<PAGE>

                                                                     EXHIBIT 4.1

             FORM OF DEBENTURE AGREEMENT AND CONVERTIBLE DEBENTURE


                              DEBENTURE AGREEMENT
                              -------------------


     THIS DEBENTURE AGREEMENT (the "Debenture Agreement"), is made and entered
into effective as of September 30, 2000, by and between ENB Bankshares, Inc.
(the "Company") and ___________________________________ ("Debenture Holder") and
provides as follows:

                                   RECITALS:

     The Company is authorized by law, and deems it necessary, from time to
time, to borrow money for its proper corporate purposes, and to that end, in
exercise of said authority, has duly authorized and directed the issue of its
Debentures under specified terms, and has duly authorized and directed the
execution of this Debenture Agreement for the benefit of the holders of said
Debentures.

     NOW THEREFORE, in consideration of the premises, and of the covenants
herein contained, and of the acquisition and acceptance of the accompanying
Debenture by Debenture Holder, the Company covenants and agrees with Debenture
Holder as follows:

                                   ARTICLE I

                                   ISSUANCE
                                   --------

     1.01  Issuance.  The Company hereby issues to the Debenture Holder its
           --------
Debenture in the principal amount of $_______________ (hereinafter referred to
as the "Debenture"), and the Debenture Holder hereby tenders $______________
cash to the Company in exchange for the Debenture.  The Debenture shall be
substantially in the form attached hereto as Exhibit "A".
                                             -----------

     1.02  Principal Payments.  Payments of principal in an amount equal to
           ------------------
twenty percent (20%) of the original principal amount of the Debenture, which
pursuant to the terms and conditions contained herein below, shall be due and
payable on December 31, 2006, 2007, 2008, 2009, and the remaining principal
amount and accrued unpaid interest shall be due and payable on December 31, 2010
(the "Maturity Date").

     1.03  Interest Payments.  Interest on the Debenture will accrue from
           -----------------
September 30, 2000, at the rate equal to nine and one half percent (9.5%) per
year.  Accrued interest shall be paid quarterly beginning on March 15, 2001 and
continuing on each June 15, September 15, December 15 and March 15 until the
entire principal amount is paid in full.  Principal and interest on each
Debenture will be paid to the person in whose name the Debenture is registered
at the close of business on the first day of the calendar month during which
such payment is to be made and will be paid by check mailed to the registered
address of the holder of the Debenture.

     1.04  Prepayment.  This Debenture may be prepaid, in whole or in part,
           ----------
without penalty at the option of the Company at any time after December 31,
2002.  All prepayments are to be applied first to satisfy unpaid interest and
then against principal.  To prepay, the Company must first notify the Debenture
Holder of its intention to prepay and of the prepayment date which must be at
least thirty (30) days from the notice of prepayment.

     1.05  Additional Debenture Holders. The Debenture Holder acknowledges and
           ----------------------------
agrees that additional Debentures may be issued to other parties
contemporaneously with the issuance of the Debenture and in the future to other
parties. The Debenture Holder specifically agrees that such other Debentures may
be issued by the Company without the signature, consent, or any other action of
the Debenture Holder, and that the terms and provisions of this Debenture
Agreement shall continue to apply to the Debenture and such other Debentures.
<PAGE>

                                  ARTICLE II

                                   TRANSFERS
                                   ---------

     2.01  Requirements of Transfer. No transfer of the Debenture shall be valid
           ------------------------
unless and until the transferor executes a separate power of attorney indicating
his intent to transfer ownership, delivers such Debenture to the Company along
with transfer instructions, and such change of ownership is recorded in the
Company's debenture records.

     2.02  Registration of Transfer.  Transfer of the Debenture shall be
           ------------------------
registered upon the Company's debenture records following the Company's receipt
of such Debenture, (properly endorsed or accompanied by a notarized separate
power of attorney indicating intent to transfer ownership), and receipt of
written transfer instructions signed by the transferor and the transferee which
shall state the name, mailing and residence address of the transferee, and the
desired effective date of transfer.  The above may be either personally
delivered by the transferor or transferee, or mailed to the Company in
accordance with Section 10.02 hereof.

