why at higher substrate concentration the enzyme reactions are zero order while at very low concentration of substrate the enzyme catalysed reactions are first order??

As substrate concentration increases it has less and less effect - and eventually the rate reaches a maximum. Increasing the concentration will make no difference no difference to the rate of the reaction and therefore the reaction will be zero order.

After a certain concentration of substrate is reached, every enzyme molecule present in the mixture is involved in the reaction. If the substrate concentration increases further, there won't be any enzyme molecules to help the extra substrate molecules to react.