reformulated their parenteral multivitamin products to provide 150 meg of vitamin K in accordance with FDA recommendations. There is a parenteral multivitamin formulation available without vitamin K (e.g., for patients who require warfarin therapy), but standard compounding of PN formulations should include a parenteral multivitamin that contains vitamin K unless otherwise clinically indicated. 

The right side of the illustration shows the way in which the redox potential E is dependent on the composition (the proportion of the reduced form as a %) in two biochemically important redox systems (pyruvate/lactate and NAD /NADH+H see pp.98, 104). In the standard state (both systems reduced to 50%), electron transfer from lactate to NAD" is not possible, because AE is negative (AE = -0.13 V, red arrow). By contrast, transfer can proceed successfully if the pyruvate/lactate system is reduced to 98% and NAD /NADH is 98% oxidized (green arrow, AE = +0.08 V). 

Biochemical reactions are basically the same as other chemical organic reactions with their thermodynamic and mechanistic characteristics, but they have the enzyme stage. Laws of thermodynamics, standard energy status and standard free energy change, reduction-oxidation (redox) and electrochemical potential equations are applicable to these reactions. Enzymes catalyse reactions and induce them to be much faster . Enzymes are classified by international... 

---