\documentclass[12pt,border=1pt,tikz,class=scrartcl,multi=false]{standalone}
\usepackage{times}
\usepackage[T1]{fontenc}
\usepackage[latin1]{inputenc}
\usepackage{mathtools,amssymb}
\usepackage{pgfplots}
\pgfplotsset{compat=1.12}
\usetikzlibrary{positioning,calc,arrows,fit,backgrounds,decorations.text,shapes}
 
\usepackage{chemfig,chemmacros}
\renewcommand*{\familydefault}{\sfdefault}
\renewcommand*\printatom[1]{\ensuremath{\mathsf{#1} } }
\setatomsep{1.7em}
\setdoublesep{.6ex}
\setarrowdefault{,1,ultra thick}
\setbondstyle{very thick,cap=but}
\definesubmol\nobond{-[,0.2,,,draw=none]}
 
\pagestyle{empty}
\thispagestyle{empty}
\begin{document}
\renewcommand{\baselinestretch}{.5}
\begin{tikzpicture} [>=latex,inner sep=2pt]
	\node (l1) {\chemfig{
		O=[::30](-[::-60]OH)-[::60](-[::-60]NH_2)
		<:[::60]-[::60]
		*5(-*6(-=-=-=)--[,,,2]HN-=)
	} };
	\node at ($(l1)+(8.5,0)$) (r1) {\chemfig{
		O=[::30](-[::-60]OH)-[::60](-[::-60]NH_2)
		<:[::60]-[::60](=[::60,,,,red]\color{red}{O})
		-[::-60]*6(=-=-=(-NH-[::-60](-[::-60,,,,red]\color{red}{H})=[::60,,,,red]\color{red}{O})-)
	} };
	\node at ($(l1)-(0,5)$) (l2) {\chemfig{
		O=[::30](-[::-60]OH)-[::60](-[::-60]NH_2)
		<:[::60]-[::60]
		*5(-*6(-=(-[,,,,red]\color{red}{H}&#x7c;\color{red}{O})-=-=)--[,,,2]HN-=)
	} };
	\node at ($(l2)-(0,5)$) (l3) {\chemfig{
		NH_2-[:-150,,,,red]-[::-60,,,,red]-[::60]
		*5(-*6(-=(-HO)-=-=)--[,,,2]HN-=)
	} };
	\node at ($(l3)-(0,5)$) (l4) {\chemfig{
		\color{red}{CH_3}-[:-150,,,,red](=[::60,,,,red]\color{red}{O})-[::-60,,,,red]\chemabove{N}{H}
		-[::60]-[::-60]-[::60]
		*5(-*6(-=(-HO)-=-=)--[,,,2]HN-=)
	} };
	\node at ($(l4)-(0,5)$) (l5) {\chemfig{
		CH_3-[:-150](=[::60]O)-[::-60]\chemabove{N}{H}
		-[::60]-[::-60]-[::60]
		*5(-*6(-=(-O-[::60,1.5,,,red]\color{red}{H_3C})-=-=)--[,,,2]HN-=)
	} };
	\node at ($(l2)+(8.5,0)$) (r2) {\chemfig{
		O=[::30](-[::-60]OH)-[::60](-[::-60]NH_2)
		<:[::60]-[::60](=[::60]O)
		-[::-60]*6(=-=-=(-\color{red}{N}&#x7c;\color{red}{H_2})-)
	} };
	\node at ($(l3)+(8.5,0)$) (r3) {\chemfig{
		O=[2](-[::-60]OH)-[::60] 
		*6(-=-\color{red}{C}(-[4,.6,,,draw=none]\color{red}{OH})
		-[,,,,draw=none]\color{red}{C}(-[4,.7,,,draw=none]\color{red}{HO_2})-(-NH_2)=)
	} };
	\node at ($(l4)+(8.5,0)$) (r4) {\chemfig{
		*6(-[,,,,red]\color{red}{N}
		=(-(-[::60]OH)=[::-60]O)
		-(-(-[::-60]OH)=[::60]O)
		=-=)
	} };
	\node at ($(l5)+(8.5,0)$) (r5) {\chemfig{
		*6(-N
		=[,,,,red]-[,,,,red](-(-[::-60]OH)=[::60]O)
		=-=)
	} };
	\node [below=.1em of l5] {\textbf{melatonin} };
	\node [below=.1em of r5] {\textbf{niacin} };
	\draw [->,very thick] (l1.east) -- (r1.west)
		node [pos=.5,above] {\footnotesize Indoleamine}
		node [pos=.5,below] {\footnotesize 2,3--dioxygenase};
	\draw [->,very thick] ($(l1.south)-(0,.2)$) node [left] {\textbf{L-tryptophan} } -- ($(l2.north)-(0,.5)$)
		node [pos=.5,right,text width=5em,text centered] {\footnotesize tryptophan hydroxylase};
	\draw [->,very thick] ($(l2.south)-(0,.2)$) -- ($(l3.north)-(0,.7)$)
		node [pos=0,left,text width=5em] {\footnotesize5--hydroxy--tryptophan}
		node [pos=.5,right,text centered,text width=6em] {\footnotesize aromatic amino acid carboxylase};
	\draw [->,very thick] ($(l3.south)-(0,.2)$) -- ($(l4.north)-(0,.7)$)
		node [pos=0.1,left=1,text width=5em] {\footnotesize5--hydroxy--tryptamine\\(5-HT)}
		node [pos=0,above right=1,text width=5em] {\textbf{serotonin} }
		node [pos=.5,right,text centered,text width=6em] {\footnotesize N--acetyl transferase};
	\draw [->,very thick] ($(l4.south)-(0,.2)$) -- ($(l5.north)-(0,.7)$)
		node [pos=0.1,left=1,text width=4em] {\footnotesize N--acetyl 5--HT}
		node [pos=.5,right,text centered,text width=8em] {\footnotesize5--hydroxyindole--\\O--methyltransferase};
	\draw [->,very thick] ($(r1.south)-(0,.2)$) -- ($(r2.north)-(0,.5)$)
		node [pos=.5,right,text centered,text width=6em] {\footnotesize formamidase};
	\node [right=0pt of r1,text width=5em] {\footnotesize N'--formyl--\\kynurenine};
	\draw [->,ultra thick] ($(r2.south)-(0,.2)$) -- (r3.north);
	\node [right=0pt of r2,text width=5em] {\footnotesize kynurenine};
	\draw [->,very thick] ($(r3.south)-(0,.2)$) -- ($(r4.north)-(0,.5)$)
		node [pos=.5,right,text centered,text width=6em] {\footnotesize non-enzymatic\\cyclization};
	\node [right=0pt of r3,text width=6em] {\footnotesize2--amino--3--(3--oxoprop--1--enyl)--fumaric acid};
	\draw [->,ultra thick] ($(r4.south)-(0,.2)$) -- (r5.north);
	\node [right=0pt of r4,text width=5em] {\footnotesize quinolinate};
\end{tikzpicture}
\end{document}