how do enzymes control the cell cycle
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The cell possesses an extraordinary array of enzymes , each specialized to carry out an important function in the cell. However, in many cases it is critical that the enzymes only be active at certain times and not others. For example, the digestive enzymes secreted by cells lining the stomach and intestine must only be active once they have been secreted and not before. If they were active prior to secretion they would degrade the proteins within the very cells that synthesized them. Or consider the enzymes that carry out the many activities of cell division. If these are not held in tight control, a cell will divide inappropriately and may become cancerous. Thus, it is critical for the cell to be able to control the activities of many of its enzymes, and a number of intricate mechanisms have evolved to do just that. In most cases the activity of an enzyme is achieved via changes in its conformation, or shape, and the four most common ways of achieving this are regulation by small molecules; regulation by phosphorylation ; regulation by protein-interactions; and regulation by proteolytic cleavage. Regulation of an enzyme can occur by the binding of a small molecule to a site distant from the active site , which is the binding site for the enzyme's substrate . This is called allosteric regulation (from the Greek allo, meaning "other," and steric, meaning "site"). Because the small molecule does not bind the active site, it does not function by blocking access to the substrate. Instead, it acts by changing the conformation of the protein. A classic example of this occurs with an enzyme called aspartate transcarbamoylase (AT-Case) from the bacterium E. coli. ATCase is the first enzyme in a series of enzymes whose end product is cytidine triphosphate (CTP), which is used to make ribonucleic acid (RNA) and deoxyribonucleic acid (DNA). CTP has been found to bind to ATCase and inhibit its activity. The binding of CTP to ATCase changes the conformation of the active site such that the affinity for substrates is decreased by up to 90 percent. Thus, the buildup of CTP shuts the entire pathway off, thereby maintaining a fairly constant supply. This type of inhibition, in which the end product of a reaction inhibits its own synthesis, is called feedback inhibition and is a common regulatory mechanism in biologic pathways. In a more extreme case, the amino acid tryptophan goes so far as to inhibit the synthesis of the mRNAs encoding the biosynthetic enzymes that synthesize it. Thus, in that case, the enzymes themselves are not even synthesized until they are needed. An important class of proteins regulated by small molecules are the G-proteins. They are called G-proteins because they bind and are activated by guanosine triphosphate (GTP). G-proteins have intrinsic GTPase activity, meaning they convert the bound GTP molecule to GDP (guanosine diphosphate). Typically, when GDP is bound, the conformation of the protein is such that the molecule is inactive. A protein called a GTP exchange factor (GEF) stimulates the exchange of GDP for GTP, thus reactivating the G-protein. The ras protein is a G-protein found in a number of different organisms. Research has shown that ras activates proteins involved in cell growth and division. It was first discovered in a virus that causes tumors in mice. It causes tumors when it is mutated such that its GTPase activity is defective. This causes the protein to always be bound to GTP (instead of GDP) and hence it remains active. Thus, overactive ras causes uncontrolled cell proliferation and cancer. It has since been found that up to 15 percent of human cancers involve a mutation in ras that inhibits its GTPase activity, making it an important protein in human disease. The normal protein is only activated when stimulants outside the cell, such as growth factors, signal it to grow and proliferate. Following a short period of activity, inherent GTPase activity of the ras returns it to the inactive form. Interestingly, in the fruit fly, Drosophila melanogaster, ras serves a different function. The structure and regulation of ras in D. melanogaster is similar to that in mammalian cells, but instead of participating in a pathway signaling cell proliferation, it is involved in a pathway leading to the differentiation of a certain type of cell in the eye, the photoreceptor cell. The regulation of protein activity by a GTP-GDP switch is apparently evolutionarily ancient and has been adapted to serve a variety of different cellular functions. Phosphorylation means the addition of a phosphate group, PO43-. Phosphorylation of certain amino acids in a protein can occur via the action of a group of proteins called kinases . Kinases are classified into two broad classes based on the amino acid they phosphorylate: the serine/threonine class and the tyrosine class. All three of these amino acids contain a hydroxyl (-OH) side chain, to which a phosphate group can be attached. Phosphorylation often occurs on more than one amino acid in a protein, and the result is a conformational change that affects the