this can be done via post-routing table and masquerade:
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
previous rule will nat all traffic trough the eth0 interface and change the source address (if that is the interface with the public ip address.) if you want to nat a single ip:
iptables -t nat -A POSTROUTING -src 192.168.1.45 -o eth0 -j SNAT --to 112.100.4.34
If you want to persistent the previously rule add the rule removing iptables
prefix in to /etc/sysconfig/iptables
file of your centos.
To have a detail of the iptables rules take a read here