Suppose the molecular weight of enzyme = x
0.5% by weight means in 100 g of enzyme weight of Se = 0.5 g
$\therefore$ In xg of enzyme weight of Se $\frac {0.5}{100} \times x$
Hence, $78.4 = \frac {0.5\times x}{100}$
$\therefore \, \, \, \, \, \, x = 15680 = 1.568 \times 10^4$