What Advantages Do Multienzyme Complexes Provide
During evolution, some enzymes evolved to reach a virtual catalytic perfection, namely, for such enzymes nearly every collision with their substrate results in catalysis. Examples are:
- fumarase , which catalyzes the seventh reaction of the citric acid cycle, the reversible hydration/dehydration of fumarates double bond to form malate
- acetylcholinesterase , which catalyses the hydrolysis of acetylcholine, a neurotransmitter, to choline and acetic acid, which in turn dissociates to form an hydrogen ion and acetate
- superoxide dismutase , which catalyzes the conversion, and then the inactivation, of the highly reactive superoxide radical, , to hydrogen peroxide and water
- catalase , which catalyzes the degradation of H2O2 to water and oxygen.
- The diffusion of substrates and products in the bulk solvent is minimized, then their dilution and decrease of concentration, too. This leads to the production of high local concentrations, even when their intracellular concentration is low. In turn this leads to an increase in the frequency of enzyme-substrate collisions.
- The time required by substrates to diffuse between successive active sites is minimized.
- The probability of side reactions is minimized.
- Chemically labile intermediates are protected from degradation by the solvent.
Type I Fatty Acid Synthase
Fatty acid synthase or FAS catalyzes the synthesis of palmitic acid using malonyl-CoA, the product of the reaction catalyzed by acetyl-CoA carboxylase, as a donor of two-carbon units.There are two types of FAS.In animals and fungi, it is a multifunctional enzyme, and is called type I. In animals it is an homodimer, and each polypeptide chain contains all seven enzymatic activities plus acyl carrier protein or ACP. In yeast and fungi FAS consists of two multifunctional subunits, called and , arranged in an 66 heterododecameric structure.In most prokaryotes and in plants, fatty acid synthase, called type II, it is not a multifunctional enzyme but a multienzyme complex, being composed of distinct enzymes plus ACP.
Structure Of Dihydrolipoyl Transacetylase Or E2
Three functionally distinct domains can be identify in the structure of dihydrolipoyl transacetylase: an N-terminal lipoyl domain, a peripheral subunit-binding domain, and a C-terminal catalytic domain or acyltransferase domain. These domains are connected by 20- to 40 amino acid residues rich in alanine and proline, hydrophobic amino acids that are interspersed with charged residues. These linkers are highly flexible and largely extended, that allows the three domains to kept away from each other.
Note: flexible linkers are present in E3BP, too.
an interchange of the acetyl groups between the lipoyl groups of the dihydrolipoyl transacetylase core the exchange of both acetyl groups and disulfides between the tethered arms.
- PSBD is composed of 35 amino acid residues arranged to form a globular structure that binds to both pyruvate dehydrogenase and dihydrolipoyl dehydrogenase, that is, it holds the multienzyme complex together.
- The C-terminal catalytic domain, which, of course, contains the active site, is composed of 250 amino acid residues arranged to form a hollow cage-like structure containing channels large enough to allow substrates and products to diffuse in and out. For example, CoA ad lipoamide, the two substrate of dihydrolipoyl transacetylase, bind, in their extended conformation, at the opposite ends of a channel located at the interface between each pair of subunits in each trimers.
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Rs System And Multiple Chiral Centers
When two or more chirality centers are present in a molecule, each center is analyzed separately using the rules previously described.Consider 2,3-butanediol. The molecule has two chiral centers, carbon 2 and carbon 3, and exists as three stereoisomers: two enantiomers and a meso compound. What is the RS configuration of the chiral centers of the enantiomer shown in figure?
Consider carbon 2. The order of priority of the groups is OH > CH2OHCH3 > CH3> H. Rotate the molecule so that the hydrogen, the lowest priority group, is pointed away from the viewer. Tracing a path from OH, the highest priority group, to CH3, the lowest priority group, we move in a clockwise direction: the configuration of the carbon 2 is, therefore, R. Applying the same procedure to carbon 3, its configuration is R. Then, the enantiomer shown in figure is -2,3-butanediol.
Chirality In The Absence Of A Chiral Center
Chirality can also occur in the absence of a chiral center, due to the lack of free rotation around a double or a single bond, as in the case of:
- allene derivatives, organic compounds in which there are two cumulative double bonds, namely, two double bonds localized on the same carbon atom
- biphenyl derivatives.
In this case, chirality is due to the presence of an axis of chirality.
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How To Manipulate Fischer Projection Formulas
Since Fischer projections represent three-dimensional molecules on a two-dimensional sheet of paper, some rules must be respected to avoid changing the configuration.
- The projections must not be lifted out the plane of the paper, because this causes enantiomer is converted into the other enantiomer.
