Document:

<PAGE>

                                                                 Exhibit 10.56.3

                                                                  EXECUTION COPY

            AMENDMENT NO. 1, dated as of May 27, 1999 (this "Amendment"), to the
Open-End Mortgage, Security Agreement and Assignment of Leases and Rents, dated
as of March 18, 1999 (the "Mortgage"), by EME HOMER CITY GENERATION L.P., a
Pennsylvania limited partnership (the "Mortgagor"), having its principal office
at c/o Edison Mission Holdings Co., 18101 Von Karman Avenue, Suite 1700, Irvine,
California, to UNITED STATES TRUST COMPANY OF NEW YORK, as collateral agent to
the Secured Parties (as defined in the Credit Agreement referred to below) (in
such capacity, the "Mortgagee"), having an address at 114 West 47th Street, 25th
Floor, New York, New York 10036, Attn: Corporate Trust Division.

                              W I T N E S S E T H :

            A. The Mortgagor executed and delivered the Mortgage to the
Mortgagee on March 18, 1999, and recorded such Mortgage on March 23, 1999, in
Book 601, Page 341, in the Records Office of Indiana County, Pennsylvania.

            B. Pursuant to Section 8.2.1(e) of the Credit Agreement, dated as of
March 18, 1999 (the "Credit Agreement"), among Edison Mission Holdings Co., a
California corporation, the financial institutions from time to time parties
thereto and Citicorp USA, Inc., as Administrative Agent, the Mortgagor intends
to issue $300 million in aggregate principal amount of Senior Secured Bonds due
2019 and $530 million in aggregate principal amount of Senior Secured Bonds due
2026 (collectively the "Bonds"), the proceeds of which will be used, in part, to
repay the $800 million outstanding aggregate principal amount of the 364-Day
Term Loans (as defined in the Credit Agreement).

            C. The Mortgagor has requested and the Mortgagee has agreed that the
maximum amount of the advances provided for in the Mortgage be increased by $30
million, from $1,100,000,000, to $1,130,000,000, in order to reflect the $30
million increase in the principal amount of Senior Debt (as defined in the
Credit Agreement) caused by the issuance of the Bonds.

            NOW, THEREFORE, in consideration of Ten Dollars ($10.00) and other
good and valuable consideration, the receipt of which are hereby acknowledged,
the parties hereto agree as follows:

            1. Amendment of Cover Page. The second caption on the cover page of
the Mortgage is hereby amended by deleting the amount "$1,100,000,000.00" in the
second line and substituting in lieu thereof "$1,130,000,000.00."
<PAGE>

            IN WITNESS WHEREOF, this Amendment has been duly executed by the
Mortgagor and the Mortgagee as of the day and year first above written.

ATTEST:                                EME HOMER CITY GENERATION L.P.,
                                       a Pennsylvania limited partnership

By: /s/ Michelle Johnson               By: MISSION ENERGY WESTSIDE, INC.,
    ------------------------------         a California corporation, its general
        Name: Michelle Johnson             partner
        Title: Assistant Secretary

                                           By: /s/ Steven D. Eisenberg
                                               ---------------------------------
                                                   Name: Steven D. Eisenberg
                                                   Title: Vice President

[corporate seal]

ATTEST:                                UNITED STATES TRUST COMPANY
                                       OF NEW YORK, as Mortgagee

By:                                    By:
   -------------------------------        --------------------------------------
        Name: Lisa A. Geary                    Name: Christopher J. Grell
        Title: Assistant Secretary             Title: Assistant Vice President

[corporate seal]
<PAGE>

Acknowledged and Consented to by:

ATTEST:                                CITICORP USA, INC., as Administrative
                                       Agent

By: /s/                                By: /s/ Anita J. Brickell
   -------------------------------        --------------------------------------
        Name:                                  Name: Anita J. Brickell
        Title: Assistant Secretary             Title: Attorney-in-Fact

[corporate seal]

ATTEST:                                UNITED STATES TRUST COMPANY OF
                                       NEW YORK, as Trustee

By:                                    By:
   -------------------------------        --------------------------------------
        Name:                                  Name: Christopher J. Grell
        Title: [Assistant]                     Title: Assistant Vice President
                  [Secretary]
                  [Treasurer]

