Protein Families

Orthologs are homologous proteins found in different species that evolved from the same ancestral protein and typically perform similar functions. Paralogs are homologous proteins produced by gene duplication and can exist in the same or different species with potentially divergent functions. For example, alpha hemoglobin in humans and mice are orthologs, while alpha and beta hemoglobin in humans are paralogs.

Gene duplication creates an extra copy of a gene within an organism's genome. The original and duplicate gene copies can then mutate independently, coding for different amino acid sequences. Over time, these diverging sequences produce proteins with variations in structure and function while maintaining core similarities, forming a protein family.

Yes, proteins within a family may have sequence identity as low as 30 percent. Despite minimal amino acid sequence similarity, family members often retain similar overall protein structure and conservation of protein domains over different proteins, allowing them to perform related functions like oxygen binding in hemoglobin and myoglobin.

A protein superfamily comprises two or more families that evolved from a more distant common ancestor than members of a single family. Superfamily members show larger variations in structure and function but retain significant structural features. Each superfamily can contain several protein families with more closely related structures and functions.

By comparing a protein with known amino acid sequence but unknown function to other members of its family, researchers can hypothesize its three-dimensional structure and biological role. Since family members typically share similar structures and functions, this comparison provides testable predictions about the novel protein's function.

The immunoglobulin superfamily (IgSF) is one of the largest protein superfamilies with over 700 members in the human genome. All members contain immunoglobulin domains with a unique three-dimensional structure composed of two anti-parallel beta-sheets. The IgSF includes antigen receptors, cell adhesion molecules, cytoskeletal proteins, and growth-factor receptor families.

Scientists use online databases like Pfam to search for known protein families and classify newly discovered proteins. By entering an amino acid sequence into these databases, researchers can identify sequence similarities to known families, providing testable hypotheses about the novel protein's structure and function based on family relationships.