- If you rotate the projections in the plane of the paper, you obtain the same enantiomer if you rotate the structures by 180° in either direction, because the vertical groups must lie below the plane of the paper, whereas the horizontal groups above. Conversely, the rotation by 90° or 270° in either direction causes an enantiomer is converted into the other enantiomer.
- An odd number of exchanges of two groups leads to the other enantiomer.
Osmotic Pressure Osmosis And Plasma Membranes
Osmosis can be defined as the net movement or flow of solvent molecules through a semipermeable membrane driven by osmotic pressure differences across the membrane, to try to equal the concentration of the solute on the two sides of the membrane itself.In biological systems, water is the solvent and plasma membranes are the semipermeable membranes. Plasma membranes allow water molecules to pass, due to protein channels, known as aquaporins, as well as small non-polar molecules that diffuse rapidly across them, whereas they are impermeable to ions and macromolecules. Inside the cell there are macromolecules, such as nucleic acids, proteins, glycogen, and supramolecular aggregates, for example multienzyme complexes, but also ions in a higher concentration than that of the extracellular environment. This causes osmotic pressure to drive water from outside to inside the cell. If this net flow of water toward the inside of the cell is not counterbalanced, cell swells, and plasma membrane is distended until the cell bursts, that is, an osmotic lysis occurs. Under physiological conditions, this does not happen because during evolution several mechanisms have been developed to oppose, and in some cases even exploit, these osmotic forces. Two of these are energy-dependent ion pumps and, in plants, bacteria and fungi, the cell wall.
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Chirality And Optical Activity
The optical activity of materials such as quartz and, more importantly, of organic compounds such as sugars or tartaric acid, was discovered in 1815 by the French scientist Jean-Baptiste Biot.Chiral molecules can be classified based on the direction in which plane-polarized light is rotated when it passes through a solution containing them.
- If a solution containing one enantiomer rotates plane-polarized light in a clockwise direction from the observers point of view, the molecule is called dextrorotatory or dextrorotary, from the Latin dexter, meaning right, and is designated by the prefixes –, or d from dextro-.
- If a solution containing one enantiomer rotates plane-polarized light in a counterclockwise direction from the observers point of view, the molecule is called levorotatory or levorotary, from the Latin laevus, meaning left, and is designated by the prefixes , or l from laevo-.
Obviously, if we consider a pair of enantiomers, one is dextrorotatory and the other levorotatory.At present it is not possible to reliably predict the magnitude, direction, or sign of the rotation of plane-polarized light caused by an enantiomer. On the other hand, the optical activity of a molecule provides no information on the spatial arrangement of the chemical groups attached to the chirality center.Note: a system containing molecules that having the same chirality sense is called enantiomerically pure or enantiopure.
Pasteur and the discovery of enantiomers
Functions Of The Pyruvate Dehydrogenase Complex
The main functions of the pyruvate dehydrogenase complex are to produce acetyl-CoA and NADH.
- The acetyl group linked to coenzyme A, an activated acetate, depending on the metabolic conditions within the cell and/or the cell type, can be:
oxidized to two carbon dioxide molecules via the citric acid cycle reactions to harvest a portion of the potential energy stored in the form of ATP or GTP utilized for the synthesis of fatty acids, cholesterol, steroids, isoprenoids, ketone bodies and acetylcholine.
It is therefore possible to state that, depending on the metabolic conditions and/or cell type, the pyruvate dehydrogenase complex commits carbon intermediates from amino acid and glucose catabolism to:
citric acid cycle, and then to the production of energy, e.g. in skeletal muscle in aerobic conditions, and, always, in cardiac muscle synthesis of lipids and acetylcholine.
- In aerobic organisms, NADH can be oxidized to NAD+ via hydride ion transfer to the mitochondrial electron transport chain that, in turn, carries the two electrons to molecular oxygen , allowing the production of 2.5 ATP molecules per pair of electrons.Note: in anaerobic organisms there are electron acceptors alternative to oxygen, such as sulfate or nitrate.
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Ambiguities Of The Fischer
The Fischer-Rosanoff convention gives rise to uncertainties with molecules with more than one chiral center. For example, considering D–glucose, the D-L system gives information about the configuration of C-2, but no information about the other asymmetric centers, namely, C-3, C-4, and C-5.
In these cases, the RS system, developed in 1956 by Robert Sidney Cahn, Christopher Ingold, and Vladimir Prelog, labeling each chiral center, allows to describe accurately the stereochemistry of the molecule. In the case of D–glucose, the molecule has the -configuration.It should also be noted that depending on the chiral center taken as the reference center, the same molecule can belong to both the D and L series.
