Turning player toward enemy after bool is true/button is pressed

So, I need my player to still be movable but turn towards an enemy while holding down a button - kinda like a lock-on. But this all has to be in a 2D environment (It's a top down brawler). Here's the code part where i need to declare my "heading":

 float heading = Mathf.Atan2(movementVector.x*(-1), movementVector.y*(-1));
 
         if (playerLockOn == true)
         {
             transform.rotation = ///NEED IDEAS HERE///;
         }else{
 
         transform.rotation = Quaternion.Euler (0f,0f,heading*Mathf.Rad2Deg);
         }
 

Any ideas on how to go on about this? I'll post my full code down below:

 using UnityEngine;
 using System.Collections;
 
 public class Player_Movement : MonoBehaviour 
 {
 
     private Vector2 movementVector;
 
     private float movementSpeed = 5;
     // Use this for initialization
 
     private float heading;
     private Quaternion rotationToHold;
 
     private bool movingX;
     private bool movingY;
 
     public bool wallTrigger;
     public bool playerLockOn;
 
     public Animation anim;
 
     public Transform Enemy;
 
     void Start () 
     {
         anim = GetComponentInChildren<Animation> ();
         Enemy = GameObject.Find ("Test Enemy").transform;
     }
 
 
 
     // Update is called once per frame
     void Update () 
     {
         //Checks, if player attacks and does animation
         if (Input.GetKeyDown ("joystick button 2")) {
             anim.Play ();
         }
 
         //Player Locking on to enemy
         if (Input.GetKey ("joystick button 0")) {
             playerLockOn = true;
         } else {
             playerLockOn = false;
         }
             
 
         movementVector.x = Input.GetAxis ("LeftJoystick_X") * movementSpeed;
         movementVector.y = Input.GetAxis ("LeftJoystick_Y") * movementSpeed;
         ///Moving the Player
         GetComponent<Rigidbody2D>().velocity = new Vector2 (movementVector.x, movementVector.y*(-1));
 
         ///turning the player and checking if defending for lockOn
 
         float heading = Mathf.Atan2(movementVector.x*(-1), movementVector.y*(-1));
 
         if (playerLockOn == true)
         {
                           transform.rotation = //???//
         }else{
 
         transform.rotation = Quaternion.Euler (0f,0f,heading*Mathf.Rad2Deg);
         }
 
         //movement checks in order to keep rotation after letting go of stick
         if (movementVector.x == 0)
         {
             movingX = false;
         }
         else{
             movingX = true;
         }
 
         if (movementVector.y == 0) {
             movingY = false;
         } else {
             movingY = true;
         }
 
 
         if (movingX || movingY == true) {
             rotationToHold = transform.rotation;
         } else {
             transform.rotation = rotationToHold;
         }
 
         ///check if Player is inside the arena and restric his movement if not
         if (wallTrigger == true) {
             movementSpeed = 0;
         } else {
             movementSpeed = 5;
         }
     }
 
     void OnTriggerEnter2D (Collider2D other)
     {
         wallTrigger = false;
     }
 
     void OnTriggerExit2D (Collider2D other)
     {
         wallTrigger = true;
 
         var magnitude = 2500;
 
         var force = transform.position - other.transform.position;
 
         force.Normalize ();
         GetComponent<Rigidbody2D> ().AddForce (-force * magnitude);
 
 
     }
 }

Thank you guys and gals for any help!