## 4 Discussion
The PIN1 protein is an enzyme (EC 5.2.1.8) catalyzing the reaction of cis-trans isomerization of peptide bonds formed by amine groups of proline and carboxyl groups of phosphorylated serine or threonine. Its malfunction is observed, e.g., in the course of the Alzheimer's disease [83], whereas its over-expression and hyper-activity play a significant role in carcinogenesis [84]. The FPR2 acronym (N-formyl peptide receptor 2) denotes a protein being a receptor of chemotactic peptides possessing a formic acid residue at the N-terminus. These receptors play a meaningful role during inflammatory states and angiogenesis [85]. The latter process is essential to wound healing and also to cancer development. A group of proteins referred to as S1 PR (Sphingosine 1-phosphate receptor) serves the function of sphingophospholipid receptors. Their hyperactivity is observed in the course of non-specific inflammatory bowel disease. Compounds that modulate the activity of S1 PR receptors have been investigated as potential drugs to be used in this disease condition [86]. The use of S1 PR receptor modulators is also considered in the treatment of multiple sclerosis [87]. The TNFRF10A (Tumor necrosis factor receptor superfamily member 10A) protein (UniProt: P50591) is a receptor of a cytotoxic ligand TRAIL that stimulates apoptosis [88]. The protein abbreviated as TK1 (EC 2.7.1.21) is a kinase accelerating the phosphorylation of thymidine nucleoside, being one of the key enzymes to the metabolism of nucleosides and nucleotides and, thereby, to DNA synthesis [89]. The DLG4 (Disks large homolog 4) compound is a homolog of proteins that play a key role in the formation of synapses [90]. The FNT acronym (FNTA; FNTB) denotes farnesyltransferase (EC 2.5.1.58), i.e., an enzyme involved in the modification of cysteine residues by attaching oligo-isoprene residues. Its malfunction contributes to the improper pattern of post-translational modifications of proteins in certain neoplasms [91]. Also, peptides built of the non-modified protein amino acids may--theoretically--serve as farnesyltransferase inhibitors [92]. The FDPS (Farnesyl diphosphate synthase) or FPPS (Farnesyl pyrophosphate synthase) (EC 2.5.1.1) enzyme is a synthetase of farnesyl diphosphate (pyrophosphate) (PubChem: No 445713), being a substrate of farnesyltransferase. Summing up data collated in Table 4, it may be concluded that most of the proteins listed therein are related to either reactions (as enzymes) or bioactivities (as receptors) of compounds containing phosphate residues (e.g., of phospholipids). The malfunctions of these proteins are observed in the course of various diseases such as, for example, neoplasms. In the case of peptides built from unmodified protein amino acids, proteolytic enzymes serve as the main group of potentially interacting proteins [93]. The highest probability of peptide-protein interactions, presented in Table 4, is lower than the analogous probability predicted for the peptides described in previous works [92; 93; 94]. The difference in the probability of interactions with proteins noted between the peptides built of the non-modified amino acids and phosphopeptides may stem from two reasons. The first entails differences in the structure and physicochemical properties triggered by modifications, whereas the other one may be due to knowledge gaps. The SwissTargetPrediction software compares the structure of analyzed compounds with data from the ChEMBL database [57]. This database provides a load of experimental data related to the interactions of short, unmodified peptides with proteins [95], while it lacks information about phosphopeptides and, therefore, has insufficient data for comparative analyses. Further studies are, hence, needed to resolve the doubts regarding the putative bioactivity of phosphopeptides.