  We introduce two subalgebras in the type A quantum affine algebra which are
coideals with respect to the Hopf algebra structure. In the classical limit q
-> 1 each subalgebra specializes to the enveloping algebra U(k), where k is a
fixed point subalgebra of the loop algebra over gl_N with respect to a natural
involution corresponding to the embedding of the orthogonal or symplectic Lie
algebra into gl_N. We also give an equivalent presentation of these coideal
subalgebras in terms of generators and defining relations which have the form
of reflection-type equations. We provide evaluation homomorphisms from these
algebras to the twisted quantized enveloping algebras introduced earlier by
Gavrilik and Klimyk and by Noumi. We also construct an analog of the quantum
determinant for each of the algebras and show that its coefficients belong to
the center of the algebra. Their images under the evaluation homomorphism
provide a family of central elements of the corresponding twisted quantized
enveloping algebra.
