What is Initial rate, saturation? — Biology

Definition 13.5 University Biology — Year 1 · Chapter 13 — Enzymes and Biochemical Catalysis

The rate $v$ of an enzyme reaction is the amount of product formed per unit time ( $\mathrm{mol}/\mathrm{s}$ , or units of enzyme: $1\,\text{µ}\mathrm{mol}/\mathrm{min}$ ). It is measured as the initial rate $v_0$ , before the substrate is depleted or the product accumulates. At a fixed enzyme concentration $v_0$ rises with the substrate concentration $[S]$ and then saturates at a maximum $V_{\max}$ : the enzyme is fully occupied and can go no faster.

An enzyme assay: the product absorbs light, and the spectrophotometer records its appearance second by second; the initial slope is v_0. — An enzyme assay: the product absorbs light, and the spectrophotometer records its appearance second by second; the initial slope is $v_0$ .

Examples

Example 13.8 (Enzymes compared)

enzyme	$K_m$ ( $\mathrm{mol}/\mathrm{L}$ )	$k_{\text{cat}}$ ( $\mathrm{s}^{-1}$ )	$k_{\text{cat}}/K_m$ ( $\mathrm{L}\,\mathrm{mol}^{-1}\,\mathrm{s}^{-1}$ )
catalase	$2.5 \times 10^{-2}$	$4 \times 10^{7}$	$1.6 \times 10^{9}$
carbonic anhydrase	$1.2 \times 10^{-2}$	$1 \times 10^{6}$	$8 \times 10^{7}$
chymotrypsin	$1.5 \times 10^{-2}$	100	$7 \times 10^{3}$
lysozyme	$6 \times 10^{-6}$	0.5	$8 \times 10^{4}$
DNA polymerase	$1 \times 10^{-5}$	15	$1.5 \times 10^{6}$

Catalase and carbonic anhydrase work at the diffusion limit: every collision with substrate is productive. Chymotrypsin is slow but needs to be — it cuts a peptide bond, a far harder job than splitting peroxide.

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Examples

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