Caspase 7

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Introduction

From food digestion to highly regulated cascade reactions are all physiological reactions that require the participation of proteases. Apoptosis is the default programmed cell death mode during embryonic development, and it is also essential for maintaining the homeostasis of adult organisms. Caspases, an evolutionary conserved family of aspartate-specific cysteine proteases, are the core elements of the apoptosis mechanism. These proteases all exist in the form of latent zymogens which have a variable-length N-terminal prodomain preceding the catalytic domain. Caspases usually signal in a two-step cascade (Figure 1): ‘initiator caspases’ with large prodomains (such as caspase-1, -8 and -9) are recruited into the large protein complex, then autoactivation occurs. Activated initiator caspases then release their "executioner caspases" of their short inhibitory prodomains, allowing them to cleave a large number of cellular substrates. Together with caspase-3 and -6, caspase-7 belongs to the subgroup of executioner caspases. It was originally cloned as ICE-LAP3, Mch3 and CMH-1 by three different groups, and later renamed caspase-7. For a long time, this protease has been considered to be functionally redundant with caspase-3, but many biochemical studies and the latest research in caspase-7-deficient mice have shown that it also plays a unique and important role in apoptosis and inflammation, and has the potential as a new target for cancer and inflammation treatment.

Figure 1. Overview of caspase activation mechanisms (Lamkanfi, M.; Kanneganti, T.D. 2010)

Structure of caspase-7

Since the crystal structure of procaspase-7 and the crystal structure of the free and inhibitor-bound forms of the active enzyme have been resolved, the three-dimensional structure of the activation mechanism of caspase-7 has been described in detail. Procaspase-7 is a protein composed of 303 amino acids, which resides in the cytosol as a pre-assembled homodimer. Structurally, the homodimer is composed of a 12-stranded β-sheet in the center and 10 α-helices around it.

Figure 2. Schematic representation of the procaspase-7 and active caspase-7 homodimer (Lamkanfi, M.; Kanneganti, T.D. 2010)

This "open α/β barrel fold" structure consists of two identical arranged enzymatic units, each of which contains an active site. Each enzymatic unit consists of a large and a small catalytic subunit of 20 kDa and 11 kDa, respectively, which are connected by a linker sequence in the caspase-7 zymogen. The linker region hinders the ordering of the active site, therefore, remove it by initiator caspases is a prerequisite for the activation of caspase-7, which is achieved by cleavage of the peptide bonds between Asp198 and Ser199 and between Asp206 and Ala207, respectively. After the linker is removed, the loops L2, L3 and L4 undergo spatial reorganization to form the active site and the substrate binding pocket. Unlike the linker region, the N-terminal propeptide does not affect the activation of procaspase-7 in vitro. Nevertheless, the prodomain has a negative effect on the enzyme activity of caspase-7 in cells, although the mechanism is not clear.

Figure 3. Three-dimensional structure of procaspase-7 (Lamkanfi, M.; Kanneganti, T.D. 2010)

Biological function of caspase-7

The death signal is transmitted to the cytoplasm through the receptor clustering, which leads to the activation of caspase-8 and -10. On the other hand, chemotherapeutic drugs and ultraviolet radiation cause DNA damage and trigger the release of mitochondrial cytochrome c into the cytoplasm. The latter binds to the adaptor protein Apaf-1 to form apoptosome. This large protein complex activates caspase -9. Once activated, caspases-8, -9, and -10 process the executioner caspases-3 and -7. Mature caspases-3 and -7 have the ability to cleave a large number of substrates, and ultimately lead to the characteristic morphology and biochemical hallmarks of apoptosis, such as phosphatidylserine exposure. In addition to being activated during apoptosis, the proteolytic maturation of caspase-7 can also be observed under inflammatory conditions. Interestingly, in macrophages stimulated with lipopolysaccharide and ATP or infected with the Gram-negative pathogens Legionella pneumophila and Salmonella typhymurium, caspase-7 activation requires caspase-1 complexes called ‘inflammasome’. And a large number of biochemical studies have shown that caspase-7 is a direct substrate of caspase-1. In the case of excessive cell death and/or inflammation leading to disease, it may be beneficial to interfere with the activation of caspase-7. Another potential application of caspase-7 is to prevent lymphocyte cell death in sepsis. Studies have shown that single nucleotide polymorphisms (SNP) in the caspase-7 gene is also closely related to insulin-dependent diabetes mellitus and rheumatoid arthritis.