# Light-Activating PROTACs in Cancer: Chemical Design, Challenges, and Applications
## 2 Protein Degradation Strategies  
Normal cellular functioning requires tight control over the relative rate of biosynthesis and degradation of intracellular proteins. This tight control is continuously monitored by the highly organized intracellular protein signaling systems that commonly comprise multi-partner proteins, communicating in a spatiotemporal way. An alteration in such a sophisticated intracellular cell system leads to catastrophic results, where overexpression of a specific intracellular protein ends up accelerating the disease state advancement. Broadly, strategies devised to eliminate the overexpressed intracellular proteins, where DNA chemical modification leads to protein of interest (POI) knockout at the _gene level_[1], or RNA interference (RNAi) eradicates the POI expression at the mRNA level [2]. However, such approaches take time to complete the POI depletion and encourage researchers to explore the protein-targeting technologies on the basis of event-driven pharmacology. Notable protein-targeting technologies are TRIM-Away [3], HyTs (low-molecular-weight hydrophobic tags) [4], and PROTACs (proteolysis targeting chimeras) [5]. These techniques utilize the protein degradation assembly of the cell to target specific POI. Moreover, these strategies are flawed with irreversible control over protein degradation, insufficient potencies, and low cellular delivery [6]. Among these, PROTAC design strategies found a wide application against several overscured proteins in cancer (EGFR [7], CDK9 [8], TRIM24 [9], Bcl-2 family proteins (Bcl-xL [10; 11], Bcl-2 [12], Mcl-1 [12; 13]), bromodomain [14; 15], tyrosine-protein kinase (c-Met [7], ALK [16]), estrogen-related receptor-\(\alpha\)[17], and Tau [18]). Furthermore, successful oral PROTAC (ARV-110) targeting androgen receptors entered into a phase-1 clinical study [19]. PROTACs use the intracellular ubiquitination to induce POI degradation. As illustrated in Figure 2, PROTACs are chemically heterobitunctional, made up of three _warheads_, represented as _Part-A_ (green), a known drug structure/bioactive molecule that has an affinity towards the _protein of interest (POI); _Part-B_ (_magenta), the linker region that, in general, works as a connector to connect part-A and part-C; _Part-C_ (orange), a chemical structure that has an affinity to E3 ubiquitin ligase (also called as \"E3 ligase ligand\", for example, cereblon (CRBN) or Von Hippel-Lindau (VHL)). However, subsequent binding of chimeric compounds such as PROTACs (or similar compounds such as SNIPERs) to POI and E3 ligase depends on cooperativity, which means that binding of either warhead to their respective proteins (POI or E3-ligase) influences the binding of the remaining warhead to its partner protein. Therefore, a proportion of bifunctional  
Figure 1: The electromagnetic (EM) spectrum range covers frequencies, radiofrequency (3 Hz-300 GHz), microwaves, IR, UV, and gamma-waves. compounds (PROTACs, SNIPERs) forms multiple binary complexes either with POI or E3-ligase, even at high concentrations, and such biophysical limitation is also known as a _hook's effect_. The success of PROTACs is hidden in their mechanism of action when compared with the mechanism of action of conventional inhibitors as a conventional inhibitor requires higher binding affinity to engage themselves to the POI, irrespective of PROTACs, which need to hook themselves with POI, allowing E3 ligase in enough proximity to induce the intracellular ubiquitination of POI (illustrated in Figure 2A). The E3 ligase is a ubiquitin protein that recruits an E2-ubiquitin-conjugating enzyme that has ubiquitin units. After identifying the targeted protein, we find the transfer of ubiquitin from the E2 to the lysine amino acids exposed to the surface of the targeted protein that eventually enables POI degradation.