Property graphs have become a fundamental data model for representing complex, interconnected data across diverse domains. Their flexibility, achieved by annotating nodes and edges with properties, enables rich and expressive data modeling. However, this flexibility often comes at the cost of data consistency and quality, as many property graph systems lack robust schema mechanisms. To address this gap, we present PG-Schema-PC (PG-Schema for Property Constraints), a principled extension of the PG-Schema language tailored to express constraints over property sets. PG-Schema-PC introduces constructs for defining structural, cardinality, and range constraints, allowing schema designers to specify alternative property groupings, control property multiplicity, and validate value-level conditions. We formalize the abstract grammar and semantics of PG-Schema-PC and demonstrate its capabilities through practical examples inspired by real-world use cases