What effect does number of subunits in an enzyme have on feedback control?
Definition
A single subunitenzyme that is regulated by negative feedback can at most decrease from 90% to about 10% activity in response to a 100-fold increase in the concentration of an inhibitory ligand that it binds (Figure 3-59, red line). Responses of this type are apparently not sharp enough for optimal cell regulation, and most enzymes that are turned on or off by ligand binding consist of symmetric assemblies of identical subunits. With this arrangement, the binding of a molecule of ligand to a single site on one subunit can trigger an allosteric change in the subunit that can be transmitted to the neighboring subunits, helping them to bind the same ligand. As a result, a cooperative allosteric transition occurs (Figure 3-59, blue line), allowing a relatively small change in ligand concentration in the cell to switch the whole assembly from an almost fully active to an almost fully inactive conformation (or vice versa).
Term
Diagram transition in feedback control in a dimer (not answered)
Definition
Term
How does it work in a multi subunit enzyme? (not answered)
Definition
I can't find the answer to this question
Term
What's phosphorylation and what does it do?
Definition
A method that is commonly used by eukaryotic cells to regulate a protein’s function is the covalent addition of a phosphate group to one of its amino acidside chains. Each phosphate group carries two negative charges, the enzyme-catalyzed addition of a phosphate group to a protein can cause a major conformational change in the protein. Removal of the phosphate group by a second enzyme returns the protein to its original conformation and restores its initial activity.
Second, an attached phosphate group can form part of a structure that is directly recognized by binding sites of other proteins
Term
How do Kinase and Phosphatase work together to promote activity?
Definition
Protein phosphorylation involves the enzyme-catalyzed transfer of the terminal phosphate group of an ATP molecule to the hydroxyl group on a serine, threonine, or tyrosine side chain of the protein (Figure 3-63). This reaction is catalyzed by a protein kinase, and the reaction is essentially unidirectional because of the large amount of free energy released when the phosphate–phosphate bond in ATP is broken to produce ADP (discussed in Chapter 2). The reverse reaction of phosphate removal, or dephosphorylation, is instead catalyzed by a protein phosphatase. Cells contain hundreds of different protein kinases, each responsible for phosphorylating a different protein or set of proteins. There are also many different protein phosphatases; some of these are highly specific and remove phosphate groups from only one or a few proteins, whereas others act on a broad range of proteins and are targeted to specific substrates by regulatory subunits. The state of phosphorylation of a protein at any moment, and thus its activity, depends on the relative activities of the protein kinases and phosphatases that modify it.
Term
Describe the basics of lipid membrane formation.
Definition
Lipid bilayers go through a self assembly process in the formation of membranes. The cell membrane consists primarily of a thin layer of amphipathicphospholipids which spontaneously arrange so that the hydrophobic "tail" regions are shielded from the surrounding polar fluid, causing the more hydrophilic "head" regions to associate with the cytosolic and extracellular faces of the resulting bilayer. This forms a continuous, spherical lipid bilayer. Forces such as Van der Waal, electrostatic, hyrdogen bonds, and noncovalent interactions, are all forces that contribute to the formation of the lipid bilayer. Overall, hydrophobic interactions are the major driving force in the formation of lipid bilayers.
Term
What roles of hydrophobic and hydrophilic interactions play in membranes?
Definition
The Phospholipid bilayer that forms the cell surface membrane consists of a hydrophobic inner core region sandwiched between two regions of hydrophilicity, one at the inner surface and one at the outer surface of the cell membrane. The 'head' is hydrophilic (attracted to water), while the hydrophobic 'tails' are repelled by water and are forced to aggregate. The hydrophillic head contains the negatively charged phosphate group, and may contain other polar groups. The hydrophobic tail usually consists of long fatty acid hydrocarbon chains
Term
Why do membrane lipids seek the most energetically comfortable position and what drives this action?
Definition
The fluidity of a lipid bilayer depends on both its composition and its temperature, as is readily demonstrated in studies of synthetic bilayers.
Term
Describe membrane composition and character.
Definition
Membranes are sheet-like structures composed mainly of lipids and proteins. All biological membranes have a similar general structure. Membrane lipids are organized in a bilayer (two sheets of lipids making up a single membrane) that is approximately 60 to 100 Å thick. The fluid-mosaic model encompasses our current understanding of membrane structure.
Term
How do the four major phospholipids that predominate in the plasma membrane differ?
