Insights into Metabolism

Abstract

The present book chapter highlights the comprehensive epitome of metabolism, explaining the regulatory and intricate mechanisms involved. It commences with the introduction which encompasses drug metabolism pathways, enumerating linear, cyclic, and branched pathways then delves into the catabolic and anabolic pathways. The catabolic pathways explore glycolysis and the Krebs cycle, and anabolic pathways involve gluconeogenesis, protein synthesis, and lipogenesis. The further section overshadows discussion on energy in metabolism. This segment of the chapter discusses ATP as the energy currency, highlighting its structure, functions, and ATP hydrolysis. Further, it emphasizes energy transfer mechanisms that involve redox reactions. The other segments of the chapter cover the role of enzymes in metabolism, explaining catalytic functions and specificity. Afterward, enzyme regulation involves allosteric regulation and feedback inhibition. Finally, the book chapter delves the metabolism regulation, focusing on hormonal regulation, nutrient availability, and genetic-epigenetic regulation. Examining these various aspects, it is quite clear that the chapter provides a framework for understanding the complex nature of metabolism and its vital role in health and disease.

INTRODUCTION

The study of drug metabolism, a branch of pharmacology and biochemistry, quickly advanced starting around 1950. It has significantly influenced pharmacokinetics, pharmacodynamics, ecotoxicology, and other biological fields.

International rules increasingly require knowledge of the metabolism of medicines and other xenobiotics. The 19th century witnessed the most noteworthy achievements. The metabolism processes, such as acetylation, the synthesis of mercapturic acid, conjugation with glycine, sulfuric acid, and glucuronic acid, hydrolysis, reduction, and oxidation, were made clear [1]. The chemical sequence oxidation-conjugation created a number of pathways for different substrates and has become the cornerstone of modern xenobiochemistry [2].

Simply, medicines are small molecules that fall under the category of xenobiotics, or compounds that are not naturally occurring in the human body. Nonetheless, a variety of endogenous compounds, such as steroids and hormones, are occasionally referred to as drugs and are used to treat certain medical conditions. The term "metabolism" refers to the process by which an enzyme changes a chemical's moiety. The most well-known enzymes involved in drug metabolism are the reductases, hydrolases, and oxidases that make up the cytochrome P450s (CYP450s). The primary objective of metabolism is the elimination of endogenous and/or exogenous chemicals from the body [3].

In the metabolic process, there will be the conversion of lipophilic molecules into hydrophilic ones or will facilitate elimination. On the other hand, the drug-metabolizing enzymes transform compounds into their pharmacologically active form in many conditions. Prodrugs are pharmaceutically inactive drugs designed to treat absorption and bioavailability difficulties. They pass through a process of transformation in the body subsequent to absorption to become active medications. To get around ampicillin's low bioavailability, pivampicillin is produced as a prodrug that has the potential to hydrolyze into ampicillin after being absorbed into the bloodstream. A significant instance of a prodrug is the use of mycophenolate mofetil to improve the oral bioavailability of mycophenolic acid [4].

The byproducts of metabolism that are called metabolites can either be biologically active or inactive. The smooth endoplasmic reticulum (ER), mitochondria, and small intestinal epithelia of hepatocytes are the primary locations for five CYP450 enzymes that are crucial for drug removal. To a lesser extent, they are also present in the proximal tubules of the kidneys. The significance and essential functions of conjugating enzymes and drug transporters are becoming increasingly clear to humans. Modifications to these pathways may have an impact on a drug's pharmacokinetics and pharmacodynamics. Drugs interact with these pathways during their absorption, distribution, metabolism, and excretion [5].

DRUG METABOLISM PATHWAYS

There are various and diverse ways that drug-food substances can be metabolized or biotransformed, including through the chemical processes of hydration, reduction, oxidation, hydrolysis, conjugation, and condensation. Studying these pathways is critical since a drug's metabolism might reveal whether or not it has pharmacological or toxicological effects. Phase I, also known as functionalization reactions, and Phase II, sometimes known as conjugative reactions, are the two stages that generally makeup drug metabolism (Table 1). The chemical processes are often connected to drug metabolism in phases I and II [6].

On the other hand, the drug and the food go through the drug metabolism process. It can undergo different types of pathways based on their structural flow and functionality. Here's an explanation of these pathways:

Linear Pathway

In linear pathways, the drug undergoes a series of sequential biochemical transformations, with each step dependent on the previous one.

By converting acetaldehyde to acetate, mammals metabolize ethanol. By conversion to acetyl CoA, acetate is absorbed by the general metabolic pool. Although other organs subsequently metabolize the majority of the acetate, the liver is the primary organ that converts ethanol to acetate. Alcohol Dehydrogenase (ADH) is the enzyme that is principally responsible for converting ethanol to acetaldehyde by catalyzing the following reaction: acetaldehyde + NADH + H+ - ethanol + NAD+. NAD+ is an essential coenzyme that accepts electrons during the oxidation of ethanol to acetaldehyde (Fig. 1).

It is reduced to NADH in the process. The availability of NAD+ is a limiting factor in ethanol metabolism. Since NAD+ is required for glycolysis, the citric acid cycle, and the electron transport chain, extensive ethanol metabolism can lead to a depletion of NAD+, disrupting these critical metabolic processes. The liver enzymes found in mammals are soluble proteins with a molecular weight of 80,000 located in the cytosolic compartment [7]. On the other hand, NADH is generated as a byproduct of the ADH-catalyzed reaction. The accumulation of NADH relative to NAD+ alters the cellular redox state, leading to several metabolic consequences. Increased NADH levels lead to various consequences. For example, high levels of NADH inhibit gluconeogenesis, which can lead to hypoglycemia, particularly in fasting individuals. The increased NADH/NAD+ ratio favors the conversion of pyruvate to lactate (lactic acidosis) and oxaloacetate to malate, reducing the availability of substrates for the citric acid...