     2.03  Effective Date of Transfer.  The effective date of the transfer
           --------------------------
recorded on the Company's debenture records shall be the date requested in the
instructions of transfer; however, the effective date shall not precede the date
of the most recent payment date of principal or interest with respect to such
Debenture.  In the event such date precedes the date of the most recent payment
of principal or interest with respect to such Debenture or if the desired date
is omitted from the instructions of transfer, the Company may in its discretion
honor the transfer, and, in such case, the effective date of transfer shall be
the first date at which the Company is in receipt of all of the items required
by Section 2.02 hereof.

     2.04  Issuance of New Certificates.  After a transfer has been recorded in
           ----------------------------
the Company's debenture records, the Company shall cancel the old certificate
representing the Debenture being transferred and shall issue a new certificate
in the transferee's name.  The terms of such new certificate shall be identical
to those of the former Debenture certificate except as to Debenture Holder's
name.  At or prior to delivery of the new Debenture certificate to the
transferee, the Company and transferee shall enter into a Debenture Agreement,
which agreement shall be signed by the Company and transferee, and its
provisions shall be identical to those of this Debenture Agreement except for
the Debenture Holder's name and the date of execution, which date will be the
same as the effective transfer date.  Such new Debenture certificate and
Debenture Agreement will be mailed or delivered by other reasonable means to
transferee.  In the event transferee's signature is not obtained on the new
Debenture Agreement, transferee's signature on the instructions of transfer will
be deemed to constitute transferee's assent to the terms of the Debenture and of
the Debenture Agreement.

                                  ARTICLE III

                        REPRESENTATIONS AND WARRANTIES
                        ------------------------------

     3.01  Representation and Warranties of the Company.  The Company represents
           --------------------------------------------
and warrants to Debenture Holder that it is a duly organized and validly
existing corporation in good standing under the laws of the State of Texas.  The
Company has the corporate power, and has been duly authorized by its Board of
Directors, to execute and deliver this instrument and the Debenture, and in so
doing, will not violate any law, its Articles of Incorporation or Bylaws, or any
other agreement or instrument binding upon the Company.

     3.02  Representatives and Warranties of the Debenture Holder.  Debenture
           -------------------------------------------------------
Holder represents and warrants to the Company the following:

     a.     Debenture Holder is aware of the Risk Factors set forth in the Proxy
Statement and Prospectus dated ______________, 2000 understanding that an
investment in the Debenture involves a high degree of risk and that there are no
assurances that the Debenture Holder will recover his investment or receive any
return on his investment at any time.

     b.     Debenture Holder is aware that there will be no public market for
the Debenture, and accordingly, the Debenture Holder will need to bear the
economic risk of his investment for a indefinite period of time and will not be
readily able to liquidate this investment in case of an emergency.
<PAGE>

     c.     Debenture Holder is aware that neither the Securities and Exchange
Commission nor any commission of corporations or other agency charged with
enforcement of state securities laws has made any finding or determination
relating to the fairness of an investment in the Debenture and that no such
entity has recommended or endorsed the Debenture.

     d.     Debenture Holder has elected to receive shares of Company common
stock in the reorganization described in the Proxy Statement and Prospectus
dated _____________, 2000.

                                  ARTICLE IV

                           COVENANTS OF THE COMPANY
                           ------------------------

     4.01  Payment.  The Company shall pay the interest on and principal of the
           -------
Debenture at the dates and places and in the manner and upon the terms specified
in the Debenture and this Debenture Agreement.

     4.02  Office of Company.  At all times, until the full and complete payment
           -----------------
of the principal and interest of the Debenture, the Company shall keep an office
or agency in the City of Dallas, State of Texas, or at such other place as the
Company may notify Debenture Holder in accordance with Section 10.02 hereof, and
all notices, requests and demands in respect of the Debenture may be served at
such office.

