MICROBIOLOGY Study Guide - 2012

CHAPTER 9. MICROBIAL PHYSIOLOGY

9.3. MICROBIAL RESPIRATION

Microbial Respiration encompasses a multitude of biochemical reactions involving electron transport, substrate oxidation, and the release of energy essential for cellular activity.

There are two types of Biological Oxidation: Direct and Indirect. In direct oxidation of inorganic substances, autotrophic soil Bacteria derive energy using oxidase Enzymes capable of oxidizing hydrogen, methane, sulfur, ammonia, etc., with atmospheric oxygen. Indirect oxidation occurs through The transfer of electrons from a donor to an acceptor. Biological oxidation of a substrate involves the simultaneous transfer of two electrons, accompanied by the Cleavage of two protons from the substrate. This substrate oxidation, coupled with the abstraction of hydrogen atoms, is referred to as dehydrogenation. Enzyme Proteins that remove hydrogen atoms from substrates are called dehydrogenases.

Aerobic and anaerobic dehydrogenation are distinguished. During aerobic dehydrogenation, microorganisms utilize atmospheric oxygen as the terminal hydrogen acceptor. Aerobic dehydrogenation involves complete and incomplete oxidation. The End products of complete oxidation are Water and carbon dioxide. Incomplete oxidation results in only a partial release of energy and The formation of incompletely oxidized compounds, such as organic acids (acetic, citric, succinic, malic, oxalic, etc.).

Anaerobic dehydrogenation differs from aerobic respiration in that microorganisms use certain oxidized Inorganic Compounds—such as nitrates, sulfates, and carbonates—rather than oxygen as hydrogen acceptors, which are readily reduced to ammonia, hydrogen sulfide, and methane.

9.3.1. Aerobic Respiration

The Complete oxidation of Organic compounds to СО₂ and Н₂О is carried out by microorganisms during aerobic respiration. The overall equation for the aerobic breakdown of glucose is given below:

Class="center">

During Glucose Catabolism, the primary metabolic intermediate is pyruvic acid (see Fig. 28), which undergoes oxidative decarboxylation According to the following equation:

In addition to NAD⁺ and CoA—SH, this reaction requires three other Cofactors: thiamine pyrophosphate (TPP), Lipoic Acid, and Mg²⁺. The resulting acetyl-CoA enters the Krebs tricarboxylic acid (TCA) cycle.

The oxidation of a single acetyl-CoA molecule in the Krebs cycle results in the release of 2 molecules of carbon dioxide and 8 protons. Protons released by the substrate are transferred to the cytoplasmic membrane in prokaryotes or The inner mitochondrial membrane in eukaryotes. Cell/29.html">The Lipid Bilayer of these membranes houses the Components of the respiratory (electron transport) chain, which facilitate the transfer of protons and electrons to oxygen. The major components of the Respiratory Chain include Flavoproteins, iron-sulfur proteins, Quinones, and Cytochromes (Fig. 26).

Fig. 26. Electron transport in the respiratory enzyme chain

NAD(P)-dependent dehydrogenases catalyze the removal of hydrogen from various substrate molecules and its transfer to the initial carrier of the respiratory chain. Once reduced, NAD(P)H₂ dissociates from the Active Site of the enzyme and moves to the membrane, where it transfers a proton to flavin dehydrogenase.

Flavoproteins are enzymes whose prosthetic group is flavin mononucleotide (FMN) or flavin adenine dinucleotide (FAD). Their reactive sites are two nitrogen atoms in the isoalloxazine ring, each binding one [H].

Iron-sulfur (FeS) proteins are redox systems that mediate electron transport. These proteins contain iron atoms bound to both the sulfur of Cysteine residues and inorganic disulfide sulfur. The [2Fe + 2S] centers involved in the respiratory chain are capable of transferring only a single electron. Iron-sulfur proteins are present in several enzyme complexes of the respiratory chain (I, II, III).

Quinones form a group of redox systems within the respiratory chain. Ubiquinone (coenzyme Q) is found in the inner mitochondrial membrane of eukaryotes and the cytoplasmic membrane of Gram-negative bacteria, whereas naphthoquinones are present in Gram-positive bacteria. Quinones are capable of transferring hydrogen or electrons. Unlike other electron carriers, quinones are not bound to specific proteins.

Cytochromes are redox systems that transfer exclusively electrons. Their prosthetic group is heme. The central iron atom of the heme ring participates in electron transfer by changing its valence:

Five cytochromes (b, c₁, с, а, a₃) have been identified in eukaryotic Mitochondria. Terminal cytochromes (а + a₃), which constitute cytochrome c oxidase, transfer electrons to molecular oxygen. Water is formed when 4 electrons are transferred to an oxygen molecule. Some cytochrome oxidases transfer only 2 electrons, resulting in The production of hydrogen peroxide, which is subsequently decomposed by catalase or peroxidase.

The transfer of hydrogen and electrons along the respiratory chain is coupled with ATP synthesis, meaning oxidation reactions are linked to phosphorylation reactions (coupling sites 1, 2, and 3 in Fig. 26). This process, known as Oxidative Phosphorylation, takes place in membranes with the participation of ATP synthase. Phosphorylation is driven by the difference in redox potential between the electron donor and acceptor (E_h). Typically, ATP formation occurs at sites with the greatest potential difference. The complete oxidation of 1 mole of glucose via the Hexose diphosphate pathway and The Tricarboxylic Acid Cycle yields a net Energy balance of 38 ATP.

The respiratory chain can be inhibited or blocked by various cellular poisons. Amobarbital and rotenone suppress FMN-dehydrogenase activity, while antimycin A blocks electron transfer between cytochromes b and c. Cyanide, carbon monoxide, and hydrogen sulfide specifically inhibit cytochrome oxidase.