Alright. So from our previous lesson video, we know that biochemists mainly focus on the initial reaction velocities of enzyme-catalyzed reactions. In this video, we're going to talk about a plot that biochemists commonly use to measure those initial reaction velocities, known as an enzyme kinetics plot. An enzyme kinetics plot graphs the initial reaction velocity of an enzyme-catalyzed reaction, or \( v_0 \), on the y-axis, and plots the substrate concentration on the x-axis. If we take a look at our image on the left hand side, you'll see an enzyme kinetics plot. Notice we have the initial reaction velocity plotted against the substrate concentration. If we follow it and make a curve, it makes a curve that looks really similar to the previous curves that we saw. However, we need to take into account what's actually on the x-axis, as simple as it might sound. You'll notice is that the x-axis shows the initial substrate concentration. In contrast to our previous graph which showed time in seconds on the x-axis. This means that in the previous graph, we monitored the progress of the reaction over time, but here, we are monitoring different reactions that have different substrate concentrations. It's essential to understand that the initial reaction velocity, or \( v_0 \), can only occur at a very specific period and very early on at the beginning of the reaction. This means we cannot monitor the initial reaction velocity over time because it would no longer be the initial velocity.

On an enzyme kinetics plot, time cannot be on the x-axis when including the initial reaction velocity. Instead of seeing this graph as a solid line, notice that we have individual data points instead. While textbooks might show a solid line, in reality, these data points come from completely separate experiments. Imagine there's one experiment in a test tube where a biochemist chooses a low initial substrate concentration, measures the initial reaction, and plots that data point. In a different test tube, the biochemist might increase the substrate concentration slightly, measure the reaction rate, and plot that data point. After collecting many data points, a curve of best fit is drawn, which you typically see in textbooks. However, drawing a solid line can be misleading as it suggests monitoring the initial reaction rate over time, which is not the case since time is not on the x-axis.

Also, note that the y-axis is different. In this graph, we have the initial reaction rate, unlike the product concentration seen in other charts. This discrepancy in axes leads to different interpretations when the graph levels out. The leveling out below signifies enzyme saturation, quite distinct from reaching equilibrium as might be interpreted in other settings where the product concentration levels out.

We also observe graphs comparing the initial reaction rate with the reaction rate after 30 seconds. The data shows that the reaction rate after 30 seconds is less than the initial reaction rate at corresponding substrate concentrations. Biochemists focus on initial reaction velocities rather than velocities at later times, like 30 seconds in, to ensure accurate representation of maximum velocity.

It's crucial that the initial reaction velocity varies with substrate concentration when other variables influencing the reaction rate, like temperature, pH, and enzyme concentration, are constant. Ensuring consistency across separate experiments allows for accurate comparisons between data points. This lesson has emphasized the importance of understanding the axes on enzyme kinetics plots, which dictate interpretation and will be revisited throughout our course in practice sessions. I'll see you guys there.