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Energy rich compounds; classification; biological significances of ATP and cyclic AMP

In a cell, chemical bonds can be used to store energy, but not all chemical bonds are equally energetic.

Energy-rich compound

In a cell, chemical bonds can be used to store energy, but not all chemical bonds are equally energetic. A broken chemical bond can release a greater amount of energy than others. The phosphate molecule is made up of three oxygen atoms attached to one phosphorus atom (PO3). It forms phosphate bonds with other molecules when they are bonded together. When you break this bond, a great deal of energy is released. These molecules are themselves composed of energy-rich compounds. Cells use these energy-rich compounds as fuel for biochemical reactions that require energy. The molecules derived from coenzyme A are another important source of energy. As an example, acetyl-CoA has a sulfur-rich thioester bond rather than phosphate bonds, so it provides energy. It takes just enough energy to make a phosphate bond in ATP when acetyl-CoA is broken down. Even though these molecules are essential for all types of metabolism, they're especially important for microbes that rely on anaerobic metabolism (fermentation), which takes place without oxygen.

Classification of Energy-rich compound

There 5 types in total

Phosphoanhydrides - In phosphoric acid, two molecules are joined to form a compound called phosphoanhydrides. Example - In ATP, these kinds of bonds occur. An ATP molecular structure consists of two high-energy diphosphate molecules (phosphoanhydride bonds). A phosphate ester bond between ribose and phosphate is not as energy-dense as an a phosphate ester bond. In most endergonic reactions, ATP serves as the principal immediate source of free energy. Example - transport, muscle contractions, and nerve impulse transmission. Proteosynthesis and gluconeogenesis rely on GTP (Guanidine triphosphate) as an energy source. Also, UTP (Uridine triphosphate) and CTP (Cytidine triphosphate) are energy sources that are used by the body to get energy from saccharides and lipids.

Enolphosphatic bond - There is a very high energy band that releases 61KJ/mole upon hydrolysis. During glucose inglycolysis, phosphoenol pyruvate forms a similar bond to that of phosphoenol pyruvate.

Acyl phosphatic bond - On hydrolysis, this bond provides 49 kJ/mol of energy. This type of bond occurs during inglycolysis of 1-3 bisphosphoglycerates

Guanidine phosphate - When phosphate is attached to guanidine, guanidine phosphate is formed. When hydrolyzed, the product releases about 43 kilojoules of energy. PC contains the same kind of bond as phosphocreatine. This substance is found in muscle cells and acts as an energy reserve in tissues.

Thioester bond - There is no energy-rich phosphate in the Thioester Bond, so it does not contain much energy. Acetyl co-A contains this type of bond.

The biological significance of ATP

The most common source of energy for cells is ATP. Adenosine triphosphate (ATP) is composed of carbon, nitrogen, hydrogen, oxygen, and phosphorus. A large amount of energy is released in reactions when ATP undergoes hydrolysis because it contains unstable, high-energy bonds. As an enzyme removes a phosphate group from ATP to form ADP, it generates a significant amount of energy, which can be used by the cell for a variety of metabolic processes. Proteins as well as other macromolecules can be synthesized with it. By removing a second phosphate group from ATP, adenosine monophosphate (AMP) is formed as well as a further energy release. AMP and ADP are added back to phosphate to form ATP when the organism does not require energy, which is hydrolyzed when needed. Therefore, ATP serves as an efficient energy source for cellular pathways.

Cyclic AMP

cyclic AMP plays a key role in hormone action through the action of epinephrine. The "Fight or flight" hormone is released by the adrenal glands in response to stress. As a result of the hormone, blood pressure increases, and glucose is broken down for energy. Engaging in physical activity helps humans deal with a situation's challenges. In response, the body displays high blood pressure, rapid heartbeat, and a dry mouth. Biochemical reactions are responsible for these responses. Epinephrine remains outside cells on membrane bound receptors when it binds to them. Adenylate cyclase manufactures cyclic AMP, the second messenger.



The adenylate cyclase enzyme system consists of two components. It catalyzes adenylate cyclase when it's bound to a hormone bound receptor and when a regulatory protein, a stimulatory G-protein (guanylate nucleotide binding protein), is bound to it and activated. As an intermediary between the receptor and cyclic AMP synthesis, the G-protein plays an important role. It depends on the guanylate nucleotide that is bound to G-proteins whether they are active or inactive. G-protein binds to GDP when it is inactive. Once the G-protein is active, GTP binds to it. In G-proteins, GTP is converted to GDP by an intrinsic GTPase activity. The G-protein becomes inactive once GTP is hydrolyzed. G-proteins are cyclical:
  • Hormones bind to receptors.
  • Hormone binding receptors bind to G-proteins, thereby causing GTP to replace GDP.
  • Adenylate cyclase interacts with GTP-bound G-protein.
  • Upon hydrolysis of GTP, G-protein returns to the ground state.
Various G-proteins can increase or decrease cyclic AMP production by either stimulating or inhibiting adenylate cyclase.



As an example, glycogen phosphorylase and glycogen synthase are not directly affected by cyclic AMP. Instead, cyclic AMP stimulates protein kinase activity, which ultimately causes a response in cells. A phosphate is carried from ATP to glycogen phosphorylase to glycogen phosphorylase via cycle AMPs, which bind to protein kinase A. Glycogen-1-phosphate is then derived from glycogen by active glycogen phosphorylase. Muscles are then able to utilize this energy source.

It's impossible to keep a cell "on" forever. They must respond to something. Each step can be reversed. A protein phosphatase hydrolyzes phosphate from a protein starting with the target protein. The enzyme phosphodiesterase hydrolyzes cyclic AMP. Hydrolysis of GTP by G-protein may be crucial to modulation. Adenylate cyclase returns to an unstimulated state after having been stimulated. This modulation feature must be present in every signaling mechanism to allow for control. Ras protein belongs to the family of membrane-bound GTPases, such as those found in mammals. If Ras GTPase activity is reduced in mammalian cells, then tumors may form (i.e., grow uncontrollably).
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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