Isozymes of glycolytic pathway in sperm: the unique sites for contraception

Abstract

Efficient sperm motility is crucial for reproductive success. Sperm capacitation and sperm activation are prerequisites for sperm-egg interaction and ovum fertilization. Hyperactivity of sperm can be observed if glycolysis is not hampered. How glycolysis, hyperactivity, and sperm fertility are functionally linked is not clear. Glycolysis is the major source of ATP required for sperm flagellar movement, capacitation, and fertilization. The unique isozymic pattern of glycolytic enzymes bound to sperm flagella with specific kinetics seems as a controlling mechanism for requisite energy output of the glycolysis. Furthermore, mammalian sperm contain also atypical isozymes of phosphoglucose isomerase, triosephosphate isomerase, phosphoglycerate mutase, enolase and others. The molecular bases for the testis-specific expression are diverse. For instance PGK-2 is encoded by different gene, while some originate from alternative promoters or by alternative splicing. An unusual feature of GAPDHS is a novel proline-rich N-terminal 105-amino acid sequence not present in other isozymes, whereas ALDOA_V2 and ALDOART1 have a novel 54-55-amino acid sequence at N-terminal region. While LDH-C and PGK-2 lack N-terminal features, Hexokinase, HK1S has an unusual N-terminal spermatogenic cell-specific sequence. This review describes the up-to-date, extended knowledge on isozymic pattern and characteristics of enzymes of pathway of glycolysis in sperm with a view to understand the mechanism of sperm motility and their future potential as targets for contraception by chemical and/or immunological methods.