1. Explain what is meant by the concept of an \"electron carrier\" as it relates
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Question
1. Explain what is meant by the concept of an "electron carrier" as it relates to the metabolism of microbes. 2. Describe the role and the chemistry of enzymes in the metabolism of microbes.Help please and thank you! 1. Explain what is meant by the concept of an "electron carrier" as it relates to the metabolism of microbes. 2. Describe the role and the chemistry of enzymes in the metabolism of microbes.
Help please and thank you! 2. Describe the role and the chemistry of enzymes in the metabolism of microbes.
Help please and thank you!
Explanation / Answer
1. Electron carriers carry the electrons from donor to acceptor. Electron transfer occurs from organic compound to one group of electron carrier mostly NAD+ and FAD. In the next few steps series of electron carriers pass electrons to the oxygen molecules or other oxidized organic or inorganic molecules. The whole procedure takes place in the plasma membrane of prokaryotes and the process is called electron transport system which is essential for oxidative phosphorylation and oxidation reduction reactions. For example, the electron transport system (ETS) of chemoorganotrophs are located in the plasma membrane. The electron transfer from one electron carrier to the another releases energy and some of this energy can be used to produce ATP from ADP through chemiosmosis. The carriers that make up ETS differ in terms of their chemical nature and the way they carry electrons. NAD+ and its chemical relative nicotinamide adenine dinucleotide phosphate (NADP+) contain a nicotinamide ring. This ring accepts two electrons and one proton from a donor ( e.g., an intermediate formed during the catabolism of glucose), and a second proton is released. Flavin adenine dinucleotide ( FAD) and flavin mononucleotide ( FMN) bear two electrons and two protons on the complex ring system. Proteins bearing FAD and FMN are called flavoproteins. Both NAD+/NADH and FAD/FADH2 are used to extract energy from sugars during chemoheterotroph catabolisms. NADP+/NADPH helps in anabolic reactions and photosynthesis. Catabolism is exergonic pathway that breaks down complex molecules into simple one and anabolism is endergonic pathway that involves in biosynthesis.
2. Enzymes can be defined as protein catalysts that have greater specificity for the reaction catalyzed and the molecules acted on. A catalyst is a substance that increases the rate of a chemical reaction without being permanently altered itself. Thus enzymes speed up cellular reactions. The reacting molecules are called substrates and the substances formed are the products. Enzymes consist of a protein called apoenzyme and a non protein component called cofactor. The complete enzyme consisting of the apoenzyme and its cofactor is called the holoenzyme. If the cofactor is firmly attached to the apoenzyme it is a prosthetic group. If the cofactor is loosely attached with apoenzyme and can dissociate from the protein after products have been formed , it is called a coenzyme. Many coenzymes can carry one of the products to another enzyme. For example, NAD+ is a coenzyme that carries electrons within the cell. When two molecules approach each other to react , they form a transition state complex. Activation energy is needed to bring the reacting molecules together in the correct way to reach the transition state. The transition state complex can then resolve to yield the products. If a reaction is endergonic, the presence of an enzyme will not shift its equilibrium so that more products can be formed. Enzymes simply speed up the rate at which a reaction proceeds toward its final equilibrium.
Microorganisms can be poisoned by enzyme inhibitors. A competitive inhibitor directly competes with the substrate at an enzyme's catalytic site and prevents the enzyme from forming products. Sulfa drugs resemble para -aminobenzoate, a molecule used in formation of folic acid. The drugs compete with para- aminobenzoate for catalytic site of an enzyme resulting in inhibition of folic acid synthesis and it ceases bacterial growth. Noncompetitive inhibitors affect enzyme activity by binding to enzyme at some location other than active site.
Enzyme regulates microbial metabolism in 3 ways :
1. Metabolic channeling : influences pathway activity by localizing metabolites and enzymes into different parts of a cell. For example, channeling through cytoplasmic matrix.
2. Regulation of the amount of synthesis of a particular enzyme : Transcription and translation can be regulated and these two processes function in synthesizing enzymes.Regulation at this level is relatively slow, but it saves the cell considerable energy and raw material.
3. Direct stimulation or inhibition of the activity of critical enzymes : This type of regulation rapidly alters pathway activity and called as posttranslational regulation because it occurs after the enzyme has been synthesized.
- Allosteric regulation : Most regulatory enzymes are allosteric enzymes. The activity of this enzyme is altered by effector or modulator. The effector binds reversibly by noncovalent forces to a regulatory site separate from the catalytic site and causes a change in the shape or conformation of the enzyme. For example, allosteric enzyme aspartate carbamoyltransferase of Escherichia coli catalyzes the condensation of carbamoyl phosphate with aspartate to form carbamoylaspartate which is the rate determining reaction of the pyrimidine nucleotide biosynthetic pathway of the bacteria.
- Reversible covalent modification can be seen in E.coli glutamine synthetase involved in nitrogen assimilation.
- The rate of metabolic pathway can be adjusted by feedback inhibition or end product inhibition. The pacemaker enzyme catalyzes the slowest or rate limiting reaction in the pathway. The end product inhibits regulatory enzyme. The E.coli carbamoyltransferase is an excellent example of this.
Bacteria uses PTS or phosphotransferase system in carbohydrate uptake and in control of carbohydrate metabolism. Here the source of energy is phosphoenolpyruvate (PEP) and this translocation is seen only in bacteria which is enzyme mediated( enzymes of plasma membrane and cytoplasm). Enzyme I, the final step of the reaction carries downstream reaction which converts PEP to pyruvate. Enzyme one is capable of reverse reaction too. The reactions can be studied well in Escherichia coli.
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