protein's activity. A second class of proteins, called phosphatases, reverses the activity of kinases by removing the phosphates, returning the proteins to their original forms. Phosphatases are also categorized by their substrate specificities, either serine/threonine or tyrosine, however, a few have the ability to dephosphorylate all three amino acid side chains. In the example of regulation by phosphorylation described below, dephosphorylation leads to activation. This is not always the case, and equally as many proteins are activated by phosphorylation. Nuclear factor of activated T cells (NFAT) is a protein regulated by phosphorylation. NFAT is a transcription factor that is found in the cytoplasm and is phosphorylated in resting cells. Dephosphorylation causes a conformational change that allows it to be transported to the nucleus , where it binds other transcription factors to activate gene transcription . NFAT was first found to activate genes in T cells (an immune cell); however, it has since been found in a number of other cell types as well. The phosphatase that dephosphorylates and activates NFAT is called calcineurin, which is of the serine/threonine class. Even when inactive, calcineurin is bound to NFAT, ensuring a rapid response upon activation. Stimulation by an inducer such as a viral infection or perhaps an organ transplant causes the activation of calcineurin, which then makes additional contacts with NFAT via its active site and dephosphorylates it. This activates NFAT, aiding the immune response by the T cell. Many enzymes are regulated by binding to another protein. For example, transcription factors, such as heat shock factor (HSF), can be activated by binding to other copies of itself (homomultimerization). In addition, inactivation of a protein can also occur by protein-protein interactions. Although HSF activation occurs by binding of identical molecules to one another, often regulation by protein-protein interactions occurs between different proteins. For example, calcineurin, the phosphatase that activates NFAT, is itself regulated by protein-protein interactions. Calcineurin is composed of two polypeptide subunits, one catalytic and one regulatory. The catalytic subunit contains the active site and is the part of the enzyme that dephosphorylates NFAT. The regulatory subunit binds the catalytic subunit and keeps it inactive by blocking the active site until a stimulus is detected. The stimulus for calcineurin activation is increased cytoplasmic calcium levels. Calcium, together with a small protein called calmodulin, binds calcineurin, which results in the displacement of the regulatory subunit, exposing the active site and allowing it to dephosphorylate NFAT. Thus, calcineurin is an example of an enzyme that is regulated by both small molecules (calcium) and proteins (the regulatory subunit and calmodulin). Another example of this occurs with a kinase called cAMP-dependent protein kinase, which in resting cells consists of a complex of two catalytic and two regulatory subunits. As with calcineurin, the reg
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Answers:Why do living organisms need to control energy? > Organisms need energy to fuel life. It takes energy to move, breath, think, etc. If the process of burning energy weren't controlled, the breakdown of food would be literally explosive. (E=mc^2 anyone?) Cell respiration, the breakdown of food, is more efficient because it doesn't release all the energy at once. How do they synthesize the molecules needed for energy flow? > I have no idea what you mean by the "flow" of energy. I think you are referring to cellular respiration? If so, here's an explanation in a nutshell. Food is ingested and various enzymes break it down into glucose, the most basic sugar. Glucose then undergoes glycolysis, where the molecule is sort of broken in two. Then, it goes through the Krebs cycle. In this cycle, NAD+ and FAD+ take electrons from the former glucose molecule, becoming NADH and FADH. The leftover carbon from the food is exhaled as CO2 and is gone from the system. The NADHs and FADHs, in the inner membrane of the mitochondria, donate their electrons to a series of proteins that pump ions into the intermembrane space. These ions build up like a body of water, to use a rough analogy. Then, ATP synthase, a special protein, opens up and the ions flow through. They provide energy much like water flowing over a waterwheel. This energy allows the protein to make ATP out of ADP. (ATP is the "charged' form of ADP, much like charged batteries that can be reused.) ATP is the "battery" that provides energy to everything a living organism does. How do they cause metabolism to flow quicklyin the proper direction? > No idea what you're talking about How do organisms maintain complexity in a universe that favors simplicity? > The universe doesn't favor simplicity. Obsessive humans minds do. I'm not saying you're obsessive, I'm saying it's human nature to try to reduce things to understand them. The universe favors entropy, which is separate from complexity. Life slows down entropy, however, as explained above. That is part of its majesty.
