Studies on the Metabolism of Threonine and Related Substrates in Neurospora crassa

Citation

Reissig, José Luis (1952) Studies on the Metabolism of Threonine and Related Substrates in Neurospora crassa. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/J2RA-YB37. https://resolver.caltech.edu/CaltechTHESIS:02092018-110126859

Abstract

We have investigated the following enzyme systems in extracts of a wild type strain of Neurospora:

1) Threonine deaminase. It catalyzes the reaction giving rise to alpha-ketobutyric acid and ammonia from threonine. It was concluded that alpha-ketobutyric acid, glutamic acid or a deaminated alpha-ketobutyric acid precursor in equilibrium with alpha-ketobutyric could not be intermediates in this reaction. Pyridoxal phosphate activates the system, and a number of methods were tested in order to improve the resolution of enzyme and coenzyme in the preparations.

2) Serine deaminase. Yields pyruvic acid and ammonia from serine, and is also activated by pyridoxal phosphate. The responses of serine and threonine deaminases to pyridoxal phosphate are at variance, suggesting that two different enzymes are involved.

The effect of p^H and temperature on serine and threonine deaminase was investigated.

3) Glutamic-alphaketobutyric transaminase, which is activated by pyridoxal phosphate.

4) A system forming alpha-aminobutyric from threonine, possibly as a result of the summation of activities 1) and 3).

5) A system forming an unidentified blue fluorescent product by incubation with threonine, but not with any of a number of related metabolites (serine included).

6) Alpha-ketobu tyric decarboxylase, which is acti­vated by cocarboxylase, and has a pH optimun1 of 5.5.

The threonine deaminase activities of a number of threonineless strains and of a B₆-less strain were com­pared with those of wild type, using cultures grown under different conditions. The significance of the variability in activity encountered is discussed. Mutant 35423, which requires threonine for growth but is unable to use alpha­ ketobutyric or alpha-aminobutyric acid,has the ability of converting threonine into those acids in vitro.

Mutant 44104 cannot utilize alpha-ketobutyric acid in place of alpha-aminobutyric to initiate early growth, but its glutamic-alpbaketobutyric transamtnase is as active in vitro as that of wild type.

A new scheme of threonine biosynthesis is presented to account for the information available.

An attempt is made to find a common denominator to the mechanisms of the diverse coenzymatic activities of pyridoxal phosphate, and schemes for those mechanisms are proposed.

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