diff --git a/src/frontend/enzymes.jl b/src/frontend/enzymes.jl
index 9749dc48..6fdcbcd8 100644
--- a/src/frontend/enzymes.jl
+++ b/src/frontend/enzymes.jl
@@ -104,11 +104,13 @@ function enzyme_constrained_flux_balance_constraints(
     # allocate all variables and build the system
     constraints = flux_balance_constraints(model; interface, interface_name)
 
-    constraints += sign_split_variables(
-        constraints.fluxes,
-        positive = :fluxes_forward,
-        negative = :fluxes_reverse,
-    )
+    constraints +=
+        :fluxes_forward^unsigned_positive_contribution_variables(constraints.fluxes)
+    constraints *=
+        :fluxes_reverse^unsigned_negative_contribution_constraints(
+            constraints.fluxes,
+            constraints.fluxes_forward,
+        )
 
     constraints += enzyme_variables(;
         fluxes_forward = constraints.fluxes_forward,
@@ -117,27 +119,21 @@ function enzyme_constrained_flux_balance_constraints(
         isozyme_reverse_ids,
     )
 
-    return constraints *
-           :directional_flux_balance^sign_split_constraints(;
-               positive = constraints.fluxes_forward,
-               negative = constraints.fluxes_reverse,
-               signed = constraints.fluxes,
-           ) *
-           enzyme_constraints(;
-               fluxes_forward = constraints.fluxes_forward,
-               fluxes_reverse = constraints.fluxes_reverse,
-               isozyme_forward_amounts = constraints.isozyme_forward_amounts,
-               isozyme_reverse_amounts = constraints.isozyme_reverse_amounts,
-               kcat_forward,
-               kcat_reverse,
-               isozyme_gene_product_stoichiometry,
-               gene_product_molar_mass,
-               capacity_limits = capacity isa Real ?
-                                 [(:total_capacity, gene_ids, C.Between(0, capacity))] :
-                                 [
-                   (Symbol(k), Symbol.(gs), C.Between(0, cap)) for (k, gs, cap) in capacity
-               ],
-           )
+    return constraints * enzyme_constraints(;
+        fluxes_forward = constraints.fluxes_forward,
+        fluxes_reverse = constraints.fluxes_reverse,
+        isozyme_forward_amounts = constraints.isozyme_forward_amounts,
+        isozyme_reverse_amounts = constraints.isozyme_reverse_amounts,
+        kcat_forward,
+        kcat_reverse,
+        isozyme_gene_product_stoichiometry,
+        gene_product_molar_mass,
+        capacity_limits = capacity isa Real ?
+                          [(:total_capacity, gene_ids, C.Between(0, capacity))] :
+                          [
+            (Symbol(k), Symbol.(gs), C.Between(0, cap)) for (k, gs, cap) in capacity
+        ],
+    )
 end
 
 export enzyme_constrained_flux_balance_constraints