                                   ARTICLE V

                                  CONVERSION
                                  ----------

     5.01  Exchange Ratio.  Unpaid principal and accrued interest owed under the
           --------------
Debenture will be convertible into the Company's common stock at the option of
the Debenture Holder at any time prior to December 31, 2006. After December 31,
2006, the Debenture shall not be convertible into the Company's common stock
except upon the occurrence of an event identified in Sections 5.03(a) and
5.03(b) of this Debenture Agreement. At conversion, the Debenture Holder shall
be entitled to receive a number of shares of common stock of the Company equal
to the quotient of (a) amount of the outstanding principal and accrued interest
being converted, divided by (b) $15.00, rounded down to the next whole share,
all as subject to adjustment as provided in Section 5.03(d).

     5.02  Manner of Exercise.  To exercise the conversion, the Debenture Holder
           ------------------
must surrender to the Company the Debenture along with the attached Form of
Conversion duly executed by him.

     5.03  Other Provisions Regarding Convertibility.  The following provisions
           -----------------------------------------
shall also apply to the convertibility of the Debenture:

     (a)    If the Debenture is called for prepayment, it may be converted into
            shares of common stock of the Company at any time prior to the close
            of business on the fifth (5th) day preceding the date fixed for
            prepayment.

     (b)    In the event of any merger, consolidation, recapitalization, split
            up, spin off, liquidation, sale of substantially all of the assets
            of the Company, or any other business combination or acquisition to
            which the Company may be a party, the Company shall give written
            notice of such event to the Debenture Holder. Upon receipt of such
            notice, the Debenture may be converted into shares of common stock
            of the Company at any time within thirty days of such notice. If
            this Debenture is not so converted, the Debenture Holder shall have
            no further conversion rights into the Company's common stock.

     (c)    To exercise the conversion privilege, the Debenture Holder must
            surrender the Debenture to the Company and must also execute and
            deliver to the Company the Form for Exercising Election to Convert
            at the end of the Debenture.

     (d)    In the event of a stock dividend, stock split, recapitalization, or
            any other change affecting the number of issued and outstanding
            shares of common stock of the Company, an adjustment shall be made
            in the total number and
<PAGE>

            class of shares and the conversion price so as to reflect such
            change. Any such computation shall be rounded down to the nearest
            whole share and no such modification shall require the issuance of
            fractional shares.

     (e)    The shares of Common Stock into which the Debenture may be converted
            shall only be issued to the Debenture Holder, and the Debenture
            Holder shall not be entitled to any rights or privileges as a
            stockholder of the Company until such time as he shall have
            irrevocably exercised his rights, as set forth above, to convert the
            Debenture into shares of Common Stock.

     (f)    If the Debenture Holder shall exercise his option to convert the
            Debenture into shares of common stock, the Company shall promptly
            issue to the Debenture Holder certificates evidencing the shares of
            common stock into which the Debenture is to be converted. On the
            date the Debenture is surrendered to the Company for conversion, the
            right of the Debenture Holder as a holder thereof shall cease and
            thereafter the Debenture Holder will not be entitled to any interest
            on this Debenture not theretofore due and payable and the Debenture
            Holder shall not be entitled to any principal payments with respect
            to the Debenture.

     (g)    The Company covenants that it will at all times reserve and keep
            available out of its authorized common stock, solely for the purpose
            of issue upon conversion of the Debenture, the maximum number of
            shares of Common Stock remaining issuable upon conversion of the
            Debenture.

     (h)    The Company further covenants that, in the event the Debenture
            Holder converts the Debenture into shares of common stock, all
            shares of common stock which shall be issued to the Debenture Holder
            shall be, when issued, duly and validly issued and fully paid and
            non-assessable.

