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  <xsd:element name="PrepaidManufacturingOfClinicalMaterials" id="ptgx_PrepaidManufacturingOfClinicalMaterials" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:balance="debit" xbrli:periodType="instant" />
  <xsd:element name="ScheduleCashEquivalentsAndAvailableForSaleSecuritiesTable" id="ptgx_ScheduleCashEquivalentsAndAvailableForSaleSecuritiesTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="CollaborativeArrangementDisclosureAbstract" id="ptgx_CollaborativeArrangementDisclosureAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element name="JanssenLicenseAndCollaborationAgreement26May2017Member" id="ptgx_JanssenLicenseAndCollaborationAgreement26May2017Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration" />
  <xsd:element id="ptgx_JanssenBiotechIncMember" name="JanssenBiotechIncMember" nillable="true" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" xbrli:periodType="duration" />
  <xsd:element id="ptgx_RevenueSourceAxis" name="RevenueSourceAxis" nillable="true" type="xbrli:stringItemType" substitutionGroup="xbrldt:dimensionItem" abstract="true" xbrli:periodType="duration" />
  <xsd:element id="ptgx_RevenueSourceDomain" name="RevenueSourceDomain" nillable="true" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" xbrli:periodType="duration" />
  <xsd:element id="ptgx_SecuritiesPurchaseAgreementWithAccreditedInvestorsMember" name="SecuritiesPurchaseAgreementWithAccreditedInvestorsMember" nillable="true" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" xbrli:periodType="duration" />
  <xsd:element id="ptgx_StockIssuedDuringPeriodValueWarrantsExercised" name="StockIssuedDuringPeriodValueWarrantsExercised" nillable="true" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" xbrli:balance="credit" />
  <xsd:element id="ptgx_StockIssuedDuringPeriodSharesWarrantsExercised" name="StockIssuedDuringPeriodSharesWarrantsExercised" nillable="true" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" />
  <xsd:element id="ptgx_UsTreasuryAndAgencySecuritiesMember" name="UsTreasuryAndAgencySecuritiesMember" nillable="true" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" xbrli:periodType="duration" />
  <xsd:element id="ptgx_CashEquivalentsAndMarketableSecuritiesAmortizedCost" name="CashEquivalentsAndMarketableSecuritiesAmortizedCost" nillable="true" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" xbrli:balance="debit" />
  <xsd:element id="ptgx_CashEquivalentsAndMarketableSecuritiesGrossUnrealizedGain" name="CashEquivalentsAndMarketableSecuritiesGrossUnrealizedGain" nillable="true" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" xbrli:balance="credit" />
  <xsd:element id="ptgx_CashEquivalentsAndMarketableSecuritiesGrossUnrealizedLoss" name="CashEquivalentsAndMarketableSecuritiesGrossUnrealizedLoss" nillable="true" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" xbrli:balance="debit" />
  <xsd:element id="ptgx_CashEquivalentsAndMarketableSecuritiesFairValue" name="CashEquivalentsAndMarketableSecuritiesFairValue" nillable="true" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" xbrli:balance="debit" />
  <xsd:element id="ptgx_CashEquivalentsAndMarketableSecuritiesAbstract" name="CashEquivalentsAndMarketableSecuritiesAbstract" nillable="true" type="xbrli:stringItemType" substitutionGroup="xbrli:item" abstract="true" xbrli:periodType="duration" />
  <xsd:element id="ptgx_PercentagesRoyaltiesOnNetProductSales" name="PercentagesRoyaltiesOnNetProductSales" nillable="true" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" />
  <xsd:element id="ptgx_CollaborativeArrangementEligibleAmountReceived" name="CollaborativeArrangementEligibleAmountReceived" nillable="true" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" xbrli:balance="debit" />
  <xsd:element id="ptgx_NoncurrentContractualMaturitiesMaximumPeriod" name="NoncurrentContractualMaturitiesMaximumPeriod" nillable="true" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" />
  <xsd:element id="ptgx_NoncurrentContractualMaturitiesMinimumPeriod" name="NoncurrentContractualMaturitiesMinimumPeriod" nillable="true" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" />
  <xsd:element id="ptgx_CurrentContractualMaturitiesMaximumPeriod" name="CurrentContractualMaturitiesMaximumPeriod" nillable="true" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" />
  <xsd:element id="ptgx_SharesWithheldForNetSettlementOfTaxWithholdingUponVestingOfRestrictedStockUnits" name="SharesWithheldForNetSettlementOfTaxWithholdingUponVestingOfRestrictedStockUnits" nillable="true" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" xbrli:balance="credit" />
