GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

11. MAIN MECHANISMS OF METABOLISM AND ENERGY CONVERSION IN MICROORGANISMS

11.2. ENERGY METABOLISM

Energy is required to drive anabolic (constructive METABOLISM) reactions. Such energy can be obtained from various sources: phototrophic microorganisms utilize light energy; lithotrophs use energy released during The oxidation of Inorganic Compounds; and organotrophs rely on energy released during the oxidation of Organic compounds, i.e., via Catabolism.

The sum of metabolic reactions that convert light energy or chemical compounds (inorganic or organic) into a form usable in constructive metabolism (high-energy compounds and reducing equivalents) is termed Energy Metabolism.

High-energy compounds. Compounds containing high-energy bonds—that is, bonds with a high Standard Free energy of Hydrolysis—are called high-energy compounds. Primarily among these is ATP, the main carrier of biological energy, as well as A number of intermediates formed during The breakdown of organic substrates (1,3-diphosphoglycerate, phosphoenolpyruvate, acyl phosphate, etc.).

ATP as a coenzyme for metabolite activation. ATP acts as The Cell's "energy currency"—a direct source of energy for synthesizing structural cell components, transporting nutrients into the cell, mechanical movement, and osmoregulation. Furthermore, many metabolic intermediates require group-transfer activation before undergoing further transformations within the cell. This activation is mediated by ATP. There are three possible modes of ATP utilization:

1. Sugars are activated by conversion into their corresponding phosphorylated derivatives:

Class="center">Glucose + ATP Glucose phosphate + ADP.

2. Ribose-5-phosphate is activated via The transfer of a diphosphate (pyrophosphate) moiety:

Ribose-5-phosphate + ATP - Phosphoribosyl diphosphate + AMP.

3. Certain inorganic acids, all Amino Acids, and inorganic sulfate are activated by attaching an AMP group with the release of pyrophosphate:

Fatty acid + ATP — Acyl-AMP + Pyrophosphate.

Amino acid + ATP - Aminoacyl-AMP + Pyrophosphate.

Sulfate + ATP — Sulfate-AMP + Pyrophosphate.

Reducing equivalents. These are protons (hydrogen atoms) and electrons generated during substrate oxidation (in redox reactions). For instance, the oxidation of organic compounds occurs via Electron transfer from a donor to an acceptor. Biological Oxidation most commonly involves the simultaneous transfer of two electrons, accompanied by the abstraction of two protons from the substrate. Such substrate oxidation, proceeding with the removal of two protons, is called dehydrogenation. Substrate dehydrogenation is catalyzed by Enzymes known as dehydrogenases. The names of many dehydrogenases often indicate their hydrogen (proton) donor, such as malate dehydrogenase or Alcohol dehydrogenase. It should be noted that the terms hydrogen donor and electron donor are used synonymously. The terms hydrogen acceptor and electron acceptor, oxidation and dehydrogenation, as well as reduction and hydrogenation, are likewise equivalent.

Pyridine NUCLEOTIDES. Many dehydrogenases transfer hydrogen to one of two Coenzymes—nicotinamide adenine dinucleotide (NAD, in its oxidized form—NAD⁺) or nicotinamide adenine dinucleotide phosphate (NADP, in its oxidized form—NADP⁺) (Fig. 11.2).

Fig. 11.2. Dehydrogenation and pyridine nucleotides

A coenzyme is a low-molecular-weight compound that, together with an enzyme, participates in binding and subsequently transferring specific substrate fragments. Prosthetic groups perform a similar role during enzyme function. However, unlike coenzymes, a prosthetic group is tightly bound to the enzyme and does not dissociate from it during the attachment and transfer of substrate fragments.

Fig. 11.2 illustrates the oxidation of ethanol to acetaldehyde, catalyzed by the enzyme alcohol dehydrogenase. The functional group that determines The activity of the NAD and NADP coenzymes is nicotinic acid amide. One hydrogen atom is transferred from the substrate along with a pair of electrons (as a hydride ion) to the pyridine ring, while the second hydrogen enters the solution. As a result of the reaction, NAD and NADP are converted into NADH and NADPH, respectively. Both coenzymes dissociate freely—meaning they detach from one dehydrogenase and transfer hydrogen to another acceptor after binding with a different enzyme. Consequently, they are referred to as hydrogen carriers, and THE CONCEPT OF "reducing equivalents" is specifically associated with NADH and NADPH.

Several logical questions arise: What is the relationship between reducing equivalents and energy metabolism? Why, when discussing the energetics of a microbial cell, do we refer to ATP and reducing equivalents? How do reducing equivalents relate to ATP? NADH delivers hydrogen to the Respiratory Chain, where its oxidation (via electron transfer from NADH to oxygen or another electron acceptor) releases energy utilized for ATP synthesis. Thus, NADH is involved in energy generation (ATP synthesis) within the microbial cell. This Mechanism of ATP synthesis, coupled with electron transport in the respiratory chain, is known as Oxidative Phosphorylation (electron transport-linked phosphorylation).

As for NADPH, it is believed to participate primarily in the reductive steps of biosynthetic pathways (constructive metabolism).