                                  ARTICLE VI

                     CONSOLIDATION, MERGER AND CONVEYANCE
                     ------------------------------------

     6.01  Continuation of Terms of Debenture Agreement.  Nothing contained in
           --------------------------------------------
this Debenture Agreement or in the accompanying Debenture shall prevent any
consolidation or merger of the Company with or into any other corporation, or
any conveyance of the business, assets and properties of the Company as a whole
or substantially as a whole, to any other corporation or other entity, provided,
that all terms and conditions of this Debenture Agreement, including payment, to
be observed and performed by the Company shall be expressly assumed by the
successor entity formed by or resulting from any such merger or consolidation,
or to which any such conveyance shall have been made.

     6.02  Rights of Successor Company.  In the event the Company or any
           ---------------------------
successor entity shall be consolidated or merged with or into, or shall make a
conveyance to, any other corporation or other entity, the entity formed by or
resulting from such consolidation or merger or to which such conveyance shall
have been made, shall succeed to and be substituted for the Company, with the
same force and effect as if it had been named in, and had executed, this
Debenture Agreement, and shall have and possess and may exercise, subject to the
terms and conditions of this Debenture Agreement, each and every power,
authority and right herein reserved to or conferred upon the Company.

     6.03  Construction.  For every purpose of this Debenture Agreement,
           ------------
including the execution, issue and use of Debentures, the term the "Company"
includes and means (unless the context otherwise requires) not only the
corporation which has executed this Debenture Agreement, but also any such
successor entity in accordance with the provisions of this Article VI.

                                  ARTICLE VII

                                    DEFAULT
                                    -------

     7.01  Event of Default.  As used in this Debenture Agreement and the
           ----------------
Debenture, the term "event of default" shall mean any one of the following:
<PAGE>

     (a)    default by the Company in the payment of the principal of the
            Debenture as and when the same shall become due and payable and the
            continuance of such default for 180 days;

     (b)    default by the Company in the payment of any installment of interest
            on the Debenture as and when the same shall become due and payable
            and the continuance of such default for 180 days; or

     (c)    institution of voluntary or involuntary bankruptcy proceedings by or
            against the Company.

     7.02  Acceleration.  After the occurrence of an event of default, Debenture
           ------------
Holder, upon ten business days prior written notice to the Company, shall have
the right to accelerate the maturity of the unpaid principal and interest of the
Debenture and make demand on the Company for payment in full.

                                 ARTICLE VIII

                                  DEFEASANCE
                                  ----------

     If, when the accompanying Debenture shall have become due and payable
(whether by elapse of time or by declaration by the exercise of the right to
accelerate as provided in Article VII hereof), the Company shall pay, or cause
to be paid, the whole amount of the principal monies and interest due upon the
Debenture, then and in that case, this Debenture Agreement shall terminate and
be of no force and effect, and the Debenture shall be marked "paid" and canceled
in the debenture records of the Company.

                                  ARTICLE IX

                     STOCKHOLDERS, OFFICERS AND DIRECTORS
                     ------------------------------------

     No recourse shall be had for the payment of the principal of the Debenture
or of the interest thereon, or for any claim based thereon or otherwise in
respect thereof, or arising from this Debenture Agreement, against any
incorporator, or against any past, present or future stockholder, director,
officer, employee or agent of the Company, as such, whether by virtue of any
constitution, statute, or rule of law, or by the enforcement of any assessment
or penalty; all such liability being by the acceptance of the accompanying
Debenture and as part of the consideration of the issuance thereof expressly
waived and released by Debenture Holder and by any subsequent owners of the
Debenture.

                                   ARTICLE X

                                 SUBORDINATION
                                 -------------

     10.01 Subordination.  The payment of principal and interest on this
           -------------
Debenture is subordinated in right of payment to the prior payment in full of
all senior indebtedness of the Company, whether outstanding on this date or
thereafter. Senior indebtedness is defined as the principal of, and premium and
interest on, indebtedness of the Company for money borrowed from persons, firms,
or corporations that regularly engage in the business of lending money. In the
event of any insolvency, bankruptcy, receivership, liquidation, or any other
marshalling of the assets and liabilities of the Company, the holders of senior
indebtedness will be entitled to receive payment in full of all principal and
interest on all senior indebtedness before the holder of this Debenture is
entitled to receive any payment on account of principal or interest.

