Final answer:
A transmembrane beta barrel is composed of parallel or antiparallel beta-pleated sheets that form a stable structure through hydrogen bonding, different from alpha-helix membrane-spanning proteins.
Step-by-step explanation:
The sequence and structure of a transmembrane beta barrel is best described as a parallel arrangement of beta strands. This structure consists of multiple beta-pleated sheets that are aligned side by side and can run in a parallel or antiparallel fashion. These sheets are characterized by extended polypeptide chains that are connected by hydrogen bonds, which provides stability to the overall structure. Transmembrane beta barrels are integral membrane proteins and are distinct from proteins with alpha-helices that span the membrane or those without any secondary structure.