Definition
Four major phospholipids predominate in the plasma membrane of many mammalian cells: phosphatidylcholine, sphingomyelin, phosphatidylserine, and phosphatidylethanolamine. Only phosphatidylserine carries a net negative charge, the importance of which we shall see later; the other three are electrically neutral at physiological pH, carrying one positive and one negative charge. Together these four phospholipids constitute more than half the mass of lipid in most membranes. All of the lipid molecules are derived from glycerol except for sphingomyelin, which is derived from serine. , almost all of the lipid molecules that have choline - (CH3)3N+CH2CH2OH - in their head group (that is, phosphatidylcholine and sphingomyelin) are in the outer half of the lipid bilayer, whereas almost all of the phospholipid molecules that contain a terminal primary amino group (phosphatidylethanolamine and phosphatidylserine) are in the inner half. Because the negatively charged phosphatidylserine is located in the inner monolayer, there is a significant difference in charge between the two halves of the bilayer.
Term
What are some of the roles for cholesterol in membranes?
Definition
Cholesterol is required to build and maintain cell membranes; it regulates membrane fluidity over a wider range of temperatures. The hydroxyl group on cholesterol interacts with the phosphate head of the membrane, while the bulky steroid and the hydrocarbon chain is embedded in the membrane. Some research indicates that cholesterol may act as an antioxidant.[2] Cholesterol also aids in the manufacture of bile (which is stored in the gallbladder and helps digest fats), and is also important for the metabolism of fat soluble vitamins, including vitamins A, D, E and K. It is the major precursor for the synthesis of vitamin D and of the various steroid hormones (which include cortisol and aldosterone in the adrenal glands, and the sex hormones progesterone, the various estrogens, testosterone, and derivatives).
Term
Describe what are saturated and unsaturated lipids are and how they alter membranes.
Definition
The hydrocarbon tail of palmitic acid is saturated: it has no double bonds between carbon atoms and contains the maximum possible number of hydrogens. Stearic acid, another one of the common fatty acids in animal fat, is also saturated. Some other fatty acids, such as oleic acid, have unsaturated tails, with one or more double bonds along their length. The double bonds create kinks in the molecules, interfering with their ability to pack together in a solid mass. It is this that accounts for the difference between hard (saturated) and soft (polyunsaturated) margarine.
Term
Describe rafting - what makes up a raft and what does rafting do?
Definition
The plasma membrane of cells is made of a combination of glycosphingolipids and protein receptors organized in glycolipoprotein microdomains termed lipid rafts.[1][2][3] These specialized membrane microdomains compartmentalize cellular processes by serving as organizing centers for the assembly of signaling molecules, influencing membrane fluidity and membrane protein trafficking, and regulating neurotransmission and receptor trafficking.[3][4] Lipid rafts are more ordered and tightly packed than the surrounding bilayer, but float freely in the membrane bilayer
Term
What are the two most common forms of membrane associated diseases? How do they work?
Definition
Many, if not most, diseases involve the membrane in some shape or form. There are two categories of what can be termed 'true' membrane-based diseases: the first caused when defects in cytoskeletal components impair membrane function, the second occurring when altered membrane lipid composition disrupts membrane trafficking. The cytoskeleton is a microscopic network of actin filaments and microtubules in the cytoplasm that gives the cell shape and coherence. It contains a set of structural proteins linked to the membrane that provide protection from the stresses of many processes.
Disruptions in the cytoskeleton can lead to a range of diseases, such as sickle cell anaemia and Duchenne muscular dystrophy
Defects in membranes have particularly profound effects on membrane trafficking, the elaborate system of transport pathways that convey cargo in and out of the cell. Researchers are investigating the role of Rab27a in the trafficking disorders choroideremia, Hermansky–Pudlak and Griscelli syndromes.
Term
Describe the membrane related mechanism of alcohol in the brain?
Definition
Alcohol affects the brain’s neurons in several ways. It alters their membranes as well as their ion channels, enzymes, and receptors. Alcohol also binds directly to the receptors for acetylcholine, serotonin, GABA, and the NMDA receptors for glutamate. Alcohol produces its effects by intercollating itself into membranes, resulting in increased fluidity of the membrane with short term exposure to ethanol. It is also theorized that long term use of ethanol results in membrane stiffness. It has long been known that membrane fluidity is critical to the normal functioning of receptors, ion channels, and other membrane processes
Term
What non-membrane mechanism might alcohol work through? (not answered)
Definition
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Term
Describe how sidedness occurs in membranes. How does sidedness regulate? Why is it important in cell death?
Definition
The two sides of any cellular membrane is in contact with two distinct environments. For example, one side of a plasma membrane is in contact with the cytoplasm of the cell, while the other side is in contact with other cells, or with then plasma, etc. For this reason, different proteins are expressed on each side to reflect the unique functions associated with them.
