about "do" expression

Hallo friends,
Why does not this do expression run?
1    (do ((x 1 (+ x 1))
2             (y x x))
3           ((> x 5))
4          (format t "(~A ~A) " x y))
Error: Attempt to take the value of the unbound variable `X'.
[condition type: UNBOUND-VARIABLE]

I think that the variable y in row 2 can refer to the variable x. In the 
first iteration has x been intialised to 1, while do is parallel 
evaluation, that menas, y can refer to x and assign y to 1. Why does 
intepreter give wrong report.

why does this do expression run?
(let ((x 'a))
         (do ((x 1 (+ x 1))
             (y x x))
           ((> x 5))
          (format t "(~A ~A) " x y))
(1 A)
(2 1)
(3 2)
(4 3)
(5 4)
NIL

I have only added a "let" expression and in "let" initialised x with 'a. 
 In previous "do" expression I have also initialised x with value 1. 
Whether can not  y direkt refer to x without "let" to initialise x in 
outmost enviroment?   What difference is there in this "do" expression 
with "let" and previous "do" expression without "let"?

Best Greetings
Xu