Adapted from my presentation at the VIVE Genesis Performance conference in Dallas, Texas (September 21st 2019)

Catechol-O-methyltransferase (COMT) degrades catecholamines: dopamine and its metabolites, epinephrine and norepinephrine. A functional polymorphism in the COMT gene (val158met) results in a 400% increase above baseline variation in enzyme activity.  
Interestingly, the COMT allele status has also been shown to functionally alter DA activity in the PFC wherein COMT Met (low-activity; high dopamine, "worrier") allele carriers outperform Val (high-activity; low dopamine, "warrior") allele carriers on a variety of cognitive tasks.  
However, when it comes to stress stress handling, ‘Worriers’ displayed higher reactivity to averse stimuli and sympathetic nervous system activity and were “less able to process emotional stimuli and are more likely to have an elevated risk for emotion-related psychopathology (Tartar et al. 2019).
A little bird told me that in the very near future there will be a study which elucidates how this gene polymorphism affects MMA fighters- stay tuned for more cool stuff coming out of Nova Southeastern University Florida
Learn more about Jaime Tartar PhD, Professor and Research Director in the Department of Psychology and Neuroscience at NSU's College of Psychology here. Dr Tartar is also the President of NeuroSports, a non-profit academic society that aims to connect researchers in the fields of neuroscience and exercise science. 
Hill, LD, Lorenzetti, MS, Lyle, SM, Fins, AI, Tartar, A, Tartar, JL (2018) Catechol‐O‐methyltransferase Val158Met polymorphism associates with affect and cortisol levels in women. Brain Behav 8:e00883.
Goldman D, Weinberger DR, Malhotra AK, Goldberg TE. The role of COMT Val158Met in cognition. Biol Psychiatry. 2009;65(1):e1–e4.
Jose Martinez Serrano, Jonathan B. Banks, Thomas J. Fagan & Jaime L. Tartar (2019) The influence of Val158Met COMT on physiological stress responsivity, Stress, 22:2, 276-279, DOI: 
Michael N. Smolka, Gunter Schumann, Jana Wrase, Sabine M. Grüsser, Herta Flor, Karl Mann, Dieter . Braus, David Goldman, Christian Büchel, Andreas Heinz Catechol-O-Methyltransferase val158met Genotype Affects Processing of Emotional Stimuli in the Amygdala and Prefrontal Cortex
Journal of Neuroscience 26 January 2005, 25 (4) 836-842;