Functions of apoproteins are/is
Structural component of lipoprotein
Enzyme cofactors
Enzyme inhibitors
All of the above
Apolipoproteins carry out several roles:
(1) they can form part of the structure of the lipoprotein, for example, apo B;

(2) they are enzyme cofactors, for example, C-II for lipoprotein lipase,  A-I  for  lecithin:cholesterol  acyltransferase,  or  enzyme inhibitors, for example, apo A-II and apo C-III for lipoprotein lipase, apo C-I for cholesteryl ester transfer protein; and 
(3) they Apolipoproteins carry out several roles:
(1) they can form part of the structure of the lipoprotein, for example, apo B;
(2) they are enzyme cofactors, for example, C-II for lipoprotein lipase,  A-I  for  lecithin:cholesterol