[corporate seal]<PAGE>

                                                                Exhibit 10.58.1

     AMENDMENT NO. 1, dated as of May 27, 1999 (this "AMENDMENT"), to the
Security Deposit Agreement, dated as of March 18, 1999 (the "AGREEMENT"),
among (i) EDISON MISSION HOLDINGS CO., a California corporation (the
"BORROWER"), (ii) EDISON MISSION FINANCE CO., a California corporation
("EDISON MISSION FINANCE"), (iii) HOMER CITY PROPERTY HOLDINGS, INC., a
California corporation ("HOMER CITY HOLDINGS"), (iv) CHESTNUT RIDGE ENERGY
COMPANY, a California corporation ("CHESTNUT RIDGE"), (v) MISSION ENERGY
WESTSIDE, INC., a California corporation ("MEW"), (vi) EME HOMER CITY
GENERATION L.P., a Pennsylvania limited partnership ("EME HOMER CITY"; and,
together with the Borrower, Edison Mission Finance, Homer City Holdings,
Chestnut Ridge and MEW, the "LOAN PARTIES"), and (vii) UNITED STATES TRUST
COMPANY OF NEW YORK, as collateral agent for certain senior secured creditors
of the Borrower (in such capacity, the "COLLATERAL AGENT").

                                  RECITALS

          A. The Borrower has entered into a Credit Agreement, dated as of
March 18, 1999 (as the same may be amended, supplemented or otherwise
modified from time to time, the "CREDIT AGREEMENT"; the loans made
thereunder, the "LOANS"), with certain financial institutions (collectively,
the "LENDERS") and Citocorp USA, Inc., as administrative agent for the
Lenders (in such capacity, the "ADMINISTRATIVE AGENT").

          B. Pursuant to the Agreement, certain accounts of the Loan Parties
were established with the Collateral Agent, together with instructions for
deposits into, and payments from, each such account and each Loan Party
pledged and assigned to the Collateral Agent certain security interests
related to such accounts for the benefit of the senior secured creditors to
the Borrower.

          C. The Borrower wishes to issue $300 million in aggregate principal
amount of Senior Secured Bonds due 2019 and $530 million in aggregate
principal amount of Senior Secured Bonds due 2026 (collectively, the
"BONDS"; the trustee to the holders thereof, the "TRUSTEE"), the proceeds of
which will be used to refinance a portion of the Loans and to repay a portion
of the Borrower's direct parent's equity investment in the Borrower.

          D. The Loan Parties have requested, and, upon this Amendment
becoming effective, the Collateral Agent has agreed, that certain provisions
of the Agreement be amended in the manner provided for in this Amendment.

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                                                                             2

NOW, THEREFORE, the parties hereto hereby agree as follows:

          1. AMENDMENT TO DEFINITION OF "NET CASH PROCEEDS." The definition of
"Net Cash Proceeds" is hereby amended by deleting the current definition in
its entirety and substituting in lieu thereof the following:

          "NET CASH PROCEEDS" means (a) in connection with any Recovery
      Event, the proceeds thereof in the form of cash and cash equivalents of
      such Recovery Event, net of any expenses reasonably incurred in
      respect of such Recovery Event, including attorneys' fees, amounts
      required to be applied to the repayment of Indebtedness secured by a
      Lien expressly permitted under the Credit Agreement and the other
      Financing Documents on any asset which is the subject of such Recovery
      Event (other than any Lien pursuant to a Security Document) and net of
      taxes paid or reasonably estimated to be payable as a result thereof
      (after taking into account any available tax credits or deductions and
      any tax sharing arrangements), to the extent that, in the case of
      Recovery Events relating to property or casualty insurance claims, the
      amount of such proceeds exceeds $5,000,000 with respect to any asset of
      any Loan Party and (b) in connection with any issuance or sale of debt
      securities or other incurrence of Indebtedness, the cash proceeds
      received from such issuance, sale or incurrence, net of attorneys'
      fees, investment banking fees, accountants' fees, underwriting
      discounts, commissions, administrative agency fees, upfront fees,
      arrangement fees and other customary fees and expenses actually
      incurred in connection therewith.

          2. AMENDMENT TO DEFINITION OF "SECURED PARTIES." The definition of
"Secured Parties" is hereby amended by deleting the current definition in its
entirety and substituting in lieu thereof the following:

          "SECURED PARTIES" means the Collateral Agent, the Administrative
      Agent, the Lenders, the Trustee, each Person that provides any
      Refinancing Indebtedness, Ordinary Course Letter of Credit
      Indebtedness, DSR Letter of Credit Indebtedness, Additional
      Indebtedness or Swap Indebtedness and each Person that serves as
      indenture trustee, collateral agent, lenders' representative or in any
      similar capacity for Person that provide any Senior Debt.