During embryogenesis, left-right sidedness is established by asymmetric expression of laterality genes. A recent model predicts the presence of a functional midline that divides the left side of the embryonic disc from the right side, separating left- and right-inducing signals. These midline cells are fated to die, and dead cells remain in the midline during gastrulation. Inhibition of midline cell death compromises the early expression of laterality genes, such as shh and nodal and randomizes the direction of heart looping. We suggest that cell death along the primitive streak midline is a novel mechanism involved in the regulation of left-right asymmetry during early embryogenesis.
Term
Briefly describe the role of Phospholipids in signal transduction?
Definition
Some types of phospholipid can be split to produce products that function as second messengers in signal transduction.
Term
What are some of the side effects of having glycolipids in the membrane?
Definition
Whatever their normal function, some glycolipids provide entry points for certain bacterial toxins. The ganglioside GM1, for example, acts as a cell-surface receptor for the bacterial toxin that causes the debilitating diarrhea of cholera. Cholera toxin binds to and enters only those cells that have GM1 on their surface, including intestinal epithelial cells. Its entry into a cell leads to a prolonged increase in the concentration of intracellular cyclic AMP, which in turn causes a large efflux of Na+ and water into the intestine
Term
How can membranes associate with proteins?
Definition
Associated with each membrane is a set of membrane proteins that enables the membrane to carry out its distinctive activities. Membrane proteins can be classified into two broad categories—integral (intrinsic) and peripheral (extrinsic)—based on the nature of the membrane-protein interactions.
Term
How can you determine if a protein might be a trans-membrane by looking at its amino acid sequence?
Definition
Particular amino acids tend to occur at different positions relative to the surface or interior of the bilayer in transmembrane segments of integral proteins. Transmembrane a-helices that line a water-filled channel might have polar amino acid R-groups (side-chains) facing the lumen, and non-polar amino acid R-groups facing lipids or other hydrophobic a-helices
Term
what are roles for beta barrels in transmembrane proteins?
Definition
The β barrel proteins are abundant in the outer membrane of mitochondria, chloroplasts, and many bacteria. Some are pore-forming proteins, generating water-filled channels that allow selected hydrophilicsolutes to cross the lipid bilayer of the bacterial outer membrane.
Term
Why are many transmembrane proteins made up of alpha helixes?
Definition
The vast majority of multipass transmembrane proteins in eucaryotic cells and in the bacterial plasma membrane are constructed from transmembrane α helices. The helices within these proteins can slide against each other, allowing the protein to undergo conformational changes that can be exploited to open and shut ion channels, transport solutes, or transduce extracellular signals into intracellular ones.
Term
Why are many proteins glycosolated?
Definition
Most cells secrete proteins, and this makes the protein glycosylated. The glycosylated proteins are mostly found in the blood serum. Being one of the components in the cell membrane, they are responsible for combining cells or even combining sperm and eggs.
Term
How do detergents take membranes apart?
Definition
Detergent breaks apart the cell and nuclear membrane by distrupting the liquid and protein structure. The cellular contents, including DNA, are then released.
Term
Why are membrane ghosts useful? What can you study with them?
Definition
Instead of the protoplast being sheared off to produce a round ghost, the plasma membrane tears and collapses onto the slide, ordering the actin bundles in the process. One implication of this model would be that cortical microtubules provide structural support to the plasma membrane of the protoplast so that only in their absence do the type II ghosts form.
Term
Describe the Spectrin skeleton of an RBC - why is it important? How does it change in sickle cell disease?
Definition
The skeleton thus is intermediate in complexity between a multisubunit enzyme and an organelle such as the endoplasmic reticulum. High resolution electron microscopy of the stretched membrane
skeleton has provided striking images ofa regular latticelike organization with five to six rod-shaped spectrin moleculesattached to short actin filaments 30-50 nm in length to form asheet of five and six sided polygons. Spectrin, the major structural component of this network, is a flexible rod-shaped molecule comprised of two subunitsaligned side-to-side to form heterodimers and head-to-head into tetramers. Spectrin requires additional proteins toform a membrane skeleton.
Term
What are transmembrane protein pumps and what are they used for?
Definition
Protein pumps function to pump out or in solutes or ions from a low concentration region to high concentration region....as this is normally cant hapen so they exploits energy from ATP. these pumps are transmembrane proteins and are very dynamic in conformation ( 3d structure----need to change their structure by ATP for pumping action)
