Cellular Respiration and Enzymes

The substances that form at the end of the reaction are called end products. They cause feedback inhibition.

Enzyme immobilisation may be defined as confining the enzyme molecules to a distinct phase from the one in which the substrates and the products are present.

Cytochrome a3 helps in the transfer of electrons to oxygen. The oxygen has great affinity to accept the electrons and in the presence of protons, a water molecule is formed.

HMP pathway generates NADPH molecules which are used as reductants in biosynthetic process under conditions when NADPH molecules are not generated by photosynthesis. It is, therefore, important in non- photosynthetic tissues such as in differentiating tissues, generating seeds and during periods of darkness. Production of NADPH is not linked to ATP generation in pentose phosphate pathway.

Acetyl-CoA is an important biochemical molecule in cellular respiration. It is produced in the second step of aerobic respiration after glycolysis and carries the carbon atoms of the acetyl group to the TCA cycle to be oxidised for energy production. It is produced by decarboxylation of pyruvate in the mitochondrial matrix.

During glycolysis, NAD (Nicotinamide adenine dinucleotide) removes electrons from 1, 3- diphosphoglyceric acid using diphosphoglycrealdehyde dehydrogenase. NAD changes to NADH2 and this is either utilised as such in anaerobic respiration or in the presence of oxygen.

Mitochondrion is the organelle which bears various enzymes participating in Krebs cycle. Each mitochondrion is covered by a double membrane. The inner membrane is selectively permeable and forms foldings called cristae. The inner membrane bears oxysomes, enzymes of fatty acids, succinate dehydrogenase (of Krebs cycle) and the electron transport system. All other enzymes of Krebs cycle are present in the mitochondrial matrix.

The last step of aerobic respiration is the oxidation of reduced coenzymes, i.e. NADH₂ and FADH₂ by molecular oxygen through FAD, ubiquinone, cyt. f, cyt. c, cyt. c, cyt. a and cyt. ay. By oxidation of 1 molecule of NADH, 3 ATP molecules are produced and by oxidation of 1 molecule of FADH₂, 2 ATP molecules are produced. So, the greatest number of ATP molecules are produced in the electron transport chain.

Respiration is an energy-liberating, enzymatically-controlled multistep catabolic process of stepwise breakdown of organic substances (hexose sugar) inside the living cells. Aerobic respiration includes the 3 major processes - glycolysis, Krebs cycle and electron transport chain. The substrate is completely broken down to form CO₂ and water. A large amount of energy is released stepwise in the form of ATP.

Glycolysis or EMP pathway is the breakdown of glucose to two molecules of pyruvic acid through a series of enzyme mediated reaction releasing energy. Pyruvic acid is a 3-carbon compound. In glycolysis, net gain of 2ATP and 2 NADH2 molecules occurs.

Cellular respiration is the process where cells break down glucose to produce energy (ATP), releasing carbon dioxide and water as byproducts. This is the primary biological pathway for converting glucose to CO2.

Heme biosynthesis requires succinyl CoA, glycine, and ferrous ion as key substrates. Propionic acid is not involved in heme synthesis - it's a fatty acid that can be converted to succinyl CoA via propionyl CoA, but is not a direct reactant.

Aminolevulinic acid (ALA) synthase is the rate-limiting enzyme in heme synthesis, and its transcription can be induced by certain drugs (like barbiturates) through cytochrome P450 induction. It requires pyridoxal phosphate (not NAD), is inhibited by heme (feedback inhibition), and steroids don't directly inhibit its synthesis.

In heme synthesis, the first three enzymes (ALA synthase, ALA dehydratase, and porphobilinogen deaminase) are cytosolic, while the later enzymes (including coproporphyrinogen oxidase, protoporphyrinogen oxidase, and ferrochelatase) are mitochondrial. ALA dehydratase (also known as porphobilinogen synthase) catalyzes the condensation of two ALA molecules to form porphobilinogen in the cytosol, making it the correct answer.

Ferrochelatase is the final enzyme in the heme biosynthesis pathway, inserting iron into protoporphyrin IX to form heme. ALA synthase, while rate-limiting, is not considered the regulatory enzyme in erythroid cells where feedback inhibition by heme is less prominent.