  <xsd:element id="ptgx_SharesWithheldForNetSettlementOfTaxWithholdingUponVestingOfRestrictedStockUnitsShares" name="SharesWithheldForNetSettlementOfTaxWithholdingUponVestingOfRestrictedStockUnitsShares" nillable="true" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" />
  <xsd:element id="ptgx_SharesWithheldForNetSettlementsOfTaxWithholdingUponVestingOfRestrictedStockUnits" name="SharesWithheldForNetSettlementsOfTaxWithholdingUponVestingOfRestrictedStockUnits" nillable="true" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" xbrli:balance="debit" />
  <xsd:element id="ptgx_LicenseAndCollaborativeAgreementAmendedAndRestatedMember" name="LicenseAndCollaborativeAgreementAmendedAndRestatedMember" nillable="true" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" xbrli:periodType="duration" />
  <xsd:element name="ProceedsFromMilestonePayments" id="ptgx_ProceedsFromMilestonePayments" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" xbrli:balance="debit" />
  <xsd:element name="PrepaidLicenses" id="ptgx_PrepaidLicenses" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" xbrli:balance="debit" />
  <xsd:element name="CreditRelatedLossesToBeRecognized" id="ptgx_CreditRelatedLossesToBeRecognized" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" xbrli:balance="credit" />
  <xsd:element name="ProceedsFromExerciseOfWarrantsInExchangeForIssuanceOfPreFundedWarrants" id="ptgx_ProceedsFromExerciseOfWarrantsInExchangeForIssuanceOfPreFundedWarrants" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" xbrli:balance="debit" />
  <xsd:element name="MinimumPercentageOfCommonStockHeldByInvestorsForNotExerciseOfWarrants" id="ptgx_MinimumPercentageOfCommonStockHeldByInvestorsForNotExerciseOfWarrants" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" />
  <xsd:element name="CollaborativeArrangementMilestonePaymentEligibleToReceive" id="ptgx_CollaborativeArrangementMilestonePaymentEligibleToReceive" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" xbrli:balance="debit" />
  <xsd:element name="TakedaPharmaceuticalsMember" id="ptgx_TakedaPharmaceuticalsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="LicenseAndCollaborationAgreementMember" id="ptgx_LicenseAndCollaborationAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="PercentageOfRoyaltiesOnNetSalesPercent" id="ptgx_PercentageOfRoyaltiesOnNetSalesPercent" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" />
  <xsd:element name="CollaborativeAgreementProfitSharePercentage" id="ptgx_CollaborativeAgreementProfitSharePercentage" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" />
  <xsd:element name="WilliamDWaddillMember" id="ptgx_WilliamDWaddillMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="ExerciseOfFullOptOutRightMember" id="ptgx_ExerciseOfFullOptOutRightMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="ExerciseOfFullOptOutRightDuringInitialOptOutPeriodMember" id="ptgx_ExerciseOfFullOptOutRightDuringInitialOptOutPeriodMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="CollaborativeArrangementMilestoneAdditionalPaymentEligibleToReceive" id="ptgx_CollaborativeArrangementMilestoneAdditionalPaymentEligibleToReceive" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" xbrli:balance="debit" />
  <xsd:element name="UpcomingPotentialDevelopmentAndRegulatoryMilestonesPayments" id="ptgx_UpcomingPotentialDevelopmentAndRegulatoryMilestonesPayments" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" xbrli:balance="credit" />
  <xsd:element name="PhaseThreeClinicalTrialForRusfertideInPvMember" id="ptgx_PhaseThreeClinicalTrialForRusfertideInPvMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="FoodAndDrugAdministrationApprovalOfNewDrugApplicationForRusfertideInPvMember" id="ptgx_FoodAndDrugAdministrationApprovalOfNewDrugApplicationForRusfertideInPvMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="DevelopmentServicesMember" id="ptgx_DevelopmentServicesMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="DineshV.PatelMember" id="ptgx_DineshV.PatelMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="SingleReportableSegmentMember" id="ptgx_SingleReportableSegmentMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="EmployeeExpenseBenefitRelatedToDiscovery" id="ptgx_EmployeeExpenseBenefitRelatedToDiscovery" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" xbrli:balance="debit" />
