Anabolic reactions are those that require energy to occur. Cells can couple anabolic reactions together with catabolic ones to form an efficient energy cycle; the catabolic reactions transform chemical fuels into cellular energy, which is then used to initiate the energy-requiring anabolic reactions.
Anabolism can be thought of as a set of metabolic processes, in which the synthesis of complex molecules is initiated by energy released through catabolism. These complex molecules are produced through a systematic process from small and simple precursors. For example, an anabolic reaction can begin with relatively simple precursor molecules (created previously by catabolic reactions) and end with fairly complex products, such as sugar, certain lipids, or even DNA, which has an extremely complex physical structure. The increased complexity of the products of anabolic reactions also means that they are more energy-rich than their simple precursors.
Anabolic reactions are divergent processes. That is, relatively few types of raw materials are used to synthesize a wide variety of end products. This results in an increase in cellular size or complexity—or both. Anabolic processes are responsible for cell differentiation and increases in body size. Bone mineralization and muscle mass are attributed to these processes. Anabolic processes produce peptides, proteins, polysaccharides, lipids, and nucleic acids. These molecules comprise all the materials of living cells, such as membranesand chromosomes, as well as the specialized products of specific types of cells, such as enzymes, antibodies, hormones, and neurotransmitters.
ATP provides the energy needed for anabolism to take place. ATP is a high energy molecule that couples anabolism by the release of free energy. This energy does not come through the breakage of phosphate bonds; instead it is released from the hydration of the phosphate group.The chemical reaction where ATP changes to ADP supplies energy for this metabolic process. Anabolism is the opposite of catabolism, for example, synthesizing glucose is an anabolic process whereas the breaking down of glucose is a catabolic process. The Gibbs free energy for the synthesis of glucose is positive, meaning that the reaction is not spontaneous and will not go to completion in any time frame. However, when coupled with ATP, this reaction becomes more thermodynamically favorable, as is the case with many other endothermic reactions in the body. Anabolism and catabolism must be regulated in a way that does not allow the two processes to occur simultaneously. Each process has its own set of hormones that switch these processes on and off. Anabolic hormones include growth hormone, testosterone, and estrogen. Catabolic hormones include adrenaline, cortisol, and glucagon.
There is a need for cells to separate the metabolism process into anabolic and catabolic pathways. Anabolism requires the input of energy, which can be described as an "uphill" (energy intake) process. Catabolism is a "downhill" process which energy is released as the organism had used up energy. At certain points in the anabolic pathway, the cell must put more energy into a reaction than is released during catabolism. Such anabolic steps require a different series of reactions than are used at this point during catabolism.
Stages of anabolism
- There are three basic stages of anabolism.
Stage 1 production of precursors such as amino acids, monosaccharides and nucleotides.
Stage 2 use energy from ATP to turn the precursors into reactive form.
Stage 3 the assembly of these activated precursors into complex molecules such as proteins, polysaccharides, lipids and nucleic acids.
Examples of Biochemical Anabolic Reactions
1) Polysaccharides: Polysaccharides serve as an example of anabolism because polysacchrides are derived from their subunits of simple monosaccharides. A simple exmaple is the formation of glycogen. Glygocen is a polysaccharide that is composed of subunit glucose monosacchrides connected by glycosidic bonds.
2) Polypeptides: Polypeptides serve as an example of anabolism because polypeptides are derived from their subunits of simple peptides. Polypeptides such as hemoglobin are composed of four different protiens (essentially peptides) that come together and form a completely different protien. Peptides them selves are a result of anabolic reactions themselves due to the condensation reactions that simple amino acids go through to combine and thus form peptide chains.
3) Carbon Fixation: Carbon fixation is another example of anabolism because in photosynthetic organisms such as plants, cyanobacteria, algae and other photoautotrophic ogranisms fixate carbon dioxide into glycerate 3-phosphate which is then further converted into glucose. Beginning with photosynthesis, synthesis of carbohydrates from sunlight and carbon dioxide. This process uses ATP and NADPH produced by the photosynthetic reactions to convert CO2 into glycerate 3-phosphate. This shows anabolism because from a smaller simpler subunit of carbon dioxide, glycerate 3-phosphate is produced, a much larger and complex biochemical compound.
Examples of Anabolic hormones
Growth hormone is a protein-based peptide hormone that stimulates growth, cell reproduction, and regeneration in humans and other animals. Growth hormone is often used to treat children with growth disorders as well as adult growth hormone deficiency.
Insulin is a hormone that is essential for regulating fat and steroids metabolism in the body. It causes the uptake of glucose from the blood by cells in the liver, muscle, and fat tissue. Glucose is then stored as glycogen in the liver and muscle. Insulin also contributes to other body functions such as vascular compliance and cognition.
Testosterone is a steroid hormone found in mammals, reptiles, birds, and other vertebrates. In mammals, testosterone is mainly secreted in the testes of males and the ovaries of females. However, small amounts of testosterone are also secreted by the adrenal glands. Testosterone is the principal male sex hormone.
Estradiol is the predominant sex hormone present in females. However, it is also found in males and acts as an active metabolic product of testosterone. Estradiol has major impacts on the reproductive and sexual functions as well as other organs.  [Berg]
Side effects may occur when anabolic hormones (or steroids) are used in excessive amount. If used too much by men, it can cause a decline in testosterone secretion, testicular atrophy (wasting away of the testes), and even breast enlargement. For women, excess use of anabolic hormones may cause a decrease in estrogen secretion and the ability to ovulate, the growth of facial hair, as well as regression of the breasts.
Berg, Jeremy; Tymoczko, John L.; Stryer, Lubert. Biochemistry, 6th edition. W.H. Freeman and Company. New York. 2007