          3. AMENDMENT TO SECTION 3.4. Section 3.4 of the Agreement is hereby
amended by deleting the current section in its entirety and substituting in
lieu thereof the following:

               SECTION 3.4 PRINCIPAL ACCOUNTS. (a) The Loan Parties agree
      that, until the 364-Day Term Loans have been repaid in full, all Net
      Cash Proceeds of any Refinancing Indebtedness shall be deposited into
      the Loan Principal Account. IF, notwithstanding the foregoing, any Loan
      Party shall receive any such proceeds, it shall immediately deliver
      such proceeds in the exact form received (duly indorsed, if
      appropriate, in a manner satisfactory to the Collateral Agent) to the
      Collateral Agent for

<PAGE>
                                                                             3

     deposit into the Loan Principal Account. The Collateral Agent shall have
     the right to receive all such Net Cash Proceeds directly from the
     Persons owing the same. All such Net Cash Proceeds received by the
     Collateral Agent shall be deposited into the Loan Principal Account.

               (b) The Loan Parties agree that all Net Cash Proceeds of any
     Additional Indebtedness that is incurred for the purpose of (i)
     prepaying any Loans, Refinancing Indebtedness, Ordinary Course Letter of
     Credit Indebtedness, DSR Letter of Credit Indebtedness or Additional
     Indebtedness or (ii) repaying any Loans, Refinancing Indebtedness or
     Additional Indebtedness at its scheduled maturity, shall be deposited
     into the Principal Account related to the Indebtedness that is being
     prepaid or repaid. If, notwithstanding the foregoing, any Loan Party
     shall receive any such proceeds, it shall immediately deliver such
     proceeds in the exact form received (duly indorsed, if appropriate, in a
     manner satisfactory to the Collateral Agent) to the Collateral Agent for
     deposit into such related Principal Account. The Collateral Agent shall
     have the right to receive all such Net Cash Proceeds directly from the
     Persons owing the same. All such Net Cash Proceeds received by the
     Collateral Agent shall be deposited into such related Principal Account.

              (c) The Loan Parties agree that all Net Cash Proceeds of any
     Additional Indebtedness other than the type described in paragraph (b)
     above shall be deposited into the Revenue Account.

          4. AMENDMENT TO SECTION 4.1(a). Section 4.1(a) of the Agreement is
hereby amended by deleting the current clauses beginning with the words
"third" and "fourth" in their entirety and substituting in lieu thereof the
following:

          THIRD, (i) into each Accrued Interest Account, the Accrued Interest
     Amount calculated for such Accrued Interest Account, together with the
     amount of all deficiencies, if any, with respect to deposits of Accrued
     Interest Amounts required in all prior months, as certified in such
     Request Letter, (ii) into each Principal Account (other than (A) the
     Loan Principal Account and any other Principal Account established in
     respect of non-amortizing Senior Debt, (B) any DSR Letter of Credit
     Indebtedness Principal Account relating to non-amortizing DSR Letter of
     Credit Indebtedness (or, so long as any Loans or related Commitments
     remain outstanding, any DSR Letter of Credit Indebtedness Principal
     Account relating to any DSR Letter of Credit Indebtedness), (C) any
     Ordinary Course Letter of Credit Indebtedness Principal Accounts
     established for non-amortizing Ordinary Course Letter of Credit
     Indebtedness and (D) any Swap Indebtedness Termination Payment Account),
     an amount equal to 1/6th of the principal amount, if any, which is
     payable on or within six months following such Monthly Transfer Date on
     account of the Senior Debt with respect to which such Principal Account
     was established, together with the amount of all deficiencies, if any,
     with respect to deposits of principal required in all prior months, as
     certified in such Request Letter,

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                                                                             4

     (iii) into each Ordinary Course Letter of Credit Indebtedness Principal
     Account, the amount certified in such Request Letter to be sufficient to
     repay in full the related non-amortizing Ordinary Course Letter of
     Credit Indebtedness then outstanding, (iv) into each Swap Indebtedness
     Termination Payment Account, the amount certified in such Request Letter
     to be sufficient to repay in full the related termination payment then
     due and payable and (v) to the Administrative Agent, the amount
     specified in the Request Letter to be applied to the repayment of
     Revolving Loans;

          FOURTH, into each DSR Letter of Credit Indebtedness Principal
     Account relating to non-amortizing DSR Letter of Credit Indebtedness
     (or, so long as any Loans or related Commitments remain outstanding,
     each DSR Letter of Credit Indebtedness Principal Account relating to any
     DSR Letter of Credit Indebtedness), the amount certified in such Request
     Letter to be sufficient to repay in full the related DSR Letter of
     Credit Indebtedness then outstanding.