                                  ARTICLE XI

                                 MISCELLANEOUS
                                 -------------

     11.01 Binding Debenture Agreement.  This Debenture Agreement shall be
           ---------------------------
binding upon and inure to the benefit of the parties hereto and their respective
heirs, executors, administrators, legal representatives, successors and assigns,
except as otherwise expressly provided herein.

     11.02 Notices.  Unless otherwise provided herein, any notice, tender, or
           -------
delivery to be given hereunder by either party to the other may be effected by
(a) personal delivery in writing, or (b) by registered or certified mail,
postage prepaid, return receipt requested, and such notice, tender or delivery
shall be deemed made or received as of the earlier of actual delivery or two
business days after the date of mailing.  Notices may be given to the parties
hereto at the following addresses:

     To Debenture Holder:

     _________________________
     _________________________
     _________________________
<PAGE>

     To Company:

     ENB Bankshares, Inc.
     5006 Verde Valley
     Dallas, Texas 75240

or addressed to either party at such other address as such party shall hereafter
furnish to the other party in writing.  The address for any purpose hereof of
the Company or Debenture Holder may be changed at any time and shall be the most
recent address furnished in writing to the other party.

     11.03 Governing Law.  This Debenture Agreement shall be construed in
           -------------
accordance with and governed by the laws of the State of Texas, and all
obligations of the parties created hereunder are performable in Dallas, Dallas
County, Texas.

     11.04 Severability.  In case any one or more of the provisions contained in
           ------------
this Debenture Agreement shall for any reason be held to be invalid, illegal or
unenforceable in any respect, such invalidity, illegality or unenforceability
shall not affect any other provisions hereof and this Debenture Agreement shall
be construed as if such invalid, illegal or unenforceable provisions had never
been contained herein.

     11.05 Entire Debenture Agreement.  This Debenture Agreement and the
           --------------------------
accompanying Debenture constitute the sole and only agreements of the parties
hereto and supersede any prior understandings or written or oral agreements
between the parties respecting the within subject matter.

     11.06 Headings.  The captions used in conjunction with this Debenture
           --------
Agreement are for convenience only, and shall not be deemed a part of this
Debenture Agreement or used to construe any provision hereof.

     11.07 Inclusive of Debenture.  Reference is made to the accompanying
           ----------------------
Debenture dated of even date herewith.  The provisions of such Debenture shall
be deemed incorporated into this Debenture Agreement, and for all purposes shall
have the same effect as though fully set forth on the face hereof.

     11.08 Usury Savings Clause.   All agreements between the Company and the
           --------------------
holder hereof, whether now existing or hereafter arising, and whether written or
oral, are hereby expressly limited so that in no contingency or event
whatsoever, whether by reason of acceleration of the maturity hereof or
otherwise, shall the amount paid, or agreed to be paid, to the holder hereof for
the use, forbearance, or detention of the money loaned hereunder or otherwise or
for the payment or performance of any covenant or obligations contained herein
or in any document evidencing, securing or pertaining to the indebtedness
evidenced hereby, exceed the maximum amount permissible under applicable law.
If, from any circumstances whatsoever, fulfillment of any provision hereof or
other document at the time performance of such provision shall be due, shall
involve transcending the limit of validity prescribed by law, then, ipso facto,
the obligation to be fulfilled shall be reduced to the limit of such validity,
and if from any such circumstance the holder hereof should ever receive any
amount deemed interest by applicable law which shall exceed the highest lawful
rate, such amount which would be excessive interest shall be characterized as
expense, to the extent permitted by this Debenture and the agreements governing,
securing or relating to the indebtedness evidenced by this Debenture, rather
than interest, or be applied to the reduction of the principal amount owing
hereunder or on account of any other principal indebtedness of the Company to
the holder hereof, and not to the payment of interest, or if such excessive
payment cannot be characterized as an expense, and exceeds the unpaid balance of
principal hereof and such other indebtedness, the excess shall be refunded to
the Company.  All sums paid or agreed to be paid by the Company for the use,
forbearance or detention of the indebtedness of the Company to the holder hereof
shall, to the extent permitted by applicable law, be amortized, prorated,
allocated and spread throughout the full term of such indebtedness until payment
in full.  The terms and provisions of this paragraph shall control and supersede
every other provision of all agreements between the Company and holder hereof.
<PAGE>

     IN WITNESS WHEREOF, the parties hereto have caused this Debenture Agreement
to be duly executed as of the day and year first above written.


