Cells control or regulate the flux through metabolic pathways by means of I. allosteric control of enzymes. II. covalent modification of enzymes. III. genetic control of the concentrations of enzymes. IV. genetic expression of allosteric regulators.

I, II, III

Explanation:

Allosteric regulation is a common mechanism of regulation of enzyme activity, which generally involves key enzymes in metabolic pathways. Allosteric modulation occurs when a substance/molecule called 'allosteric regulator' binds to an enzyme at a site other than the enzyme's active site, thereby triggering a conformational change in the enzyme and thus affecting its activity. Moreover, covalent modifications such as phosphorylation, acetylation, methylation, etc, are also involved in the regulation of enzymatic activity in metabolic pathways. For example, phosphorylation is a type of reversible covalent modification of proteins consisting of the addition of phosphate groups at specific amino acid residues (i.e., serine, threonine, and tyrosine) by specific enzymes known as protein kinases. Phosphorylation alters the three-dimensional (3D) structure of proteins, thereby turning the substrate or cellular pathway to active and/or inactive. Finally, both transcriptional and post-transcriptional mechanisms are also able to control the expression of enzymes involved in signaling pathways. Transcriptional mechanisms are capable of limiting the amount of mRNA that is produced from a given gene (in this case, an enzyme encoding gene), whereas post-transcriptional mechanisms such as, for example, RNA interference pathways, control the translation of messenger RNA (mRNA) molecules into proteins.