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I have Read that we cannot digest cellulose because we do not have enzymes to digest Beta glycosidic linkages in Cellulose

Then how is it that we have an enzyme called Lactase to digest the Beta glycosidic linkages in lactose?

Also If we have evolutionarily developed Lactase ,Why didnt we develop "Cellulase " too ?

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  • $\begingroup$ Humans have β-galactosidase (lactase) which allows them to break down the β-glycosidic linkages in lactose. This enzyme is inactive against cellulose. We also haven't evolved a cellulase, probably because we didn't need to. It's quite simple. Are you asking any further questions beyond these? $\endgroup$ – S Pr Oct 30 '19 at 14:43
  • $\begingroup$ Please quote or provide a reference to what you have "read" when asking questions on this site. It is quite likely that you are misquoting or misunderstanding, and by checking the original we can put you right. Enzymes recognize their substrates — in this case not just a chemical bond, but the two sugars that it joins. These are different for cellulose (glucose only) and lactose (glucose, galactose). Also, please restrict yourself to one question at a time. The second question about why few animals have cellulases is quite separate. (It should be obvious why we evolved lactase.) $\endgroup$ – David Oct 30 '19 at 16:01
  • $\begingroup$ @SPr — I'm not sure it's quite so obvious why very few animals have not evolved cellulases. Ruminants and most insects that do "need to" use symbiotic bacteria. I think this might be an interesting question in itself. Is it something to do with the waxy coat on leaves, perhaps? I don't know. $\endgroup$ – David Oct 30 '19 at 16:06
  • $\begingroup$ Few animals have evolved cellulases, but they are a very tiny minority and they tend to be aquatic invertebrates or small insects with little gut tracts like termites; always different to large mammals. I must admit I don't understand "why didn't we develop X" questions though. I think the onus is on the person to reason that it would have been beneficial in the first place, at least to a more competitive degree than the absence of a trait. Consider free glucose in the anaerobic gut, in the presence of a diverse microbiome... Consider a lifelong lack or reduction of fiber... No thanks. $\endgroup$ – S Pr Oct 30 '19 at 16:25
  • $\begingroup$ What I meant by the Second Question was that ,When we have developed β-galactosidase ,just based on the nature of the bond making the cellulose must also be possible....that too Given the high advantage an animal with cellulase will have ,So, What property of cellulose has made the evolution of cellulase so rare? Or does the answer have nothing to do with the nature of cellulose but some factors at the organism or ecosystem level?Or is it just pure luck? $\endgroup$ – Sudhanshu Bharadwaj Oct 31 '19 at 11:26
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One simple reason is physical interaction. lactose is only composed of two sugars, while cellulose is giant network of interlocked sugars. Cellulose keeps the sugars tightly packed in three dimensions. even getting to the bonding site is difficult.

human and many other lactases are a huge molecules that would never be able to fit on to a cellulose chain, while cellulase of often a tiny molecule as enzymes go and the active site is not buried in the center. Compare the two enzymes.

enter image description here lactase

enter image description here

enter image description here two different cellulase enzymes

Note the big difference in size. Also look at cellulase in action.

enter image description here

Notice it has to peel up the cellulose strip by strip, while lactose is just floating free in solution. This even limits the speed of cellulose breakdown. which means organisms that breakdown cellulose have to spend a log time doing it, while humans have a relatively short and fast digestive timeframe, so even if we could we would not get much from it.

These functional limits also makes a cellulase enzyme harder to evolve, since it can't be too large or in organisms with fast digestive processes. We often forget the physical necessities of enzyme function. there are plenty of other reasons enzymes do not function on multiple substrates, but this is why is basically impossible to for evolution to change lactase into a cellulose cleaving enzyme.

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