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Cellular needs and conditions vary from cell to cell and change within individual cells over time. For example, the required enzymes and energetic demands of stomach cells are different from those of fat storage cells, skin cells, blood cells, and nerve cells. Furthermore, a digestive cell works much harder to process and break down nutrients during the time that closely follows a meal compared with many hours after a meal. As these cellular demands and conditions vary, so do the amounts and functionality of different enzymes.

The rates of biochemical reactions are controlled by activation energy, and enzymes lower the activation energies for chemical reactions. The relative amounts and functioning of the variety of enzymes within a cell ultimately determine which reactions will proceed and at what rates. In certain cellular environments, environmental factors like pH, and temperature partly control enzyme activity. There are other mechanisms through which cells control enzyme activity and determine the rates at which various biochemical reactions will occur.

Metabolic pathways rely on specific enzymatic products expressed by specific genes, which in turn require continuous synthesis of certain metabolites and ATP. So, these two processes must coordinate with one another as cells adapt to the changing conditions. According to the body's requirement, transcription of the genes is regulated. When there is a need for a specific enzyme to carry out an essential biochemical process, then the concerned gene gets activated and undergoes the process of transcription and translation.

Moreover, enzymes can be regulated in ways that either promote or reduce their activity. There are many different kinds of molecules that inhibit or promote enzyme function, and various mechanisms exist for doing so. For example, in some cases of enzyme inhibition, an inhibitor molecule is similar enough to a substrate that it can bind to the active site and simply block the substrate from binding. When this happens, the enzyme is inhibited through competitive inhibition because an inhibitor molecule competes with the substrate for active site binding. Some inhibitor molecules bind to enzymes in a location where their binding induces a conformational change that reduces the enzyme's affinity for its substrate. This type of inhibition is an allosteric inhibition.

Catabolic and anabolic hormones in the body also help to regulate metabolic processes. Catabolic hormones stimulate the breakdown of molecules and the production of energy. These include cortisol, glucagon, adrenaline/epinephrine, and cytokines. All of these hormones are mobilized at specific times to meet the needs of the body. Anabolic hormones are required to synthesize molecules and include growth hormone, insulin-like growth factor, insulin, testosterone, and estrogen.

This text is adapted from Openstax, Biology 2e, Section 6.5: Enzymes, and Openstax, Anatomy and physiology 2e, Section 24.1: Overview of metabolic reactions

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