diff --git a/src/builders/enzymes.jl b/src/builders/enzymes.jl
index d5a24373..db334509 100644
--- a/src/builders/enzymes.jl
+++ b/src/builders/enzymes.jl
@@ -118,7 +118,8 @@ $(TYPEDSIGNATURES)
 
 Returns a constraint tree with enzyme constrained added to it. Splits reactions into forward and backward
 
-Function inputs:
+Inputs:
+- `gene_ids` is a list of gene product IDs.
 - `isozyme_ids` takes a reaction ID and returns `nothing` if the reaction does
   not have isozymes associated with it, or an iterable container of all the
   isozyme ids.
@@ -136,6 +137,7 @@ Function inputs:
 function enzyme_constraints(
     constraints::C.ConstraintTree;
     fluxes = constraints.fluxes,
+    gene_ids = String[],
     isozyme_ids = _ -> nothing,
     kcat_forward = (_, _) -> 0.0,
     kcat_reverse = (_, _) -> 0.0,
@@ -155,12 +157,15 @@ function enzyme_constraints(
     # allocate variables for everything (nb. += wouldn't associate right here)
     constraints =
         constraints +
-        :fluxes_forward^unsigned_positive_contribution_variables(constraints.fluxes) +
-        :fluxes_reverse^unsigned_negative_contribution_variables(constraints.fluxes) +
+        sign_split_variables(
+            constraints.fluxes;
+            positive = :fluxes_forward,
+            negative = :fluxes_reverse,
+        ) + 
         :isozyme_forward_amounts^isozyme_amounts +
         :isozyme_reverse_amounts^isozyme_amounts +
         :gene_product_amounts^C.variables(
-            keys = Symbol.(A.genes(model)),
+            keys = Symbol.(gene_ids),
             bounds = C.Between(0, Inf),
         )
 
@@ -190,10 +195,11 @@ function enzyme_constraints(
         Symbol(id) => C.Constraint(
             value = sum(
                 constraints.gene_product_amounts[Symbol(gp)].value *
-                gene_product_molar_mass[gp] for gp in gps
+                gene_product_molar_mass(gp) for gp in gps
             ),
             bound = C.Between(0, limit),
         ) for (id, gps, limit) in capacity_limits
     )
-    constraints
 end
+
+export enzyme_constraints
diff --git a/src/frontend/enzymes.jl b/src/frontend/enzymes.jl
index 3c4f1c5d..18ea86b5 100644
--- a/src/frontend/enzymes.jl
+++ b/src/frontend/enzymes.jl
@@ -63,8 +63,9 @@ function enzyme_constrained_flux_balance_constraints(
 
     capacity_limits = capacity isa Real ? [("TotalCapacity", A.genes(model), capacity)] : capacity
 
-    enzyme_constraints(
+    c = enzyme_constraints(
         flux_balance_constraints(model);
+        gene_ids = A.genes(model),
         isozyme_ids,
         kcat_forward,
         kcat_reverse,