<PAGE>   1
                                                                   EXHIBIT 10.45

                              EMPLOYMENT AGREEMENT


This agreement is made and entered into as of the 1st day of December, 1998, by
and between TransTexas Gas Corporation, hereinafter called "Company" and Edwin
Donahue, hereinafter called "Employee".

1.       The term of this Agreement is for two years commencing on December 1,
1998, and ending on November 30, 2000. Thereafter Employee shall be considered
an "at will" Employee subject to termination for any cause at any time. This
agreement may be terminated prior to the end of the term pursuant to paragraph
6, 7 or 8 hereof.

2.       The Company agrees to employ Employee for the term of this Agreement,
and Employee shall have and fulfill such duties, responsibilities and
obligations as are usual and customary for the Vice President and Chief
Financial Officer, subject to the direction and control of the Board of
Directors and the Chief Executive Officer.

3.       Employee agrees Employee will faithfully and diligently serve the
Company to the best of his ability.

4.       During his service hereunder, Employee's salary shall be $300,000 per
year payable in installments in accordance with the Company's regular payroll
practices, and any bonus or other consideration as approved by the Board of
Directors.

         a.       In addition to the amounts included in clause 4 above, and on
                  consideration for successful past and current financing
                  efforts, employee shall be entitled to a bonus of $500,000.
                  One-half, or $250,000 shall be due on or before January 31,
                  1999 and the balance of $250,000 shall be due on or before
                  July 31, 1999.

5.       At such costs and eligibility restrictions applicable to all employees,
Employee shall participate in all plans and benefits generally applicable or
available to employees of the Company, including but not limited to health,
hospital and surgical benefits, disability, life insurance plans, and AD&D
insurance.

         b.       During his service hereunder, Employee shall receive four (4)
                  weeks of vacation per calendar year.

6.       This agreement shall be terminable by the Company for "cause" without
advance notice. In the event of such termination properly for cause, the salary
set forth above shall cease on the effective date of termination, however,
benefits earned to date shall be paid in full. "Cause" shall mean:


<PAGE>   2

         a.       Neglect or mismanagement by Employee of his duties,
         responsibilities and obligations which either significantly damages or
         could have significantly damaged the property or interest of the
         Company; or 

         b.       Material breach of this Agreement by Employee; or

         c.       Inability of Employee to perform his duties, responsibilities
         and obligations by reason of illness, accident or other incapacity for
         a continuous period of six months.

7.       This Agreement shall be terminable by Employee for cause. In the event
that Employee terminates this Agreement properly for cause within thirty days
after he knows or should have known of the existence of such cause, the Company
shall be obligated to pay Employee his salary up to the effective date of such
termination and severance as hereinafter provided. "Cause" shall mean:

         a.       Wrongful behavior or willful neglect by the Company, not
         caused by Employee; or

         b.       Material breach of its obligations under this Agreement by the
         Company; or

         c.       Sale, reorganization or merger of the Company resulting in a 
         change in the actual control of the Company.

         d.       Relocation of Employee for an extended period of time outside
         the greater Houston area.

7a.      This agreement shall be terminable by Employee without cause by giving
Company at least thirty (30) days written notice of such termination. The
Company shall not be obligated to pay Employee his salary past the effective
date of such termination. The Company has no obligation to pay severance under a
termination without cause of Employee.

8.       The Company shall have the right to terminate this Agreement without
cause by giving Employee written notice of such termination and paying him
through the end of his contract as set forth in paragraph 9.

9.       In the event that the Company terminates Employee other than for cause 
before the end of the term of this Agreement, or Employee terminates this
Agreement for cause before the end of the term of this Agreement, the Company
shall pay to Employee his salary for the remaining term of this Agreement plus
an additional six months salary ($150,000.00).

10.      The entire understanding and agreement between the parties has been
incorporated into this Agreement. This Agreement shall inure to the benefit of,
and shall be binding upon the Company, its successors and assigns and upon
Employee and his heirs, successors, and assigns. Employee agrees that this
Agreement may be assigned by the Company to a subsidiary, affiliate, or
successor of the Company; such assignments, however, shall not relieve the
Company of any 



<PAGE>   3

of its obligations hereunder except to the extent that such obligations are
actually paid and discharged by such subsidiary, affiliate, or successor. This
agreement may not otherwise be assigned without the prior written approval of
the other party.

11.      Any disputes concerning or arising out of interpretation or application
of this Agreement shall be determined by final and binding arbitration under and
in accordance with the rules and procedures of the American Arbitration
Association provided that the dispute is submitted to arbitration by written
demand therefor delivered to and received by the other party within sixty days
after occurrence of the event giving rise to the dispute.

12.      If there is a conflict between the Company personnel policies and this
Agreement, this Agreement shall prevail.

13.      For purposes of clause 4(a), TransAmerican Natural Gas Corporation
joins as a party to this contract and agrees to be jointly and severally liable
with respect thereto.

TransTexas Gas Corporation                  Edwin Donahue

By: /s/ John R. Stanley                     By: /s/ Edwin Donahue
    -----------------------------               ------------------------------
        John R. Stanley

Date: December 1, 1998                      Date: December 1, 1998
      ----------------------------                ----------------------------