  <xsd:element name="EmployeeExpenseBenefitRelatedToDevelopment" id="ptgx_EmployeeExpenseBenefitRelatedToDevelopment" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" xbrli:balance="debit" />
  <xsd:element name="EmployeeExpenseBenefitRelatedToGeneralAndAdministrativeAffairs" id="ptgx_EmployeeExpenseBenefitRelatedToGeneralAndAdministrativeAffairs" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" xbrli:balance="debit" />
  <xsd:element name="CostOfRevenueDiscoveryExpensesExcludingEmployeeExpenseBenefitAndAllocationShareBasedCompensationExpenses" id="ptgx_CostOfRevenueDiscoveryExpensesExcludingEmployeeExpenseBenefitAndAllocationShareBasedCompensationExpenses" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" xbrli:balance="debit" />
  <xsd:element name="CostOfRevenueDevelopmentExpensesExcludingEmployeeExpenseBenefitAndAllocationShareBasedCompensationExpenses" id="ptgx_CostOfRevenueDevelopmentExpensesExcludingEmployeeExpenseBenefitAndAllocationShareBasedCompensationExpenses" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" xbrli:balance="debit" />
  <xsd:element name="GeneralAndAdministrativeExpenseExcludingEmployeeExpenseBenefitAndAllocationShareBasedCompensationExpenses" id="ptgx_GeneralAndAdministrativeExpenseExcludingEmployeeExpenseBenefitAndAllocationShareBasedCompensationExpenses" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" xbrli:balance="debit" />
  <xsd:element name="AcceptanceOfNewDrugApplicationFiledByFoodAndDrugAdministrationApprovalForSecondIndicationMember" id="ptgx_AcceptanceOfNewDrugApplicationFiledByFoodAndDrugAdministrationApprovalForSecondIndicationMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="FoodAndDrugAdministrationApprovalOfNewDrugApplicationForSecondIndicationMember" id="ptgx_FoodAndDrugAdministrationApprovalOfNewDrugApplicationForSecondIndicationMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="NonCashLeaseExpense" id="ptgx_NonCashLeaseExpense" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" xbrli:balance="debit" />
  <xsd:element name="IncreaseDecreaseInReceivablesFromCollaborationPartner" id="ptgx_IncreaseDecreaseInReceivablesFromCollaborationPartner" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" xbrli:balance="credit" />
  <xsd:element name="FirstRegulatoryApprovalInThreeEuropeanCountriesMember" id="ptgx_FirstRegulatoryApprovalInThreeEuropeanCountriesMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="FirstRegulatoryApprovalInJapanMember" id="ptgx_FirstRegulatoryApprovalInJapanMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="NumberOfReportableSegmentsNotDisclosedFlag" id="ptgx_NumberOfReportableSegmentsNotDisclosedFlag" type="xbrli:booleanItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" />
  <xsd:element name="NumberOfTherapeuticAreas" id="ptgx_NumberOfTherapeuticAreas" type="xbrli:integerItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" />
  <xsd:element name="FoodAndDrugAdministrationApprovalOfNewDrugApplicationForThirdIndicationMember" id="ptgx_FoodAndDrugAdministrationApprovalOfNewDrugApplicationForThirdIndicationMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="AcceptanceOfNewDrugApplicationFiledByFoodAndDrugAdministrationApprovalForThirdIndicationMember" id="ptgx_AcceptanceOfNewDrugApplicationFiledByFoodAndDrugAdministrationApprovalForThirdIndicationMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="WilliamWaddillMember" id="ptgx_WilliamWaddillMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="AnnualNetSalesUpto1.5BillionMember" id="ptgx_AnnualNetSalesUpto1.5BillionMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="CollaborativeArrangementThresholdAnnualNetSales" id="ptgx_CollaborativeArrangementThresholdAnnualNetSales" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" xbrli:balance="credit" />
  <xsd:element name="AnnualNetSalesAbove1.5BillionMember" id="ptgx_AnnualNetSalesAbove1.5BillionMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="MaximumSalesMilestoneRevenue" id="ptgx_MaximumSalesMilestoneRevenue" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" xbrli:balance="credit" />
  <xsd:element name="FoodAndDrugAdministrationApprovalOfIcotydeMember" id="ptgx_FoodAndDrugAdministrationApprovalOfIcotydeMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="CostReimbursement" id="ptgx_CostReimbursement" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" xbrli:balance="credit" />
  <xsd:element name="FusfertideClinicalTrialSuppliesMember" id="ptgx_FusfertideClinicalTrialSuppliesMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
</xsd:schema>
