There are three fates of pyruvate: 2 anaerobic and 1 aerobic.
The aerobic pathway is where oxygen is present. It takes place in the mitochondria and produces the most ATP. It is known as the linked reaction because it is the entry into the TCA cycle. Pyruvate is converted to acetyl-CoA it is catalyzed by the enzyme pyruvate dehydrogenase and a byproduct is carbondioxide. There are five cofactors involved in this reaction: CoA-SH, NAD⁺, FAD, TPP and lipoate.
The other 2 anerobic reactions are fermentation reactions and do not consume oxygen. Conversion of pyruvate to lactate is catalyzed by lactate dehydrogenase. The cofactor present is NADH. Lactate is produced in muscle cells when the muscle cell is at rest no lactate is produced. It is also found in erythrocytes because they contain no mitochondria. Pyruvate is converted to acetylaldehyde and is catalysed by the enzyme pyruvate decarboxylase with the cofactors TPP and Mg₂⁺. The acetylaldehyde is then converted to ethanol and is catalyzed by the enzyme alcohol dehydrogenase with the cofactor NADH. NAD⁺ is present is small amounts and is quickly used up it needs to be regenerated for glycolysis to continue and the regeneration is done by fermentation.
I choose a video from my lecturer Mr. Jason Mathew to review because it is just a simply very good video. All of his videos are so check them out. They are simple to understand. All the objectives are clearly given and the fonts and colours used are very eye catching with cool and funny pictures.
From this video I learnt that there are 10 enzymes involved in glycolysis. G.Embden, O. Meyerhof and J. Parnas are the scientists responsible for elucidating the glycolysis pathway. Glycolysis is believed to be one of the most ancient metabolic pathways. At the end of glycolysis two pyruvate molecules are produced. There are two phases of glycolysis the energy investment phase and the energy generation phase. In the energy investment phase there are two irreversible reactions and three reversible reactions, where as in the energy generation phase there are one irreversible reaction and four reversible reactions. In the glycolysis path way there is a net gain of 2ATP and 2NADH produced for every turn.
In the energy investment phase glucose is converted to glucose 6- phosphate by the enzyme hexokinase. This is done to trap the glucose in the cell so it cannot leave and the addition of a phosphate group makes it less stable and promotes the reaction. This reaction is irreversible and consumes a ATP molecule. The other reaction where an ATP molecule is consumed is the third one where fructose 6-phosphate is converted to fructose1, 6-bisphoshate catalyzed by the enzyme PFK1. PFK1 is the most regulated enzyme of glycolysis. Aldolase splits fructose 1, 6- phosphate to glyceraldehyde3-phosphate (G3P) and dihyydroxyacetone phosphate (DHAP). DHAP is then converted to G3P catalyzed by the enzyme triose phosphate isomerase. Therefore at the end of the energy investment phase two molecules of G3P are produced.
In the energy generation phase (2) G3P are converted to 1,3-bisphosphoglycerate , two molecules of NADH are generated and it is the only oxidation reaction in glycolysis. The conversion of (2)1,3-bisphosphoglycerate to (2)3-phosphoglycerate catalyzed by phosphor-glycerate kinase, two molecules of ATP molecules are produced. The last reaction also produces two molecules of ATP by converting (2) phosphoenolpyruvate to (2) pyruvate catalyzed by pyruvate kinase and is the most energetically feasible of glycolysis. The excess energy is released as heat.
Enzymatic browning in bananas
Enzymatic browning ia a chemical process which occurs in fruits and begetables by the enzymes phenolase, peroxidase and other enzymes that create melanine and benzoquione from natural phenols resulting in a brown colour. The enzymes are both oxidoreductases which transfer electrons or hydride ions from one molecule to another. Phenolase catalyses monophenols to diphenols, diphenols ca be further catalysed to produce quione. Catecholase and monophenolase are two types of phenolases. Peroxidase is responsible for the oxidation of H₂O₂ which is its optimal substrate.
Enzymatic browning can be prevented by:
- Lowering the pH and the copper cofactor necessary for the enzyme to function.
- Blanching to denature enzymes.
