\documentclass[12pt,border=5pt,tikz,class=scrartcl,multi=false]{standalone}
\usepackage{times}
\usepackage[T1]{fontenc}
\usepackage[latin1]{inputenc}
\usepackage{mathtools,amssymb,siunitx}
\usepackage{pgfplots}
\pgfplotsset{compat=1.13}
\usepackage{circuitikz}
\usepackage{carbohydrates}
\usepackage{gensymb}
\usepackage{chemfig,chemmacros}
\renewcommand*{\familydefault}{\sfdefault}
\renewcommand*\printatom[1]{\ensuremath{\mathsf{#1} } }
\setatomsep{2em}
\setdoublesep{.6ex}
\setarrowdefault{,1,ultra thick}
\usetikzlibrary{positioning,calc,arrows,fit,backgrounds,decorations,shapes,matrix}
\pagestyle{empty}
\thispagestyle{empty}

\begin{document}
\begin{tikzpicture}[
	>=stealth',
	line width=3mm,
	enzyme/.style={text=blue,font=\itshape,minimum width=2cm,align=center},
	steps/.style={minimum width=2cm, align=center,font=\bfseries},
	info/.style={minimum width=2cm, align=center,font=\bfseries,text=brown},
%	every node/.style={text width=7mm,text centered}
%	myarrows/.style={line width=.6mm,draw=red,-triangle 45,postaction={draw=red,line width=1.5mm, shorten >=2mm, -} }
]
	\node[steps,font=\normalfont] (a1) {Acetyle-CoA};
	\node[steps,font=\normalfont] at ($(a1)-(0,2.5)$) (a2) {Acetoacetyl-CoA};
	\node[steps] at ($(a1)+(7,0)$) (a3) {3-hydroxy-3-methylglutaryl-CoA\\(HMG-CoA)};
	\node[steps] at ($(a3)-(0,2.5)$) (a4) {Mevalonic acid};
	\node[steps] at ($(a4)-(0,2.5)$) (a5) {Mevalonate-5-phosphate};
	\node[steps] at ($(a5)-(0,2.5)$) (a6) {Mevalonate-5-pyrophosphate};
	\node[steps] at ($(a6)-(0,2.5)$) (a7) 
		{Isopentenyl-5-pyrophosphate {\normalfont(PP)} };
	\node[steps] at ($(a7)-(0,2.5)$) (a8) {Geranyl PP};
	\node[steps] at ($(a8)-(0,2.5)$) (a9) {Farnesyl-PP};
	\node[steps] at ($(a9)-(0,2.5)$) (a10) {Squalene};
	\node[steps] at ($(a10)-(0,2.5)$) (a11) {2,3-oxidosqualene};
	\node[steps] at ($(a11)-(0,2.5)$) (a12) {Lanosterol};
	\node[steps,text=green!70!black] at ($(a12)-(0,2.5)$) (a13) {CHOLESTEROL};
	\node[steps] at ($(a7)-(7,0)$) (a14) {Dimethylallyl-PP};
	\node[steps] at ($(a9)-(6.75,0)$) (a15) {Geranylgeranyl-PP};
	\node[steps,text=green!70!black] at ($(a15)-(0,5)$) (a16) {PRENYLATED\\PROTEINS};
	\node[steps,text=green!70!black] at ($(a16)+(3,0)$) (a17) {HEME A \\DOLICHOL\\UBIQUINON};
	\draw [->, very thick] (a1) -- node[left,enzyme] {Thiolase} (a2);
	\draw [->, very thick] (a1) -- (a3);
	\draw [->, very thick] (a2) -- node[below,enzyme] {HMG-CoA\\synthase} (a3);
	\draw [->, very thick] (a3) -- node[right,enzyme] {HMG-CoA reductase} 
		node[below right=1mm and 23mm,info] {STATINS} (a4);
	\draw [->, very thick] (a4) -- node[left=7mm] (side1) {ATP} 
		node[right,enzyme] {Mevalonate kinase} (a5);
	\draw [->, very thick] (side1.east) to [out=0,in=90] (a5.north);
	\draw [->, very thick] (a5) -- node[right,enzyme] {Phosphomevalonate kinase} (a6);
	\draw [->, very thick] (a6) -- node[left=7mm](side2){--\ch{CO2} } 
		node[right,enzyme] {Mevalonate-5-pyrophosphate\\decarboxylase} (a7);
	\draw [->, very thick] (a6.south) to [out=-90,in=0] (side2.east);
	\draw [->, very thick] (a7) -- node[right,enzyme] {Farnesyl-PP synthase} 
		node[below right=1mm and 10mm,info] {BISPHOSPHONATES}
		(a8);
	\draw [->, very thick] (a8) -- node[right,enzyme] {Farnesyl-PP synthase} 
		node[below right=1mm and 10mm,info] {BISPHOSPHONATES}
		(a9);
	\draw [->, very thick] (a9) -- node[right,enzyme] {Squalene synthase} (a10);
	\draw [->, very thick] (a10) -- node[right,enzyme] {Squalene\\monooxygenase} (a11);
	\draw [->, very thick] (a11) -- node[left=7mm](side3){\ch{NADPH} } 
		node[right,enzyme] {Squalene\\epoxydase} (a12);
	\draw [->, very thick] (side3.east) to [out=0,in=90] (a12.north);
	\draw [->, very thick, dotted] (a12) -- node[right,enzyme] {19 reactions} (a13);
	\draw [->, very thick] (a7) -- node[above,enzyme] {Isopentenyl-PP\\isomerase} (a14);
	\draw [->, very thick] (a14) -- (a8);
	\draw [->, very thick] (a14) -- (a9);
	\draw [->, very thick] (a7) -- (a15);
	\draw [->, very thick] (a9) -- node[enzyme] {Geranylgeranyl-PP\\synthase} (a15);
	\draw [->, very thick] (a15) -- (a16);
	\draw [->, very thick] (a9) -- (a16);
	\draw [->, very thick] (a9) -- (a17);
\end{tikzpicture}
\end{document}