Catabolism vs anabolism: how growth factors govern key processes

26 August 2024

Metabolism in multicellular organisms is a complex interplay of biochemical processes that can be broadly categorized into two types: catabolism and anabolism. These processes, essential for maintaining life, enabling growth and ensuring the proper functioning of cells and tissues, are controlled by growth factors and hormones. Simply put, catabolism breaks down large molecules into smaller molecules releasing energy in the form of small molecules which cells use to perform a variety of biochemical processes. In contrast, anabolism builds larger molecules from small molecules and consumes energy.

Catabolism

Catabolism is the metabolic pathway that breaks down complex molecules into simpler ones, releasing energy in the process. This energy is often stored in the form of adenosine triphosphate (ATP), which cells use to perform various functions. Catabolic reactions are typically exergonic, meaning they release energy.

Catabolic processes include glycolysis in which glucose is broken down to pyruvate, producing ATP and NADH. The Citric Acid Cycle (Krebs Cycle) is also a catabolic process in which the oxidation of acetyl-CoA to carbon dioxide and water, generates ATP, NADH, and FADH2.

Catabolism is the process which provides the energy required for cellular activities. These include muscle contraction, nerve impulse propagation, and biosynthesis of essential molecules. For example, during periods of fasting or intense exercise, catabolic processes break down stored glycogen, fats, and proteins to meet the body’s energy demands.

Anabolism

Anabolism, on the other hand, is the metabolic pathway that constructs complex molecules from simpler ones, consuming energy in the process. Thus anabolic drugs, such as anabolic steroids, promote the formation of mass, notable muscle mass. Anabolic reactions are typically endergonic, meaning they require an input of energy, usually in the form of ATP.

Examples of anabolic processes include protein synthesis, DNA replication, glycogenesis - the formation of glycogen from glucose, which is then stored in the liver and muscles for future energy needs. Anabolic processes are vital for growth, repair, and maintenance of tissues. They enable organisms to build and maintain their cellular structures, store energy for future use, and reproduce.

What is the role of growth factors in stimulating anabolism and regulating catabolism

Growth factors play a crucial role in regulating both catabolic and anabolic processes. They act as signalling molecules that influence cell growth, differentiation, and survival. Growth factors primarily stimulate anabolic processes, promoting the synthesis of complex molecules and facilitating cell growth and tissue repair. This is best explained by looking at specific examples.

Insulin is a small globular protein hormone produced in the pancreas. It promotes the uptake of glucose by liver cells and stimulates glycogenesis (See above) thereby reducing blood sugar levels. Insulin also enhances protein synthesis and inhibits the breakdown of proteins and fats, thereby reducing catabolic activity. This dual role makes insulin a key regulator of both anabolic and catabolic processes.

Growth Hormone (GH), produced by the pituitary gland, is a protein that stimulates growth, cell reproduction, and cell regeneration. It promotes protein synthesis and increases muscle mass and bone density. GH also stimulates the liver to produce Insulin-like Growth Factor 1 (IGF-1), which further enhances anabolic processes.

Epidermal Growth Factor (EGF) stimulates cell growth, proliferation, and differentiation by binding to its receptor, EGFR. It is released locally during wound healing and tissue repair to promote the synthesis of proteins and other macromolecules necessary for cell regeneration.

Fibroblast Growth Factors (FGFs) are a family of growth factors involved in angiogenesis, wound healing, and embryonic development. They stimulate the proliferation and differentiation of various cell types, including fibroblasts, which are essential for the synthesis of extracellular matrix components.

Simplified schematic of anabolism and catabolism under control of growth factors. Food particles are ingested by cells and broken down into simple monomers by catabolism. These monomers are then built into complex polymeric structures such as proteins and storage sugars. These can be broken down again(catabolism) and rebuilt (anabolism) with the balance between the two dictated by hormone and growth factor levels. Waste products (metabolites) are generated by this process which are secreted by the cell. Image credit: Cell Guidance Systems

Growth factors regulate catabolism

While growth factors primarily promote anabolic processes, they also play a role in regulating catabolism. For instance, insulin inhibits lipolysis (the breakdown of fats) and proteolysis (the breakdown of proteins), thereby conserving energy stores and reducing catabolic activity. This regulatory function ensures that the body maintains a balance between energy production and consumption, which is crucial for homeostasis.

Integrated role of growth factors in metabolism

The interplay between catabolism and anabolism is tightly regulated by growth factors to ensure that cells and tissues function optimally. For example, growth factors such as EGF and FGFs promote cell proliferation and differentiation, which are essential for maintaining healthy tissues and organs. They also help in wound healing by stimulating the synthesis of proteins and other macromolecules necessary for tissue repair.

Growth factors provide “metabolic flexibility” by enabling cells to adapt to changing metabolic demands. For instance, during periods of high energy demand, such as exercise, growth factors can shift the balance towards catabolism to release stored energy. Conversely, during periods of rest or recovery, they promote anabolic processes to rebuild and store energy reserves.

IMAGE 3GF1 insulin-like growth factor macromolecular structure. CREDIT Nevit Dilmen