## 1 Introduction  
5-hydroxytryptamine (5-HT) is a biogenic monoamine that performs various functions in the central nervous system (CNS). 5-HT is formed by the hydrolysis of tryptophan catalyzed by tryptophan hydrolase to 5-hydroxytryptphan and then decarboxylation to 5-HT [1]. 5-HT is synthesized and stored in the catecholaminergic, serotonergic, and pineal glands. If 5-HT synthesis occurs outside the CNS, it will occur in enterochromaffin cells and may be produced and stored in platelets to a small extent [1, 2].  
The incorrect level of 5-HT in the human body affects the pathogenesis of diseases such as irritable bowel syndrome, depression, appetite disorders, migraine, hypertension, and neurodegenerative diseases, and prevents diabetes [3, 4, 5, 6, 7, 8]. Regulated 5-HT levels lead to better well-being, daily activity, and affective and cognitive functions. Therefore, serotonin reuptake inhibitors are often used in antidepressants for obsessive-compulsive disorder. Unfortunately, although they are effective drugs, they have several side effects [9].  
Appropriate levels of 5-HT reduce obesity by improving fatty acid oxidation and regulating 5-HT signaling pathways. An increased level of the signaling pathway involved in combining 5-HT and MAO-A most often causes excessive liver serotonin degradation. This affects the overproduction of reactive oxygen species in liver mitochondria, which leads to obesity [10; 11; 12; 13]. Other studies indicate that 5-HT has a vasoconstrticrator or vasodilator function, depending on the binding of 5-HT to bradycardia or tachycardia. Serotonin is often administered as a pharmacological agent, as a therapeutic compound in the treatment of heart disease, hypertension, and migraine [14; 15; 16; 17; 18]. Binding serotonin to the 5-HT4 receptor affects the proper contraction of the heart muscle, and too high 5-HT concentration may lead to ischemia and cardiac arrhythmias [19; 20; 21].  
Monoamine oxidoreductase (MAOs) are amine oxidases that break neurotransmitters such as dopamine, 5-HT, epinephrine, and noradrenaline [22; 23]. MAO-A is the dominant form of MAOs that metabolizes 5-HT. Therefore, it plays a key role in the treatment of depression at every stage of the disease, chronic low mood, but also emotional disorders and obsessive-compulsive behaviors [24; 25]. In chronic depressive disorder, increased levels of MAO-A and decreased levels of neurotransmitters are identified in the brain [26]. MAO-A increases appetite and contributes to the accumulation of adipocyte lipids and the differentiation of preadipocytes [27]. Ligands binding to the active site of MAO-A increase serotonin transmission by inhibiting MAO-A. The MAO-A inhibitors used may cause adverse effects such as drowsiness. They may interact with food ingredients and drugs [28]. Therefore, it is important to look for new, non-toxic MAO-A inhibitors.  
In this study, propenylbenzene derivatives such as diols and hydroxy ketones were used as candidates for MAO-A inhibitors. These compounds are used in various industries, e.g., in the food industry as food flavors and in cosmetics as essential oils. However, obtaining these compounds is not environmentally friendly. Therefore, an environmentally friendly chemo-enzymatic synthesis method was used, followed by whole-cell transformation, to obtain the studied compounds [29]. This study aimed to evaluate the degree of inhibition of MAO-A by propenylbenzene derivatives using molecular docking and isothermal titration calorimetry (ITC) methods. The innovation of this work is the first study of the enzymatic activity of MAO-A using a new chemo-enzymatic method of obtaining propenylbenzene derivatives.