threonine biosynthetic process

Definition

Target type: biologicalprocess

The chemical reactions and pathways resulting in the formation of threonine (2-amino-3-hydroxybutyric acid), a polar, uncharged, essential amino acid found in peptide linkage in proteins. [GOC:jl, ISBN:0198506732]

Threonine biosynthesis is a crucial metabolic pathway in living organisms, responsible for the production of the essential amino acid threonine. Threonine is an important building block for proteins and plays vital roles in various cellular processes, including immune function, collagen synthesis, and nerve transmission. The biosynthesis of threonine occurs in two main steps:

**Step 1: Aspartate to Homoserine**

* The pathway begins with aspartate, a non-essential amino acid, which undergoes a series of enzymatic reactions to form homoserine.
* The first key enzyme involved is aspartate kinase, which catalyzes the phosphorylation of aspartate to β-aspartyl phosphate.
* This phosphate group is then transferred to the 4-hydroxyl group of β-aspartyl phosphate by aspartate semialdehyde dehydrogenase, forming β-aspartyl semialdehyde.
* β-aspartyl semialdehyde undergoes a spontaneous reduction to homoserine.

**Step 2: Homoserine to Threonine**

* Homoserine is then converted to threonine by the enzyme threonine synthase.
* Threonine synthase utilizes a two-step mechanism involving a pyridoxal phosphate (PLP)-dependent reaction.
* Firstly, homoserine reacts with PLP, forming an unstable Schiff base intermediate.
* This intermediate then undergoes a transamination reaction with a donor amino group (typically from glutamate) to yield threonine.

**Regulation of Threonine Biosynthesis**

* Threonine biosynthesis is tightly regulated to meet the organism's demand for threonine.
* The key regulatory step is the phosphorylation of aspartate by aspartate kinase.
* Aspartate kinase is inhibited by high levels of threonine and lysine, ensuring that the pathway is downregulated when these amino acids are abundant.
* Conversely, low levels of threonine and lysine stimulate aspartate kinase activity, promoting threonine synthesis.

**Importance of Threonine Biosynthesis**

* Threonine is an essential amino acid for humans and other animals, meaning it cannot be synthesized by the body and must be obtained through diet.
* Threonine is required for the synthesis of proteins, collagen, and other essential molecules.
* It also plays a role in immune function, neurotransmission, and energy metabolism.'
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Proteins (3)

Compounds (3)