I recently started my GCSE Biology course where I spend the next two years learning and preparing for my GCSE. One of the questions I have is that I was told that enzymes are recycled every time they are used. So why do people sometimes refer to developing an enzyme deficiency?
Proteins are fairly fragile molecular machines. They are subjected to environmental and intentional degradation all the time. There is quite a bit of variability across different protein types in how long they last, but none are permanent. Chen et al 2016 write:
For example, RALB is a GTPase, and is involved in a variety of cellular processes including gene expression, cell migration and proliferation, and membrane trafficking. The half-life was measured to be 9.4 hours in this work. IGFBP1 is insulin-like growth factor-binding protein 1, which is a well-known secreted protein. It has a very short half-life of 0.3 hours inside the cell because it is secreted after synthesis. The distribution of all protein half-lives is shown in Fig. 4b. The bin at 9 hours (half-lives between 8 and 10 h) contains the greatest number of proteins. The majority of the proteins quantified have half-lives within the range of 4–14 hours. About 6% of all quantified proteins (49) have half-lives <4 hours, while 51 proteins have long half-lives (>14 hours); the median half-life is 8.7 hours.
When someone talks about enyzmes being "recycled" in a reaction, they're contrasting between enzymes that participate in a reaction but yet are not consumed in that reaction, and other chemical species that do in fact become consumed in the reaction. That doesn't mean that an enzyme is indestructable, just that it isn't used up in the reaction itself.
Similarly, a wine glass is not used up in the process of drinking wine. It's recycled every time it's used. And yet, in the course of being used or even just sitting on the shelf, a wine glass is subject to degradation in the environment at some rate. If you never ever purchase another wine glass, or at least don't purchase them as often as they are broken, you're likely to over time develop a deficiency.
For normally functioning proteins in normally functioning cells, the proteins that are degraded are constantly being rebuilt from scratch. To have a deficiency would require something in the process to go wrong: either degradation is too fast, or something along the chain from DNA -> RNA -> translated protein -> post-transcriptional modification and transport is slowed down or incorrect.
Chen, W., Smeekens, J. M., & Wu, R. (2016). Systematic study of the dynamics and half-lives of newly synthesized proteins in human cells. Chemical science, 7(2), 1393-1400.