Answers:1) Cannabinoids can modulate cells by slowing them down if they're beating too fast, or, speeding them up if they're too slow. 4)How cell division (and thus tissue growth) is controlled is very complex. The following terms are some of the features that are important in regulation, and places where errors can lead to cancer. Cancer is a disease where regulation of the cell cycle goes awry and normal cell growth and behavior is lost.
Answers:what sparks cell division on the molecular level is pretty well known at this point in time...the presence and ansence of different proteins at different times...usually they are cdc proteins (cell division cycle) or cyclin proteins. how to FIX this implies something is broken so I'll take it you mean cancer...cancers are usually a result of one one or more of these cyclins of cdc's being awalys turned on due to a change in the gene (UV light, chemicals, etc). So it' easy enough in tehory to fix it..you just re-change the gene...trouble is we don;t have the technology to do that...you ahve to change the SPECIFIC gene at the right place...IN EVERY CELL!!!
Answers:The nitrogen cycle is the biogeochemical cycle that describes the transformations of nitrogen and nitrogen-containing compounds in nature. Earth's atmosphere is about 78% nitrogen, making it the largest pool of nitrogen. Nitrogen is essential for many biological processes; and is crucial for any life here on Earth. It is in all amino acids, is incorporated into proteins, and is present in the bases that make up nucleic acids, such as DNA and RNA. In plants, much of the nitrogen is used in chlorophyll molecules which are essential for photosynthesis and further growth. Processing, or fixation, is necessary to convert gaseous nitrogen into forms usable by living organisms. Some fixation occurs in lightning strikes, but most fixation is done by free-living or symbiotic bacteria. These bacteria have the nitrogenase enzyme that combines gaseous nitrogen with hydrogen to produce ammonia, which is then further converted by the bacteria to make its own organic compounds. Some nitrogen fixing bacteria, such as Rhizobium, live in the root nodules of legumes (such as peas or beans). Here they form a mutualistic relationship with the plant, producing ammonia in exchange for carbohydrates. Nutrient-poor soils can be planted with legumes to enrich them with nitrogen. A few other plants can form such symbioses. Other plants get nitrogen from the soil by absorption at their roots in the form of either nitrate ions or ammonium ions. All nitrogen obtained by animals can be traced back to the eating of plants at some stage of the food chain. Such as nitrate and nitrite) into groundwater can occur. Elevated nitrate in groundwater is a concern for drinking water use because nitrate can interfere with blood-oxygen levels in infants and cause methemoglobinemia or blue-baby syndrome. Where groundwater recharges stream flow, nitrate-enriched groundwater can contribute to eutrophication, a process leading to high algal, especially blue-green algal populations and the death of aquatic life due to excessive demand for oxygen. While not directly toxic to fish life like ammonia, nitrate can have indirect effects on fish if it contributes to this eutrophication. Nitrogen has contributed to severe eutrophication problems in some water bodies. As of 2006, the application of nitrogen fertilizer is being increasingly controlled in Britain and the United States. This is occurring along the same lines as control of phosphorus fertilizer, restriction of which is normally considered essential to the recovery of eutrophied waterbodies. Ammonia is highly toxic to fish life and the water discharge level of ammonia from wastewater treatment plants must often be closely monitored. To prevent loss of fish, nitrification prior to discharge is often desirable. Land application can be an attractive alternative to the mechanical aeration needed for nitrification. During anaerobic (low oxygen) conditions, denitrification by bacteria occurs. This results in nitrates being converted to nitrogen gas and returned to the atmosphere. Nitrate can also be reduced to nitrite and subsequently combine with ammonium in the anammox process, which also results in the production of dinitrogen gas. *for a figure of the nitrogen cycle, u can contact me at this email add: email@example.com