                                    COMPANY:

                                    ENB BANKSHARES, INC.


                                    By:    __________________________
                                    Title: __________________________

                                    DEBENTURE HOLDER:


                                    _________________________________

<PAGE>

                             ENB BANKSHARES, INC.

                             CONVERTIBLE DEBENTURE
                             ---------------------

                         Certificate Number __________



$_____________
                                                               ___________, 2000


     ENB Bankshares, Inc. (the "Company"), a Texas corporation, for value
received, hereby promises to pay to _________________________________ (the
"Debenture Holder") or registered assigns at its principal place of business,
5006 Verde Valley, Dallas, Texas 75240, the principal sum of _______ DOLLARS
($______________).  The outstanding principal amount of this Debenture shall
bear interest until payment in full of the outstanding principal amount hereof
at the rate of nine and one half percent (9.5%) per year.

     Payments of principal in an amount equal to twenty percent (20%) of the
original principal amount of this Debenture shall be due and payable on each of
December 31, 2006, 2007, 2008, 2009, and the remaining principal amount and
accrued unpaid interest shall be due and payable on December 31, 2010 (the
"Maturity Date").

     Accrued interest shall begin to accrue on September 30, 2000 and shall be
payable quarterly beginning on March 15, 2001 and continuing on each June 15,
September 15, December 15 and March 15 until the Debenture is paid in full.

     THIS DEBENTURE IS CONVERTIBLE INTO SHARES OF COMMON STOCK OF THE COMPANY AS
PROVIDED IN THE DEBENTURE AGREEMENT ENTERED INTO BETWEEN THE COMPANY AND THE
DEBENTURE HOLDER WITH AN EFFECTIVE DATE OF EVEN DATE HEREWITH (THE "DEBENTURE
AGREEMENT").  REFERENCE IS MADE TO THE DEBENTURE AGREEMENT, AND THE PROVISIONS
OF SUCH AGREEMENT SHALL BE DEEMED INCORPORATED INTO THIS DEBENTURE, AND FOR ALL
PURPOSES, SHALL HAVE THE SAME EFFECT AS THOUGH FULLY SET FORTH ON THE FACE
HEREOF.

     Principal and interest on this Debenture will be paid to the person in
whose name this Debenture is registered at the close of business on the 1st day
of the calendar month during which such payment is to be paid and will be paid
by check mailed to the registered address of the person entitled thereto.

     If the Company (i) defaults under the terms of this Debenture and the
Debenture Agreement, (ii) the Debenture Holder satisfies the notice requirement
in the Debenture Agreement, and (iii) the default remains uncured for the period
specified in the Debenture Agreement, then the outstanding principal balance of
and all interest due on this Debenture will forthwith become due and payable and
the Company shall immediately pay the same.

     Issued and effective as of September 30, 2000.


                         ENB BANKSHARES, INC.



                         By:    _________________________________________
                         Title: _________________________________________
<PAGE>

                    FORM FOR EXERCISING ELECTION TO CONVERT
                    ---------------------------------------

     The undersigned holder of the attached ENB Bankshares, Inc. Convertible
Debenture (the "Debenture") surrenders $___________________ aggregate principal
amount of accrued interest and unpaid principal due under the Debenture for
conversion on the terms and conditions set forth in the Debenture, and requests
that the shares issuable on such conversion be issued to him.

     Date: ______________________.


                                         _______________________________________



Address of Holder:

_______________________________
_______________________________

Social Security Number or
Taxpayer Identification Number
of holder: ____________________