University:
California State University, Northridge
Course:
BIOL 101 | General Biology
Academic year:

2024
Views:

336

Pages:

1
Author:

Tommy B.

BIOOXIDATION AND ENERGY TRANSFER Chemical energy in cells is present in the form of high-energy organic groups. This group contains S or P. Adenosine triphosphate (ATP) is one of the high energy groups most important so if the two phosphate groups are hydrolyzed respectively produces 12000 cal/phosphate, while the third only produces phosphate 1500 cal. The energy released by ATP depends on the state of its hydrolysis especially pH and reactant levels. Although ATP contains 2 high-energy phosphates, in the reaction generally only one highenergy phosphate is used for activation. Oxidation in cells is catalyzed by enzymes that have cofactors or the prosthetic group accepts protons or electrons from the substrate and gives them away to the acceptor via an intermediary that has more redox potential (Eo'). higher than the donor. The most important electron carrier is NAD, FMN, and cytochromes. 1. Nicotinamide adenine dinucleotide (NAD) functions to receive hydrogen from the reduced substrate 2. Flavin adenine mononucleotide (FMN) and flavin adenine dinucleotide (FAD) function of This compound is receives hydrogen from NADH2 or directly from substrate. 3. Cytochromes (Cyt a, b, c) Each cytochrome has a different Eo', but all of them contains haem as a prosthetic group with Fe as an acceptor electron. Oxygen reacts with electrons to form O-ions and with ions + H forms H2O. For biological systems, n is usually 2 and Fo is replaced by F' because it is not on standard state, so: F' = 46.126 x AEo' If the potential difference of a system before and after oxidation is known, From the equation above it can be calculated the amount of energy released during oxidation. For example, if electrons pass from NADH2 to O2, AEo' = 1.14 volts, then AF' = -52,000 cal. Theoretically, this reaction produces 4 high-energy phosphates. When this reaction is studied with mitochondria isolated from mammals, yeast and fungi, it turns out that oxidation only produces 3 ATP from every oxygen atom used. Not all microbes have enzymes complete electron transport. Lactobacillus and Clostridium are absent. Cytochromes though have enzymes with pyrimidine nucleotides and groups prosthetic flavo protein. Lactobacillus has a flavoprotein-oxidase can use O2 as the final electron acceptor; but as it is O2, formed H2O2 not H2O. Several species of Streptococcus, Acetobacter, and yeast has a peroxidase enzyme (catalase) which reoxidizes it reduced substrate (e.g. reduced Cyt C, or reduced NAD) with the presence of peroxidase and H ions. The reaction results are oxidized substrate and water. An oxidation-reduction reaction is called fermentation (anaerobic respiration) if the final electron acceptor is not oxygen, and is called respiration (aerobic respiration) if the final electron acceptor is oxygen. naerobic and facultative anaerobic microbes that are grown organically Anaerobics have other ways of reoxidizing reduced hydrogen carriers. This is often done by offsetting the oxidation reaction of the substrate with another reduction reaction, thereby accumulating the final reduced product.