Question 11

Outline protein metabolism, including the effects of starvation.

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College Answer

For a good answer candidates were expected to mention that there were “essential” and “nonessential proteins”, that protein synthesis draws on amino acids sourced from the common amino 
acid pool (because there is no cellular storage of amino acids) which is the combination of amino 
acids derived from gut absorption of digested dietary protein and also the continuous turnover of 
endogenous protein, amino acids filtered via the glomerulus are reabsorbed within the renal 
tubule and that in health losses are minor, but increases with illness. Protein synthesis is a 
semicontinuous activity, the rate of which varies considerably between cells under the influence 
of factors neuro- endocrine factors, substrate availability and energy availability. Proteolysis 
occurs within nuclear and cytoplasmic proteasomes, via peptide bond disruption catalysed by 
proteases, which are then processed further into amino acids being then available for release 
into the amino acid pool or utilised immediately for protein synthesis

In relation to starvation candidates were expected to mention that it results from a severe 
ongoing restriction of protein, energy and nutrient intake. That initially, glycogen stores from the 
liver and skeletal muscle are utilised after conversion to glucose and provide energy, thus 
sparing of fat and protein stores from use as an energy source for a period of less than a day, 
following which fats are increasingly utilised and finally protein catabolism accelerates, especially 
from the liver and skeletal muscle, with relative sparing of the heart and brain.