Question

This is for the second level/semester biochemistry.

5) Within a few days after a fast begins, nitrogen excretion accelerates to a higher-than-normal level. After a few weeks, the rate of nitrogen excretion falls to a lower level and continues at this low rate. However, after lipid stores have been depleted, nitrogen excretion rises to a high level.

a) What events trigger the initial surge of nitrogen excretion?

b) Why does nitrogen excretion fall after several weeks of fasting?

c) Explain the increase in nitrogen excretion when the lipid stores have been depleted.

6) Cysteine and cysteine. Most cytosolic proteins lack disulfide bonds, whereas extracellular proteins usually contain them. Why?

Answer 1

5a) Within a few days after a fast begins, nitrogen excretion accelerates to a higher than normal level due to a particular reason and that is nothing by protein catabolism which results in the production of nitrogen due to which urinary nitrogen losses increases. when an individual is on fasting condition, in order to maintain the body's glucose levels fatty acid oxidation takes place and there is a surge in ketone body generation because brain solely depends on either glucose or ketone bodies for energy. Thus, there is a close relationship between blood ketone bodies, glucose and fatty acids and urinary nitrogen losses during fasting. Fasting disturbs the energy supply, which is made up by the mobilization of the lipid reserves and by increasing the conversion of liberated fatty acids into ketone bodies which serves as substrates for energy production. Brain and kidneys depend on them for energy. But there is a problem, that is, the mobilization of fat tissue and the continuous production of ketone bodies from the fatty acids need a few days to stabilize. During this time, the body's energy needs are met by the available labile protein reserve and serves as a precursor of gluconeogenesis in order to prevent a drop in glucose. Thus, protein reserves of the muscles, liver are used up in these initial days to make up for the energy needs of our body.

5b) As I have stated in my previous answer, that for stabilisation and mobilisation of the fatty acids for the production of ketone bodies requires a few days. So when they are available to produce ketone bodies protein reserves are again not used for energy production and the fat reserves are used for ketone bodies which acts as a source of energy for the body organs.thus the nitrogen excretion falls ater several weeks of fasting.

5c) When the lipid stores have been depleted body's energy needs are fulfilled by the proteins again especially the skeletal muscle proteins.thus amino acids are released for maintaining glucose homeostasis especially alanine and glutamine. Alanine in the liver is converted to glucose and ammonia is released by the urea cycle. High demand for glutamine accounts for the purine synthesis. During the later stages of starvation glutamine is required for gluconeogenesis in kidneys. And thus, these events trigger the nitrogen excretion when the lipid stores have been depleted.

6) Disulfide bonds are formed between two Cys residues in a protein. Cytosolic proteins lack disulphide bonds because of its reducing environment where the pk_a>8.0 and thus remain protonated at physiological pH. this is the same reason why the extracellular proteins contain them.