         5. GRANT, AND CONFIRMATION OF GRANT, OF SECURITY INTEREST. Each of
the Loan Parties hereby grants and confirms the grant of the security
interest in: (i) the Revenues and each of the Accounts (except as provided in
clauses (b), (c), (d), (e), (f) and (g) of Section 2.4 of the Agreement) and
all cash, cash equivalents, instruments, investments and other securities on
deposit therein and all Proceeds of the foregoing in favor of the Collateral
Agent for the benefit of the Secured Parties (as defined in Amendment No. 1
to the Guarantee and Collateral Agreement); and (ii) the Refinancing
Indebtedness Accrued Interest Account, the Refinancing Indebtedness
Principal Account and the Refinancing Indebtedness Debt Service Reserve
Account related to each class of Refinancing Indebtedness, and all cash, cash
equivalents, instruments, investments and other securities on deposit therein
and all Proceeds of the foregoing in favor of the Collateral Agent for the
exclusive benefit of the Persons that provide such class of Refinancing
Indebtedness; in each case, to secure the Obligations.

         6. SECURITIES INTERMEDIARY. The Securities Intermediary hereby agrees
that, upon its execution of this Amendment, it shall be bound by all of the
provisions of the Agreement as amended by this Amendment.

         7. COUNTERPARTS. This Amendment may be executed by the parties
hereto in any number of separate counterparts, and all of said counterparts
taken together shall be deemed to constitute one and the same instrument.
Delivery of an executed counterpart of a signature page to this letter by
facsimile shall be effective as delivery of a manually executed counterpart
of this Amendment.

         8. LIMITED EFFECT. Except as expressly provided herein, all of the
terms and provisions of the Agreement are and shall remain in full force and
effect.

         9. SEVERABILITY. Any provision of this Amendment which is prohibited
or unenforceable in any jurisdiction shall, as to such jurisdiction, be
ineffective to the extent of such

<PAGE>
                                                                             5

prohibition or unenforceability without invalidating the remaining provisions
hereof, and any such prohibition or unenforceability in any jurisdiction
shall not invalidate or render unenforceable such provision in any other
jurisdiction.

          10. CONDITIONS TO EFFECTIVENESS. The amendment provided for herein
shall become effective on the date the Collateral Agent shall have received
counterparts of this Amendment duly executed and delivered by each Loan Party
and consented to by the Administrative Agent and the Trustee.

          11. APPLICABLE LAW. THIS AMENDMENT AND THE RIGHTS AND OBLIGATIONS
OF THE PARTIES HERETO SHALL BE GOVERNED BY, AND CONSTRUED AND INTERPRETED IN
ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW YORK.

<PAGE>

     IN WITNESS WHEREOF, the parties hereto have caused this Amendment to be
duly executed and delivered by their respective proper and duly authorized
officers as of the day and year first above written.

                                      EDISON MISSION HOLDINGS CO.

                                      By: /s/ Steven D. Eisenberg
                                         --------------------------
                                         Name: Steven D. Eisenberg
                                         Title: Vice President

                                      EDISON MISSION FINANCE CO.

                                      By: /s/ Steven D. Eisenberg
                                         --------------------------
                                         Name: Steven D. Eisenberg
                                         Title: Vice President

                                      HOMER CITY PROPERTY HOLDINGS, INC.

                                      By: /s/ Steven D. Eisenberg
                                         --------------------------
                                         Name: Steven D. Eisenberg
                                         Title: Vice President

                                      CHESTNUT RIDGE ENERGY COMPANY

                                      By: /s/ Steven D. Eisenberg
                                         --------------------------
                                         Name: Steven D. Eisenberg
                                         Title: Vice President

                                      MISSION ENERGY WESTSIDE, INC.

                                      By: /s/ Steven D. Eisenberg
                                         --------------------------
                                         Name: Steven D. Eisenberg
                                         Title: Vice President

<PAGE>

                                      EME HOMER CITY GENERATION L.P.
                                      By: Mission Energy Westside, Inc.,
                                          its General Partner

                                      By: /s/ Steven D. Eisenberg
                                         --------------------------
                                         Name: Steven D. Eisenberg
                                         Title: Vice President

                                      UNITED STATES TRUST COMPANY OF
                                      NEW YORK, as Collateral Agent

                                      By: /s/ Christopher J. Grell
                                         --------------------------
                                         Name: Christopher J. Grell
                                         Title: Assistant Vice President

                                      UNITED STATES TRUST COMPANY OF
                                      NEW YORK, as Securities Intermediary

                                      By: /s/ Christopher J. Grell
                                         --------------------------
                                         Name: Christopher J. Grell
                                         Title: Assistant Vice President

<PAGE>

Acknowledged and Consented to by:

CITICORP USA, INC., as Administrative Agent

By: /s/ Anita J. Brickell
   ----------------------------
   Name: Anita J. Brickell
   Title: Attorney-in-Fact

UNITED STATES TRUST COMPANY OF NEW YORK, as Trustee

By: /s/ Christopher J. Grell
   ----------------------------
   Name: Christopher J. Grell
   Title: Assistant Vice President

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