Substrate — Science definition

The specific molecule (or molecules) an enzyme acts on.

Where it is taught

In use

Each enzyme folds so that part of its surface forms an active site, a chemically selective region that binds its substrate or substrates (the reactant molecules it acts on); shape and interactions can change during binding.

Enzymes

Questions using substrate

Problem 1

Rate increases with substrate concentration then levels off while enzyme amount is fixed. Give a plausible explanation. Explain the reasoning in your own words.
Answer and explanation

Enzyme saturation can explain the plateau; the design must rule out other limiting conditions.

At low substrate, more substrate increases encounters with active sites.

At high substrate, available sites can become occupied much of the time.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 2

An inhibitor resembles the substrate and occupies the active site. What mechanism fits? Show the reasoning, not just the conclusion.
Hints

A falling rate does not establish the exact molecular mechanism without additional tests.

Answer and explanation

Competitive inhibition is consistent with competing for the substrate-binding site.

Problem 3

The region of an enzyme that binds the substrate is the
  1. active site
  2. peptide bond
  3. phosphate head
  4. nucleotide
Answer and explanation

active site

The active site is a pocket shaped by the protein's folding; its shape and charge distribution determine which substrate fits.

Problem 4

Adding more substrate increases the rate at first, then stops increasing it. Explain why.
Answer and explanation

At low substrate concentration many active sites are empty, so adding substrate increases the number of productive collisions. Once nearly every active site is occupied, the enzyme molecules are saturated and the rate is limited by how fast each enzyme can process substrate, so further substrate makes no difference. Adding more enzyme would raise the plateau.

Response criteria

  • State a biologically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

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