Reaction Of Dihydrolipoyl Transacetylase Or E2
In the reaction sequence catalyzed by components of the pyruvate dehydrogenase complex, dihydrolipoyl transacetylase catalyzes the third step, namely, the transfer of the acetyl group from acetyl-dihydrolipoamide to CoA to form acetyl-CoA and dihydrolipoamide, the fully reduced form of lipoamide, the dithiol.It should be noted that the acetyl group, initially bound by ester linkage to one of the SH group of lipoamide is next bound to the SH group of coenzyme A, again by ester bond, hence the term transesterification.
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What Size Cake Stand Do I Need
You will want to choose a cake stand that is the same size or bigger than the bottom tier of your cake. Most tiered cakes are created on a cake board and, ideally, your cake board and cake stand will be the same size.
If you have chosen a simple cake design with minimal decorations, a cake stand that is the same size as the bottom tier of your cake will create a seamless transition from the cake to the cake stand. If your cake design has embellishments, such as flowers, you will want to use a cake stand that is bigger than the bottom tier of your cake to provide a small ledge for the embellishments.
I recommend never using a cake stand that is smaller than your cake board in order to ensure that your cake is as stable as possible. If you have any questions about the size of your cake or cake board, it is best to ask your baker.
Extra Tip When choosing a cake stand, make sure the top plate is completely flat so that it supports your cake properly. Any dips in the top plate can cause your cake to sag and ruin the design. All of Sarahs Stands cake stands feature a flat top plate for this reason.
What Else Can You Use A Cake Stand For
Cake stands are great to use for decoration when not using for desserts. Put one at the center of the table and place candles and seasonal decor on it. You can also put some fruits on it for easy access to enjoy as a snack or after a meal. Let your creative juices flow to make your own unique decoration with a cake stand.
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Energy Yield Of Glycogen Stores Under Aerobic Conditions
Under aerobic conditions, the oxidation of glucose to CO2 and H2O via glycolysis, pyruvate dehydrogenase complex, Krebs cycle, mitochondrial electron transport chain, and oxidative phosphorylation yields about 30 molecules of ATP.Below, the yield of ATP from aerobic oxidation of glucose released from glycogen by the action of glycogen phosphorylase and debranching enzyme is considered.
Examples Of Multienzyme Complexes
From what was said above, it is not surprising that, especially in eukaryotes, the multienzyme complexes, like multifunctional enzymes, are common and involved in different metabolic pathways, both anabolic and catabolic, whereas there are few enzymes that diffuse freely in solution. Below are some examples.
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Amino Acids And Gliceraldeide
In the Fischer-Rosanoff convention, all the proteinogenic amino acids are L-amino acids. In the RS system, with the exception of glycine, that is not chiral, and cysteine that, due to the presence of the thiol group, is -cysteine, all the other proteinogenic amino acids are -amino acids.Threonine and isoleucine have two chirality centers, the -carbon and a carbon atom on the side chain, and exist as three stereoisomers: two enantiomers and a meso compound. The forms of the two amino acids isolated from proteins are -threonine and -isoleucine, in Fischer-Rosanoff convention, L-threonine and L-isoleucine.In the RS system, L-glyceraldehyde is -glyceraldehyde, and, obviously, D-glyceraldehyde is -glyceraldehyde.
Regulation Of The Pyruvate Dehydrogenase Complex By Phosphorylation/dephosphorylation
Unlike prokaryotes, in mammals the activity of the pyruvate dehydrogenase complex is also regulated by covalent modifications, i.e., phosphorylation and dephosphorylation of three specific serine residues of the subunit of pyruvate dehydrogenase, the enzyme that catalyzes the first, irreversible step of the overall reaction sequence.Note: as mammalian pyruvate dehydrogenase is an heterotetramer, there are six potential phosphorylation sites.
Phosphorylation, which inactivates pyruvate dehydrogenase, and then blocks the overall reaction sequence, is catalyzed by pyruvate dehydrogenase kinase. Two of the aforementioned serine residues are located on the more C-terminal loop, at the entrance of the substrate channel leading to the respective active site, and the phosphorylation of only one of them inactivates the pyruvate dehydrogenase, hence demonstrating the out of phase coupling between its active sites.Conversely, in the dephosphorylated state the complex is active. Dephosphorylation is catalyzed by a specific protein phosphatase, the pyruvate dehydrogenase phosphatase.The activities of pyruvate dehydrogenase kinase and pyruvate dehydrogenase phosphatase are in turn subject to allosteric regulation by several modulators.
Regulation of pyruvate dehydrogenase kinase
The activity of pyruvate dehydrogenase kinase depends on the ratios of /, /, and / , as well as on the pyruvate concentration, in the mitochondrial matrix .
Regulation of pyruvate dehydrogenase phosphatase
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