 Proteins are present in diet as polypeptides which have lot of amino acids connected by peptide bonds in between the amino acids. Digestion of proteins requires breakdown of these peptide bonds which are present between the amino acids. So this releases individual amino acids from the protein. Now the enzymes responsible for protein digestion are present in stomach and pancreas. So digestion of proteins unlike that of carbohydrates and fats does not begin in mouth instead it starts in stomach. In stomach, cheat cells in the gastric glands release the inactive precursor pepsinogen and it is the hydrochloric acid released by the parietal cells which acts upon this pepsinogen, cleaves a portion of it and it releases the active enzyme pepsin. This pepsin then digest proteins to proteases and peptones. By the way the optimum pH for the action of pepsin enzyme is less than 5 and we know that there is acidic pH in stomach so as the pH decreases activity of the enzyme increases. Now these digestion products that is the proteases and peptones move to intestine where they are further acted upon by pancreatic enzymes. The pancreas releases a number of enzymes for digestion of proteins. Some are endopeptidases that is which act on peptide bonds in between the polypeptide chain and the others are exopeptidases which act on the terminal peptide bond. So endopeptidases include the trypsinogen, chymotrypsinogen, proelastase while the exopeptidase include the procarboxypeptidase A and procarboxypeptidase B. Now it contrasts to pepsin which acts in acidic environment. The pancreatic enzymes require alkaline environment so their optimum pH is around 8. You can understand that because the ordinal pH is alkaline only and like pepsin these pancreatic enzymes are also secreted as inactive precursors. So once they are released in the lumen what happens trypsinogen is activated by another enzyme which is present on the intestinal brush water and this enzyme is enterokinase. So enterokinase activates trypsinogen to trypsin and once this trypsin is formed what happens that this trypsin is actually autocatalytic that is it can act on trypsinogen further and release more and more trypsin. Also it activates others enzymes as well so chymotrypsinogen is activated to chymotrypsin proelastase to elastase procarboxypeptidase A and B to carboxypeptidase A and B respectively. By the way why do you think that so many different types of enzymes are required for protein digestion? Well different peptidases act on bonds between different amino acids we saw that there are endopeptidases and exopeptidases also in these endo and exopeptidases they act in bonds between different amino acids for example carboxypeptide A acts on peptides which have a neutral amino acid at C terminal while carboxypeptidase B acts on peptides with basic amino acid at C terminal. Fine so that was luminal digestion of proteins however you see what happens that this action of pepsin and pancreatic enzymes does not release all the amino acids from the polypeptide chain instead there is release of certain amino acids but still there remains certain dipeptides and tripeptides so the digestion need to continue and this digestion is brought about by the enzymes present on apical membrane of intracytes that is the brushwater enzymes so the brushwater has enzymes like dipeptidases which will break down the peptide bond between two amino acids then amino peptidases which will break the peptide bond at the amino terminal and there are carboxypeptidases again which will break the bond at the carboxy terminal. Fine but remember again despite the action of these brushwater enzymes still there remain some di and tripeptides and these are actually digested within the cell so that means there are the amino acids which need to be absorbed and these di and tripeptides which are remaining they are also absorbed within the cell and further digestion takes place within the cell so within the cell again there are di and tripeptidases which break the peptide bond and further amino acids are released which are finally absorbed into the blood but for this these amino acids which have been already released in the lumen and the di and tripeptides which have already been released need to be absorbed right so how is that well there are various types of transporters which are present on the apical membrane of intracytes and five of these transporters have been found to be sodium amino acid co-transporters so that is by secondary active transport and it is dependent on sodium so absorption of amino acid is dependent on sodium like that of glucose also is dependent on sodium if you remember sodium glucose transporter so five of the transporters have been recognized as sodium amino acid co-transporter while other two are for the transport of di and tripeptides and these are not sodium dependent instead they require presence of hydrogen ions so that's the way these amino acids and di and tripeptides enter into the cell and as I told inside the cell there is intracellularization of these di and tripeptides by the respective di and tripeptidases and finally the amino acids enter into blood from the basoletrocyte by means of basoletro amino acid transporter so in summary digestion of proteins occurs in lumen by pepsin and pancreatic enzyme then the brush border enzymes continue the digestion and the digestion is completed intracellularly which is quite contrast from that of the digestion of the carbohydrates and the fats and absorption absorption occurs mainly by secondary active transport now let's finish by discussing some problems of digestion and absorption of proteins well clinically you see hardly there have been seen any problems related to the mal absorption of proteins rather even an extensive mucosal inflammation disease hardly we see any problem in protein absorption only there are certain genetic diseases which are very rare which cause defect in the digestion absorption of protein so first is deficiency of the enzyme entrokinase so again there is genetic manifestation of that and as you know we have discussed that entrokinase causes the activation of trypsinogen to trypsin so if this enzyme is not there digestion of proteins cannot proceed in a small intestine then there is a heart nerve disease in which there is defect in transport of neutral amino acids and then there is cystinuria in which there is defect in transport of dibasic amino acid as I said that there are five different types of sodium amino acid co-transporter so these co-transporters are transporting different types of amino acid so depending on which transporter is affected we will get different diseases so that's all about digestion absorption of proteins thanks for watching the video if you liked it do press the like button share the video with others and don't forget to subscribe to the channel physiology open thank you