- Low temperatures reduce the rate of reaction
- Addition of chemicals such as citrates
- Addition of nitrogen gas to prevent the oxygen from reacting
showing apple without and with enzymatic browning
Allosteric enzymes are multi subunit proteins with an active site on each subunit. The binding of substrate at one active site induces conformational change in the enzyme which alters the affinity of the other active sites for substrates. Therefore more than one active site cooperatively binds substrate molecules. Effectors regulate allosteric enzymes. They bind non covalently at a site other then the active site and the effector is located on a subunit that is not catalytic.
Positive effectors increase enzyme activity whereas negative effectors inhibit enzyme activity. Allosteric enzymes catalyze the committed step early in a reaction pathway. Homotrofic Effectors is when the substrate itself serves as an effector.
An example of cooperative substrate binding I the binding of oxygen to heamoglobin. Positive and negative effectors of allosteric can affect either Vmax or km or both.
There are four types of reversible inhibition: competitive inhibition, non-competitive inhibition, uncompetitive inhibition and mixed inhibition.
Competitive inhibition occurs when the inhibitor binds reversibly to the same site that the substrate would usually occupy and competes with the substrate for that site. The inhibitor resembles the substrate. Vmax is unchanged and Km increases.
Non- competitive inhibition occurs when the inhibitor and aubstrate binds at different sites on the enzyme. The inhibitor binds to the free enzymes or enzyme substrate complex and the inhibitor does not resemble the substrate. Km is constant and Vmax is decreased.
Uncompetitive inhibition occurs when the inhibitor binds only to the enzyme substrate complex at a separate site from the active site and not with the free enzyme. The inhibitor does not resemble the substrate. Both Vmax and Km are reduced to the same amount.
Mixed inhibition occurs when the inhibitor binds at a separate site from the active site to either the free enzyme or the enzyme substrate complex. The inhibitor resembles non- competitive inhibition. Km maybe increased or decreased but Vmax is reduced.
line weaver burk plot for mixed inhibition
Substrate concentration, enzyme concentration, temperature and pH affect the rate of an enzyme catalyzed reaction.
Substrate concentration- the rate of reaction increases with substrate concentration until a maximal velocity (Vmax) is reached. At low substrate concentration the active sites of the enzymes molecules are not used up. There are not enough substrate molecules to occupy all the active sites. As the substrate concentration increases more and more active sites come into use until all are being used (saturation) .any further increase in substrate concentration cannot increase the rate of the reaction.
Enzyme concentration- the active site of an enzyme maybe used over and over. Enzymes work efficiently at low concentrations. The rate of enzyme reaction is proportional to the enzyme concentration once substrate concentration is high and pH and temperature are kept constant.
Temperature- the reaction velocity is increased until a peak velocity is reached. This is due to an increased number of molecules having the activation energy to pass over the energy barrier. Also there is an increase in collision frequency of the molecules. There is a decrease of velocity with higher temperature because the high temperature results in denaturation of the enzyme. 35 ⁰C – 40⁰C is the optimum temperature required for human enzymes.
pH-enzymes have a optimum pH range at which they work best and they function within a narrow pH range. The optimum pH is that where the maximum rate of reaction is achieved. When pH is altered above or below this value the rate of enzyme reactivity decreases. As ph decreases the acidity increases. Therefore increasing the number of positive charge. Changes in pH alter the ionic charge of the acidic and basic side groups. This disrupts the bonding that maintains the specific shape of the enzyme. Therefore leading to a change in shape of the enzyme and active site. Extremes in pH cause the enzyme to be denatured.
Substrates are the substances which enzymes act on. The enzyme active is the region where the substrate binds and catalyzes the chemical reaction.
The Fischer’s lock and key hypothesis and Koshland’s induced fit hypothesis are two hypotheses suggested to explain catalyses and specificity of enzymes.
The lock and key hypothesis us where the fit between the substrate and active site is very specific like that of a lock and key. A temporary enzyme substrate complex is formed and the products with a different shape from the substrate once formed escapes from the active site leaving it free to attach to another substrate molecule. This explains enzyme specificity and loss of activity when the enzymes. However it is too rigid.
Koshland’s induced fit hypothesis suggests in the presence of the substrate the active site may change in order to fit the substrates change. The enzyme is flexible and molds to fit the substrate molecule like gloves fitting one’s hand or clothing on a person. The enzymes initially have a binding configuration which attracts the substrate. On binding to the enzyme the substrate disturbs the shape and causes it to assume a new configuration. The active site is then molded into a precise conformation.
induced fit hypothesis
References : http://youtu.be/FPKAJlgMCbE