The pathogenesis of two metabolic disorders caused by enzyme defects in the red blood cell leading to hemolytic anemia, and in some cases of glucose phosphate isomerase (GPI) deficiency additionally to neurological impairment was investigated. Rheological studies were performed to determine the influence of a shortage of energy on the deformability of the erythrocytes. The functions of the enzymes were determined by studying the enzyme kinetics, the temperature dependence of the enzyme activity and the migration of the proteins in an electric field. A detailed molecular genetic analysis of the gene encoding for the given protein allowed the detection of mutations involving amino acid exchanges which cause alterations of the protein structure. For both enzyme deficiencies, a good correlation was found between the structural changes (usually caused by single point mutations in the gene), the altered function of the enzymes and the severity of the clinical picture. The exchange of amino acids close to either the active site or the regulatory domain results in a decreased turnover as well as an alteration of the regulatory properties of the enzymes; this usually leads to an increased severity of the disease. Increased concentrations of glucose-6-phosphate (G-6-P), found in all red blood cells of patients suffering from hemolytic anemia caused by pyruvate kinase (PK) and GPI deficiency, correlate well with the severity of the clinical picture, apparently reflecting the degree of the perturbation of glycolysis. This results in a lack of the energy donor adenosine triphosphate (ATP); this leads then to a destabilization of the red cell membrane which causes earlier lysis of the red blood cell, which in turn gives rise to hemolytic anemia of variable degrees. One patient with neurological symptoms has been studied so far biochemically and at the molecular genetic level. The point mutations found in this patient's GPI gene support the idea that GPI may have a neurological function in addition to its role in the carbohydrate metabolism; this is due to the presence of a monomeric sequence analogue called neuroleukin (NLK). The mutations apparently lead to the incorrect folding of this neurotrophic factor, and thus destroy the neurological activity.