Km: if this is a constant, does this mean that k1, k2, and k3 are fixed in relation to one another?
k1, k2, and k3 are rate CONSTANTS. They are fixed values for any particular enzyme and substrates and products. They are defined by how the rate of a part of the reaction depends on the concentration of reactants, such that: k1[X] describes the influence of the concentration of X on the rate of disappearance of X in a reaction of X < --> Y. Since X can also be formed from Y as well as turn into Y, the total description of how X changes with time is given by dX/dt = k1[X] - k2[Y], where the lower case k's are the rate constants. The k's are not rates, they are multiplication factors: the rate constants must be multiplied by the concentration(s) of reactants present to get the individual rates for each part of the reaction.